Over this past year, Zoom presentations have become a normal way of offering training classes while the country (and world) has grappled with the COVID-19 pandemic. One of our more popular classes is on the subject of hydronic system components.
Of all the components we use in a hydronic system, the most frequent questions I get during Zoom presentations are about diaphragm expansion tanks and pressure-reducing valves (PRVs) and how they interact with each other. Consistently, the contractors have been asking about what the proper settings are for each device and why.
Before we start applying answers to those questions, it is important to really understand what the functions of these components in a hydronic heating system are.
Pressure-reducing valves
A pressure-reducing valve is pretty much like its name implies—it takes the incoming street pressure and reduces it down to what is needed inside a particular building where it has been installed. The hot water heating system functions when the boiler and all of the piping and radiation are filled with water that comes from the city water main located in the basement.
This question comes up often…how do we know how much water is needed to fill the system? By using the pressure gauge on the boiler, we can determine when the system is completely filled with water. Water has weight and as you stack more water on top of itself, it weighs more. By using the pressure gauge on the boiler, we can determine how high up into the system the water has gone. The pressure gauge reads in pounds per square inch or psi. We know a column of water that is 2.31′ (or 28″) tall weighs 1lb per square inch at the bottom of that column.

 
 
 
 
 
 
 
 
 
 
 
The key to using a pressure gauge in determining the height of a column of water is the expression pounds “per square inch.” Whether the piping system has ¾” copper pipes or 4″ steel pipes, the measurement on the pressure gauge is the same…per square inch. A square inch is a square inch, it doesn’t change; therefore, a column of water 2.31′ tall weighs one pound per square inch.
To properly fill a hydronic system, measure from the boiler pressure gauge/PRV location up to the highest piece of piping or radiation (whichever is highest) in the building. Then take that number and divide by 2.31′ to convert to pressure in pounds per square inch.
However, don’t stop there—the pressure reading would ensure that the water is all the way to the top of the system, but what would the pressure be in the system at the highest point? It would be zero pounds per square inch, and if you had any high vents located at the top, how effective would they be? There would be 0lbs of pressure inside the system and 0lbs of pressure (atmosphere) on the outside of the system.
There is no motive force for any air bubbles to vent out of the system. To ensure that high vents will be able to do their job, the industry has standardized on adding an additional 4psi to the number required to get water up to the highest point. To establish the proper PRV setting for each application, measure in feet the distance between the boiler pressure gauge/PRV location to the highest pipe/radiation in the building, divide by 2.31′ and to that number add 4psi. The result will be the proper cold water fill pressure for that system. The key is to fill the system when it’s cold so that you will have an accurate reading from the pressure gauge.
Expansion tanks
Expansion tanks play a very important role in the proper operation of a hot water heating system; that function is very different from the PRV’s job but for both to be effective, they work together. To appreciate this relationship, you want to have a good understanding of what the expansion tank’s role is and how it does what it does.
When a system is completely filled with water and then is heated to the operating control’s high limit, there is anywhere from 3.5%–5.0% more water in the system because when heated, it expands. Here’s the problem—water is not compressible and so when this increase occurs, if there is no place to put this extra water, the relief valve on the boiler will open up and dump system water onto the floor.
This is where those diaphragm tanks come into play; air is a gas which is compressible and so the expansion tank becomes the place where the expanded water goes while keeping the pressure in the system below the relief valve’s setting. The air in the diaphragm tank acts like a spring, allowing the system water to push against it as it is heated and expands. The air in the tank is separated from the system water by using a butyl rubber that is flexible.
These tanks are different from the older-style steel compression tanks—in those tanks, the system water and air cushion came into direct contact with each other, and because of that, the tanks were larger than the diaphragm tanks. With a diaphragm tank, the air side is fully expanded, pushing the rubber diaphragm all the way against the other side of the tank. When connected to the system, the air side pressure is now seeing the system’s fill pressure. Remember, when cold, there should be no system water in the tank; for that to occur, the diaphragm tank’s air charge pressure must match the system’s fill pressure.
Diaphragm tanks
Diaphragm tanks are sized to accept the volume of expanding water in the system while keeping the pressure in the system below the relief valve’s setting. Normally PRVs and diaphragm expansion tanks come pre-set at 12 psi since most of the applications are for two-story buildings. If you have a system in a building that requires a higher pressure setting, the expansion tank must be pre-charged to the higher fill pressure setting.
If you did not match the air charge to the fill pressure, once the tank was attached to the system, a certain amount of cold system water would enter the tank. Remember, there should be no water in the expansion tank when the system is cold. The net result is the expansion tank acts like it is too small, causing the relief valve to open, discharging the excess pressure.
The only proper way to check the tank’s pre-charge setting is while it is disconnected from the system. If you were to check the pressure while the tank is attached to the system, it would be a faulty reading because the water pressure from the system is “squeezing” against the diaphragm. The gauge would just be reading the system pressure.
As you can see, each of these components has its own “job to do,” but to do them properly, they have to work together.
If you have any questions or comments, e-mail me at gcarey@fiainc.com, call me at FIA 1- 800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM

The landscape in the heating industry has drastically changed. Over the years, when you entered a typical boiler room in the Northeast, it was very common to find a sectional cast iron boiler(s) and usually some type of indirect heating appliance for domestic hot water. It could be a side-arm heat exchanger with a storage tank or a storage tank with the heating coil installed directly inside.
Nowadays, it has become quite common to find high efficiency modulating and condensing gas-fired boilers (either liquefied petroleum [LP] or natural). These boilers are not your average “run of the mill” atmospheric gas boilers, either, because their efficiencies range between 90%–96%. They use a Neg/Reg gas valve and fan assembly, which means the amount of gas that flows into the burner for combustion is regulated by the fan assembly’s blower speed. The blower speed is controlled by an on-board micro- processor that is performing several internal calculations to determine the appropriate amount of British thermal units (BTUs) needed to satisfy the call. Hence the modulating part—it only uses the amount of gas necessary to satisfy whatever load it is currently seeing.
Most of the residential models have a “turn down” ratio of 10:1, meaning they can fire down to 10% of their total capacity and, of course, all the way up to 100% of their capacity. It has become quite common in larger residences to install two or more small “mod/con” boilers, that, when combined, can handle the home’s total load. However, more importantly during the normal course of the heating season, when the home is operating at “part load,” the boiler plant consumes just the amount of energy needed to satisfy the current load that the house is seeing.

The same holds true for commercial applications such as apartment buildings, condominiums, churches and schools. The larger commercial “mod/con” boilers also offer turn down ratios 10:1. That means with a couple of commercial boilers, you can fire down to 5% of the total BTU capacity of the boiler plant. With this type of turn down, building owners are experiencing fuel savings in range of 35%–40% and higher!
Unique features
Another unique feature of these boilers is the venting options. The blower motor is designed to not only bring combustion air into the burner assembly but also vent the residual products out of the building. Most of the “mod/con” boiler manufacturers have approved their boilers to use several different vent materials. They are approved to be vented with PVC, CPVC, polypropylene and stainless steel vent pipe.
Each manufacturer provides very detailed instructions on the dos and don’ts of how to properly vent their boilers. Following these instructions is critical to allow the boilers to operate efficiently. Of course, all of this piping needs to be sealed tight to meet the venting codes.
These boilers encourage the condensing of their flue products, which is the exact opposite  of traditional boilers. Their heat exchangers are designed to withstand the corrosive nature of the condensate that forms when the combustion products are condensed. Of course, this condensing action is where the additional efficiency points are obtained.
Some of the by products of this condensing can gather in the boiler’s heat exchanger. If allowed to accumulate, they will negatively impact the boiler’s efficiency performance, which is why most manufacturers suggest an annual inspection and cleaning of the heat exchanger, if necessary. Also, the venting should be inspected to make sure nothing has changed that could negatively impact the operation of the boiler. This means every “mod/con” boiler needs to be inspected every year.
One of the oilheat industry’s shining stars has been its reputation for service and maintenance. The need for these high efficiency boilers to be maintained is a perfect opportunity for a company that has a service department to offer service contracts to homeowners, commercial property owners and management companies. Most of the boiler manufacturers or their local representatives offer classes on servicing these new “mod/con” boilers.
ECM circulators
A new style of “smart” pump has made its way into the North American hydronics mechanical rooms. These new circulators are ECM pumps. ECM stands for electronically commutated motor and they are very different from the PSC (permanent split capacitor) motors we are accustomed to with wet rotor pumps. The rotor in this ECM motor has permanent magnets instead of wire windings that are  separated from the system fluid. The magnets are located inside a stainless steel rotor can and react to the magnetic forces created by electromagnetic poles in the stator.
A microprocessor, which “sits on-board” the pump, reverses the polarity of the stator poles rapidly (within milliseconds), forcing the rotor to be rotated in the proper direction. The faster these poles reverse their polarity, the faster the rotor spins, meaning the faster the impeller spins.
ECM circulators can provide four times more starting torque compared to a PSC wet rotor pump and incorporate a microprocessor that has software on-board, allowing the pump to perform many functions.
For example, one application may call for a constant pressure differential where the building is zoned with zone valves. Normally as valves close, the pump would develop additional head pressure across the remaining open zones, causing an increase in flow rate through these zones. This wastes energy as well as creates potential noise problems due to increased velocity. With this constant differential in pressure capability, as valves close, the pump momentary senses an increase in differential pressure and quickly slows down the pump’s speed to eliminate the change in pressure. The result is no change in flow rate through the remaining open zones, no wasted energy and no velocity noise problems.
Another application that the microprocessor can control is called proportional differential pressure. The circulator control is set for a specific design head loss for a system. When the zone valve (or valves) then closes, once the pressure differential starts to climb, the circulator reduces its motor speed. The difference here is proportional control instead of maintaining a set differential. It will lower the speed and thus pressure differential proportionally to the reduction in flow rate. The result is an increased reduction in energy consumption.
The efficiency of these Greener circulators is designed to meet the ever-increasing efficiency standards that have made their way to North America. Their “wire to water” efficiency is higher than the current PSC wet rotor circulators. In addition, their multiple application capabilities with on-board microprocessors, and their reduction in wattage use, make them a compelling alternative to current offerings. Contact me at gcarey@fiainc.com or 1-800-423-7187. ICM

This happens every year—an older hot water boiler fails and needs to be replaced. Not always, but most times, the other components accompanying the boiler get replaced as well. The circulators get upgraded; the flow control valves and the air separator get replaced; and, of course, a new expansion tank replaces the old one.
In most of these systems, there is an existing (albeit older) diaphragm expansion tank. In the process of upgrading the tank, typically you just get the model number of the existing tank and replace it with the same, newer version.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
What happens if that hot water system you are working on has one of those old steel compression tanks? You know the ones—installed up near the ceiling, usually suspended with strapping of some sort in between the floor joists? We call them “old-style” because they were invented before the concept of using a flexible butyl membrane was introduced.
These steel tanks had air and water “touching” each other in the tank. The job of the air volume was to act like a spring on the system to maintain adequate pressure throughout the closed system. It was important to always have a certain volume of air in the tank to allow for the expanding system water to “squeeze” against while keeping the pressure below the relief valve’s setting. We certainly don’t want water splashing onto the boiler room floor from the relief valve every time the boiler heats up the system on a call for heat.
When the time comes to replace this “old-style” tank, you have two options. However, before you replace it, you must confirm a few things: the original tank worked properly (and was therefore sized properly) and there will be no changes to the application. Once these are confirmed you can:
1) Replace the old tank with the exact same size and style tank; or
2) Replace it with the common “diaphragm style” expansion tank that the industry has been frequently using for the last 40+ years. This style tank design has separate compartments for the air and system water that are separated by the flexible butyl diaphragm.
Terminology
When expansion tanks are sized properly, formulas are used to come up with the correctly-sized tank and sometimes they can be intimidating and hard to follow. When converting from the old-style tank to a modern diaphragm tank, a lot of the “heavy lifting” has already been done for us—that is, how the original tank was selected. We just have to apply some information that would be pertinent to our particular system to select the correct diaphragm tank.
Before we get there, let’s talk about some terminology that deals with expansion tanks. Here are two common terms:
1) Full Acceptance Tanks: the tank is big enough to accept all of the system’s expanded water volume while keeping the pressure range within working conditions (fill and relief valve pressure). This would include the old steel tanks as well as most commercial bladder-style tanks.
2) Partial Acceptance Tanks: the diaphragm style has a limited amount of expanding system water storage capability. This amount of water that can be stored is called the acceptance volume. This style tank is the most common type used in residential applications.
It quickly becomes apparent that when sizing a diaphragm tank, there are two criteria that need to be satisfied:
1) The total tank volume has to be large enough to keep the system pressure within operating range.
2) The acceptance volume has to be as large as the system’s expansion volume. The actual amount of expansion volume must be known. Fortunately, the original tank was sized with this piece of information.

 
 
 
 
 
 
 
 
 
 
 
 
A sizing example
Let’s walk through a sizing example to see how you can select a replacement diaphragm tank once you know the size of the original old-style steel tank. For this example, the system’s fill pressure will be 12 pounds per square inch gauge (psig) and the relief valve setting is 30psig.
1) The first formula establishes the total tank volume:
a. The formula is Vt pressurized = Vt standard (Pa/Pfill)
b. Where Vt psi = Total tank volume of pressurized tank
c. Vt standard = Size of existing old-style steel tank in gallons
d. Pa = Atmospheric pressure
e. Pfill = Fill pressure in absolute pressure (gauge pressure + atmospheric pressure)
i. 60 gals (old-style tank volume) x 14.7/(14.7 + 12)
ii. 60 gals x 14.7/26.7 = 60 gals x .551 = 33 gallons
iii. Vt psi = 33 gallons
2) The second formula establishes the actual system expansion volume:
a. Ve = acceptance volume
b. Ve = Vt (Pa/Pfill) – (Pa/Pmaxop)
c. Vt = Size of existing old steel tank in gallons
d. Pa = Atmospheric pressure
e. Pfill = Fill pressure in absolute pressure (gauge pressure + atmospheric pressure)
f. Pmaxop = Maximum Operating Psi (Relief Valve Setting + atmospheric pressure)
i. 60 (14.7/26.7) – (14.7/44.7)
ii. 60 x (.551 – .329)
iii. 60 x .222 = 13.3 gallons
g. Ve = 13.3 gallons
In this example, if the existing old-style steel tank had a volume of 60 gallons, the fill pressure requirements were for 12psig (most two-story residential applications) and the boiler’s relief valve was set for 30psig, the replacement diaphragm tank specifications would require a tank with a:
• Total tank volume of 33 gallons
• Acceptance volume of 13.3 gallons
Not that this happens every day, but here at FIA, we do come across this question often enough. Also, remember to pre-charge the diaphragm tank to the system’s required fill pressure before it gets connected to the heating system.
See Chart 1 for details on the diaphragm tanks’ acceptance volume and total tank volume for the various sizes; see Chart 2 for details on the dimensions and gallons for the old steel compression tanks. If you have any questions or comments, e-mail me at gcarey@fiainc.com, call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_GCarey. ICM

A contractor asked me to visit an apartment building that was giving the property management company a lot of headaches with nuisance service calls, so I met him on the job.
What we saw when we walked into the boiler room was quite remarkable. The first thing that got my attention was the six, old pressure-reducing valves (PRVs) sitting on top of the boiler. The next item of interest was the expansion tank or lack thereof…there was a 3/4″ copper line piped off the top of the boiler and it went straight up into the sheet-rocked ceiling. However, we couldn’t see any expansion tank, only a piece of pipe!

A drainage system that led to a floor drain in another room, built by a janitor to deal with a leaky boiler, was only a temporary fix. A much bigger problem loomed.

Like a one-man band, the connections of a one-pipe steam heating system need to be positioned correctly for everything to work harmoniously.

Steam systems with various problems have been coming at me fast and furious, even though it is still the summertime as I write this column. I have noticed though, with all of the jobs, there is one constant—the person wrestling with the problem really doesn’t understand the nuances that a steam system brings to the table. These include:
• A steam system is filled with air anytime the system is off. If you want heat, you have to get rid of all the air before the steam can get in and heat the radiation.
• A steam system operates nothing like a water system…the steam, when manufactured in the boiler, desperately wants to turn back into water and it will whenever it touches something cooler than itself. If you don’t make enough steam, it will never reach the furthest radiators.

The boiling point of water actually depends on the pressure
system it is in.

In order to make dry steam, the water must be free of oil and
debris.

In our world of providing comfort and energy efficiency to our customers, there are certain formulas that are used on a regular basis. The most important one, when talking about a hydronic heating system, is GPM (gallons per minute). Heat is distributed from the boiler room out to where the people are via water. How much water determines the flow rate; the term we use is called GPM. An accurate heat loss reading in a building is very important to establish the design load conditions. Once the load is established, we can calculate the necessary flow rate.

GPM = Heat Load/ 500 ^T

GPM is the flow rate in gallons of water per minute. The heat load is expressed as BTU/H (British Thermal Units per hour), which is the heat loss of the building at design conditions. ^T is the temperature difference that occurs from the supply to the return when the water is circulated through the radiation. Five hundred is the constant for standard water properties at 60°F and it comes from multiplying the weight of one gallon of water at 60°F, which is 8.33 pounds by 60 minutes (one hour).
The complete calculation is:

GPM = BTU/H
8.33lb./gal x 60 min x ^t°F

The formula indicates a water temperature of 60°F. However, since 60°F water is too cool for a hot water heating system, and too warm for a chilled water system, you would think that to calculate the correct flow rate, the formula should be based upon more appropriate water temperatures for each type of system—for instance, based on things such as the specific heat of the water, the density changes that occur as the water changes in temperature and the water volume changes when it gets hotter or as it cools down. As you can see from the following example, the differences are so minimal that the standard formula works fine for all of our heating and cooling applications.

In hydronic heating systems, the heat is distributed through water.

8.04 x 60 x 1.003 x 20 = 9,677 BTU/H

Under pressure
The net effect is not significant, but there is another factor that needs to be considered for a complete evaluation. As water temperature rises, water becomes less viscous, and therefore the pressure drop is reduced. When water is circulated at 200°F, the corresponding pressure drop, or “head loss,” is about 80% of water at 60°F for a typical small hydronic system.

When calculated using a system curve, the flow increases by about 10.5%. Now you can multiply the new heat conveyance just calculated by the percentage of flow increase:

1.105 x 9677 = 10,693 BTU/H

As you can see with regard to heat conveyance, the simple “round number” approach will result in design flows very close to the “temperature-corrected” flows, providing that the results from the “round number” approach isn’t corrected from the original 60°F base for both the heat conveyance and piping pressure drop. The plus and minus factors very closely offset one another.
The right circulator
GPM plays a major role in ensuring that your heating system performs as expected. You need the right sized circulator to be able to move the heat from the boiler and deliver it out to where the people live. In selecting the proper circulator, not only do you need to know the correct GPM, you also need to know the required pressure drop to circulate the necessary GPM.
As water flows through the pipes and radiation, it “rubs” against the pipe wall causing frictional resistance. This resistance can affect the performance of the heating system by reducing the desired flow rate from circulating, thus reducing the heating capacity of the system. By knowing what this resistance will be, you can select a circulator that can overcome the system’s pressure drop.
Typically, in today’s systems, we use “feet of head” to describe the amount of energy needed so that the required GPM is delivered out to the system. There are pipe sizing charts that have calculated the pressure drop in foot head of energy loss for any flow rate through any size pipe. There are standard piping practices in which the industry references that limit the amount of GPM for a given pipe size.

When calculating gallons per minute (GPM), don’t forget about water pressure.

Tell me if you have ever heard someone say something like this: I think the room is under-heating because the water is moving too fast through the baseboard. If I use a smaller pump or at least throttle down on the pump’s flow rate, the baseboard will start heating better.
I have heard this statement numerous times over the years from technicians that were having problems with their customer’s heating system. I guess you could reason that with the water moving so fast, it doesn’t have enough time while inside the baseboard to “give up” its heat.

When cold air blows, it moves heat away from our bodies much faster than normal, causing the wind chill factor. Photo by Juho Kuva/Getty Images.

1. Surface contact area
2. Temperature difference (between the water and the inside wall of the tubing) and
3. The convection coefficient (which is calculated based upon the properties of the fluid, the surface area’s shape and the velocity of the fluid)

Instead of spending too much time with the math formulas, use your mind’s eye to visualize the following: as the stream of water is flowing through the baseboard, the outer edge of this stream is in direct contact with the tube’s inner wall. This “rubbing” against the wall creates drag, which means the water that is touching the inner wall of the tubing is moving slower than the “core” or inner stream. Because of this, the temperature of this outer layer of water becomes cooler than the inner stream. In fact, this drop in temperature impacts the rate of heat transfer—it slows it down.
Remember, one of the factors that affects convection is temperature difference. A good visual is to think of this outer layer as an insulator that impacts the rate of heat transfer from the hotter inner stream of water to the tubing’s inner wall. This is especially true when the speed of the water approaches laminar flow instead of turbulent flow. So, in effect, the faster the water flows through the tubing, the outer boundary layer or insulator becomes thinner, thus increasing the rate of heat transfer from the hotter inner “core” water to the tubing’s inner wall.

 
 
 
You can confirm this by taking a look at any baseboard manufacturer’s literature and checking out its capacity charts. They will typically publish their BTU output per linear foot based upon two flow rates: 1gpm (gallon per minute) and 4gpm. The BTU output is always higher in the 4gpm column.
Wind chill factor
Here is another way to consider this concept of more speed (faster flow rate) equaling more heat transfer (higher output). Consider the size of a hot water coil used in an air handler and the amount of BTUs it can provide. Now think about how much fin-tube baseboard you would need to install to provide the equivalent amount of BTUs.
The difference is that the speed at which the fan blows the air across the coil is much faster than the air that flows naturally across a baseboard. Everyone experiences this phenomenon each winter. The weatherman refers to it as the “wind chill” factor. He will tell you the actual temperature outside, but because of the wind chill, it will feel “X” degrees cooler. Why? The cold air is moving across our bodies much faster. This takes heat away from us much faster, so it feels colder than it really is.
A trick of the trade passed on to fellow technicians over the years is to turn up the Aquastat setting on the boiler to increase the water temperature. The reason technicians will do this is—if they have a room or a zone that isn’t quite heating up to the thermostat’s setting—by increasing the water temperature, they can increase the amount of BTUs per linear foot available from the baseboard (of course, this will only work if the boiler is big enough to offset the home’s actual heat loss).
Does faster water = more heat?
Knowing that a baseboard can provide more heat with hotter water, follow this next example to see why moving the water faster and not slowing it down will provide more heat.
Let’s use average design conditions when sizing for the amount of baseboard you would need to offset a room’s heat loss. Typically, we design a hot water heating system for a residential home at a 20°F temperature drop. That means the water would enter the radiation at 180°F and exit 20°F cooler at 160°F. The average water temperature in the radiation would then be considered 170°F. You would then check the BTU/h rating of the baseboard at this average water temperature and determine how many feet of baseboard the room needs to offset the heat loss.
What would happen if we increased the flow rate so that the water only took a 10°F temperature drop? If it entered the radiation at 180°F and came out at 170°F, then average water temperature would be 175°F. At this higher average water temperature, the baseboard would be capable of providing more BTU/h output. Taking it one step further, if the system only took a 5°F temperature drop, the average water temperature would be 177.5°F in the radiation, yielding even more BTU/h output.
You can now see that water cannot be moving too fast through your baseboard to prevent the BTUs from jumping off where needed. Of course, to achieve these tighter temperature drops while delivering the right amount of BTU/h, the flow rate has to increase accordingly.
Here comes the trade-off—with higher flow rates through a given pipe size, the pressure drop increases quite dramatically. The result can be a need for very large, high-headed circulators that cost more and use more electricity. Also, the higher flow rates increase the velocity of the water, and at some point, the velocity will create noise issues.
The point is not to design a system around these high flow rate/small temperature drops, but rather recognize that the next time someone tells you that they think the water is moving too fast and that is why the room is under-heating, you’ll know that is not the cause of the problem.
If you have any questions or comments, e-mail me at gcarey@fiaince.com or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM

Waste not, want not. This expression dates back to around 1772 and it basically means if you don’t waste something you will never have a want for it. And those who waste will want. Waste and want or save and have. When it comes to heating systems, you don’t want to create a system that uses the fuel source inefficiently.Waste not, want not!
Over the years I have seen several systems that violate this principle. With energy costs on everybody’s radar screen, it is in your best interest to provide or improve your customer’s existing hot water to minimize their waste. There are several steps that you can take to eliminate wasted energy in a heating system. This includes two very obvious ones, which have little to do with the actual heating system:
1. Increase the insulation in a home and
2. Upgrade the windows (whatever heat the system produces—keep it inside the building for as long as possible).
Regarding the heating system, short-cycling is a major efficiency killer. It strikes in multiple areas, including mechanically and economically.
The mechanical problems occur because of the rapid cycling on-and-off of the boiler. All of the various components found on an oil-fired boiler have an expected life cycle. When a boiler is short-cycling, the components are seeing all these cycles in a very short time span.
This leads to premature control failures, nuisance lock-outs, service calls and frustrated customers. If you want to kill a brand new boiler in less than five years and frustrate yourself and your customers along the way…short-cycle it!
The economical problem is often unknown and is certainly under-appreciated. There is an old rule-of-thumb that states: A short-cycling boiler will operate at least 15% points below its rated efficiency when said boiler is not short-cycling. The loss of fuel efficiency can be staggering, which means the wasted fuel consumption is paid for by the unsuspecting homeowner with the new high-efficiency boiler.
So if you want to prevent short-cycling, what can you do? The first step is to ensure that the boiler is not oversized. When a boiler is too big, it will always produce more energy (BTUs) than the system can receive/use (without overheating the space). By being too big it reaches its high limit very fast, not allowing the boiler/burner to operate in a “steady state fashion.”
The best way to make sure that a new boiler is not too big is to perform an accurate heat loss assessment on the house. There are several software-based heat loss programs available that will help you establish the heat loss of any building. By using this information you can then select the right size boiler for the house instead of relying on someone who came before you and installed the previous, and perhaps wrong-sized, boiler.
Unfortunately, no good deed goes unpunished. A boiler can still short-cycle even when it is sized properly. Do you know why? Load and zoning are the reasons. A properly sized boiler is sized for design conditions. This means that when it is very cold outside (design outdoor temperatures), the boiler is capable of keeping the occupants at design indoor temperatures which is usually around 70°F. However, these outdoor design conditions exist for less than 5% of the total heating season, which means for the remainder of the heating season, even the properly sized boiler is too big and can lead to short-cycling.
Hydronic systems and zones
An advantage and/or selling feature of a hydronic system is that it can be zoned very easily. Most homeowners like the idea of being able to control sections of their house even right down to a room-by-room control. This unfortunately can also lead to short-cycling.
If one or two small zones are calling and the boiler fires in response, the energy output of the boiler is greater than the needs of the smaller zones. The high limit is reached very rapidly and the boiler shuts off. The zones continue to call and the water temperature drops. The limit control responds and the boiler fires up again. Of course, the high limit is reached quickly and the boiler turns off. So even though the hydronics industry promotes its zoning capabilities and homeowners enjoy the comfort and control offered by zoning, it can really foul up the boiler’s potential efficiency rating.

While new windows and proper insulation help with heating costs,
there are other measures a homeowner can take.

In the business of hydronic heating and cooling, there are certain formulas that are used on a regular basis. An important one deals with a system that uses water as its means to deliver comfort in GPM (gallons per minute). Water is the way in which heat is distributed from the boiler room out to where the people are.
How much water determines the flow rate and GPM. An accurate assessment of heat loss in a building is very important to establish the design load conditions. Once the load is established, then we can calculate the necessary flow rate.

GPM = Heat Load/ 500 ^T

GPM describes the flow rate; the heat load is expressed as BTU/H, which is the heat loss of the building at design conditions. ^T is the temperature difference that occurs from the supply to the return when water is circulated through the radiation. Five hundred is the constant for standard water properties at 60°F and it comes from multiplying the weight of one gallon of water at 60°F which is 8.33 pounds x 60 minutes (1 hour).

The complete calculation is then:

GPM = BTU/hr
8.33lb./gal x 60 min x ^t°F

The formula indicates a water temperature of 60°F. However, since 60°F water is too cool for a hot water heating system and too warm for a chilled water system, to calculate the correct flow rate, the formula should be based upon more appropriate water temperatures for each type of system, such as the specific heat of the water or the density changes that occur with changing water temperature. Also, the volume of the water changes when it gets hotter or as it cools down. As you can see from the example that follows, the differences are so minimal that the standard formula works fine for all of our heating and cooling applications.
An example
The formula we use to determine system flow rate assumes a mass flow rate of 500 lbs per hour for each GPM, which means at a 20°^T, 1 GPM will convey 10,000 BTUH (500 x 20) referenced to 60°F water. What happens to the heat conveyance of 1 GPM at 20°F ^T when the circulated water has an average temperature of 200°F? Water at 200°F has a density of 8.04 lb/gallon instead of 8.33 as at 60°F; however, its specific heat goes up to 1.003 from 1.0 as at 60°F. The heat conveyance for 1 GPM at 20°^T will then be:

8.04 x 60 x 1.003 x 20 = 9677 BTUH

The net effect is not significant, but there is another factor that needs to be considered for a complete evaulation. As water temperature rises, it becomes less viscous, and therefore its pressure drop is reduced. When water is circulated at 200°F, the corresponding pressure drop or “head loss” is about 80% of water at 60°F for typical small hydronic systems. When calculated using a system curve, the flow increases by about 10.5%. Now you can multiply the new heat conveyance just calculated by the percentage of flow increase:

1.105 x 9677 = 10,693 BTUH

As you can see, with regards to heat conveyance, the simple “round number” approach will result in design flows very close to the “temperature corrected” flows, providing the results from the “round number” approach aren’t corrected from the original 60°F base for both the heat conveyance and piping pressure drop. The plus and minus factors very closely offset one another.

This article provides an accurate formula to calculate the
flow rate in gallons per minute (GPM) for hydronic heating
and cooling systems.

Bell & Gossett’s The System Syzer is helpful to determine
gallons per minute (GPM).

When designing a radiant heating system, it becomes obvious that this system has characteristics different from the conventional baseboard-type of heating system. One quick difference is the temperature of the water circulated through the tubing. Most radiant systems can be classified into two types.

The first is a “wet system” in which the tubing is installed into concrete. The second type is a “dry system” where the tubing is either stapled-up from underneath the floor or laid down on a sub-floor and the final flooring placed on top of it.

The average water temperatures are 110°–120°F for the concrete type and 130°–140°F for the staple-up application; of course, there are exceptions where hotter or cooler water may be required. Unfortunately, most oil-fired boilers can’t operate at such low temperatures without experiencing flue gas problems. The best way of overcoming this problem is by using some type of mixing device, which lowers the supply temperature to the radiant zone(s) while allowing the boiler loop to maintain a temperature high enough to satisfy its requirements. There are numerous mixing methods available.

Mixing concerns
Here are some common concerns regarding the subject
of mixing:

What is mixing?
Mixing is when you take some cooler return water and “mix” it with some hot boiler water to supply a temperature of water that is lower than the boiler temperature but warmer than the return water.

Are there different methods available for mixing?
You can use a two-way valve, a three-way valve, a four-way valve or a circulator. All four devices can be used to supply a mixed water temperature.

How does each of these methods work?
1. A two-way valve works on the injection principal. There is a boiler loop with a circulator and the radiant loop with its own circulator. These two circuits are inter-connected through a supply pipe and return pipe that are spaced close together. A two-way valve is located on the supply pipe and has a controller that measures the radiant loop’s supply water temperature. The controller will cycle the valve open and closed based on the water temperature in the radiant zone. When the valve is opened, it injects bursts of hot water into the radiant loop. There, it mixes with some cool return water from the radiant zone.

2. A three-way valve mixes cool return water with hot boiler water to supply a “mixed” temperature. It has three ports, one for the return water from the radiant zone, one for hot water from the boiler loop and a mixed port to supply the radiant zone. These valves can be manually set to maintain a fixed temperature or they can have an actuator that repositions the valve according to the load.

3. A four-way valve is very similar to a three-way valve except it has four ports instead of three. Two ports go to the boiler and two ports go to the radiant zone. This valve can be set manually or used with an actuator to modulate the water temperature based on the zone load.

4. The last method is with an injection pump. This method has been used since the early 1960s. Back then, a controller would cycle the pump on and off to inject bursts of hot water into the radiant zone. Today there are control companies that will control the speed of a water-lubricated, impedance-protected, wet rotor pump. Instead of turning the pump on and off, the control increases or decreases the speed of the pump.

How to choose
Here are some general concerns for mixing:

Is one mixing method preferred over the others?
Not really, all of these methods work, but each method does come with its own benefits as well as its own limitations.
1. For example, two-way valves should only be used for small loads where the amount of the injected flow rate is a small percentage of the radiant zone’s total flow rate, typically less than 25%.
2. Three-way, self-contained, thermostatic valves are relatively inexpensive but can only provide one fixed temperature. This causes the zone’s thermostat to cycle the zone pump on and off. This type of operation is fine for a small radiant zone but not recommended when the zones become larger.
3. Injection pumping with a variable speed controller has become popular over the past few years. This method of mixing, which uses common wet-rotor circulators, provides many benefits to a radiant system such as full temperature modulation and boiler return protection from cold water. It is limited only by the pumping capacity of these wet-rotor circulators, which are typically around 35–40 gpm. In a typical radiant system, that flow rate equates to approximately 1,000,000 BTU/H.
4. Three-way and four-way valves, when used with actuating motors, have been installed in many radiant systems very successfully for years. The actuator adjusts the valve’s position to supply the appropriate mixed water temperature based on the heating load of the zone. The only real limitation to this method—compared to the cost of a wet rotor circulator—is that the valve and actuator are more expensive than an injection pump.

What happens if I use only one pump with the mixing device?
There will only be one mixing point, which will control the supply water temperature to the radiant zone, but not the temperature of the water returning to the boiler. Besides, the flow rate through the boiler will vary, decreasing the efficiency of the boiler.

Why should I use two pumps?
With two pumps and a mixing device, you create two mixing points. This allows you to control the temperature of the water going back to the boiler as well as to the radiant zone. In addition, the second pump provides constant flow through the boiler, increasing the boiler’s efficiency.

Why should I be concerned with the temperature of the water going back to the boiler?
Most oil-fired boilers are of the non-condensing type. This means it is important that the flue gases, released from the combustion process, are vented out of the boiler. When the water in the boiler is at a temperature below the dewpoint of these flue gases, the gases will condense back into water inside the boiler. The results can be very damaging. In commercial applications, boiler thermal shock is another important reason to control the return water temperature.

Is there a preferred way to pipe the mixing devices and the two pumps?
The preferred method is to use primary-secondary pumping. This method, which has been around since the 1950s, prevents pumps from pumping in series with each other and prevents valves from having a hard time opening or closing against high head pumps. This piping technique also allows the valves and injection pumps to be properly sized to the loads they are intended to control.

What is primary-secondary pumping?
It is a pumping technique that is simple in both theory and application. It is based upon a simple rule that states: When two circuits are interconnected, flow in one will not cause flow in the other if the pressure drop in the piping common to both is eliminated.

How do you eliminate the pressure drop in the common pipe?
This is achieved by keeping the supply and return tees to the secondary circuit very close together! (Maximum four pipe diameters) This means you can have two circuits interconnected, (for example, a boiler loop and a radiant loop, each with its own pump) but the pumps from each circuit will not cause flow to occur in the other loop.

How do I properly size the mixing device?
The size of the pump or valve is based upon the required flow rate from the high temperature loop. This flow rate will then blend with a portion of the cooler return water to supply the desired “mix” water temperature. This is an example to calculate the necessary flow rate:

 1. Radiant zone load =100,000 BTU/H designed with a 20°F temperature drop.
2. Design radiant zone flow rate =10 gpm
3. Radiant design supply temperature = 120°F
4. Return temperature = 100°F.
5. Boiler loop supply temperature = 180°F
6. The temperature difference between the boiler loop supply and the radiant return loop is 80°F. To calculate the flow rate required; divide the BTU/H load of the radiant zone by the temperature difference (delta T) x 500. 100,000/ 80 x 500 = 2.5 gpm.
7. The required flow rate is only 2.5 gpm of 180°F boiler water. This water will mix with 7.5 gpm (10 gpm–2.5 gpm) of 100°F radiant return water to supply a design water temperature of 120°F of 10 gpm. Therefore, the control valve or injection pump should be sized for a flow rate of 2.5 gpm.

If you have any questions or comments, e-mail me at gcarey@fiainc.com or call me at FIA 1-800-423-7187 or follow me on Twitter at @Ask_Gcarey. ICM