Written on: March 25, 2022 by George Carey
There have been many condensing boilers installed during the past few years that operate with natural or propane gas as their fuel source. They are called “high-efficiency” boilers because they have efficiency ratings in the 90s…some as high as 95%. To attain such high efficiency numbers, they intentionally condense some of the flue products that are formed as the result of combustion.
Normally, we would make sure that the boiler never condenses its flue products because, if allowed to, the condensate could damage the boiler and vent piping. When combustion occurs, energy in the form of heat is transferred through a heat exchanger (pinned cast-iron sections, copper-finned tubes, cast-aluminum and stainless-steel heat exchangers) to flowing water, which is on the other side of the flame. When the flame is produced, combustion gases are formed. These gases contain water in the form of vapor.
Generally speaking, we want these vapor gases to vent out of the boiler, up into the venting/chimney system and out into the atmosphere. Condensing boilers are designed to allow the flue gases to condense right inside the heat exchanger. They even provide a condensate drain to allow the condensate to escape from the heat exchanger and drip through a neutralization kit into a drain or condensate pump.
The benefit of condensing these combustion gases is that they contain heat or energy that we normally lose up the chimney. Just like in a steam system, when the water in the form of vapor (combustion gases) condenses, it gives off a lot of latent heat. How much? For every pound of water vapor that condenses back to liquid, 1,000 BTUs of latent heat are released. The heat exchanger “catches” this heat and transfers it over to the system water. This is how these condensing boilers achieve higher efficiency ratings. More of the unit of fuel goes into heating the system water rather than up the flue pipe.
Inner workings
Just how are these flue gases condensed? It is the exact opposite method of how we prevent these same gases from condensing in a non-condensing boiler. Condensing occurs naturally—when the combustion vapors cool below dewpoint, they will condense back to liquid. Water temperature has the greatest impact on whether the flue gases in a boiler will condense or not. It’s all related to the dewpoint of the combustion gases. Oil-fired systems want to keep the water above 140°F to prevent the gases from condensing, while gas systems generally want to be above 130°F to prevent condensing.
In a condensing boiler, the water temperature in the return needs to be 130°F or less for the flue gases to condense. With return water in that temperature range, the flue gases will condense and the boiler will operate at or near the published efficiency rating. Whenever the return water climbs above 130°F, which is higher than the dewpoint of the combustion vapors, those gases will not condense in the heat exchanger.
The particulars
The question often is, Can I use a high-efficiency modulating and condensing boiler in a system that incorporates traditional high temperature baseboard? Some people in the industry would say, No you can’t! The water needs to be hot enough to satisfy the baseboard’s requirements, which are too hot to allow any condensing of the flue gases! If you aren’t condensing the flue gases, why use a condensing boiler?
A large portion of these condensing boilers are sold in retrofit applications for both residential and commercial buildings. The boilers usually are cast iron oil-fired or atmospheric vented gas boilers and they serve high temperature terminal units such as copper baseboard or cast iron radiators.
When these systems were first installed, the radiation was sized so that on a design cold day, with water circulating through the radiation at 180°F, the room temperature could be maintained at 70°F. Due to these design conditions, the above comments can be made about compatibility issues between condensing boilers and baseboard radiation.
However, how often during the heating season do we actually encounter design conditions? Up here in the Northeast, design conditions make up about 3–5% of heating. The conditions are somewhere less than design during the rest of the year. In fact, through Bin Data collected by the National Weather Bureau, 80% of the heating season requires 50% or less of the BTUs needed for design conditions. In effect, the heating system, including the boiler and the installed radiation, is oversized for most of the heating season.
Proper application of Outdoor Reset
Outdoor Reset calculates the right water temperature for the radiation based upon the load that the house or building is experiencing. What has the greatest impact on a building’s load? The outdoor temperature! By simply incorporating the outdoor reset function that comes with the condensing boiler’s operating control, the boiler can start delivering the appropriate water temperature needed at the given set of outdoor conditions.
As you look into design conditions, reset curves and Bin Data, you see that for a large majority of the heating season, the boiler can lower the water temperature so the return temperatures coming back to the boiler are below combustion gas dewpoint levels. The boiler flue gases are now condensing; the boiler is operating at or near its rated efficiencies and the apartment building or house is comfortable.
When designing around 180°F water, 70°F indoors and a design outdoor temperature of between 0°F and 10°F, you will find that, until it gets down to 25°F or colder, the reset curve will calculate a water temperature that provides return temperatures below the dewpoint of the flue gases, ensuring the boiler is operating in a condensing mode.
Another benefit to this style of boiler is that, in addition to the condensing feature, the burner can modulate. This means that as the load changes, the boiler will consume only the necessary fuel to meet that load. Unlike traditional “On/Off” boilers—where, if they are firing, they are consuming 100% capacity—the modulating boiler can fire down to as low as 10% of its capacity and then modulate all the way up to 100%.
In a perfect situation, when using a condensing boiler, the radiation chosen should be able to provide all the BTUs needed with low temperature water. For that to occur, all of the existing homes and commercial buildings would have to change and/or upgrade their existing radiation. In some rare instances, that actually does happen—but the majority of the time it doesn’t.
Will the boiler condense all the time? No, but it will for the majority of the heating season. It will also modulate its firing rate to match the load that the building is experiencing. All of these features add up to reduced fuel consumption and more comfortable heating systems. If you have any questions or comments, e-mail me at gcarey@fiainc.com or call me at 1-800-423-7187. ICM
