Written on: June 1, 2021 by Timmie McElwain
As we look into some more modern systems and what specific problems they present, it is important to understand the basic fundamentals associated with these systems.
Most of our modern heating equipment in some way or another involves electronics, and with the electronics, the use of Flame Rectification as a safety and flame-proving system.
It does not matter if it is a forced warm air furnace or a forced hot water boiler—the same basic system is used to perform safe ignition and then consistent operation throughout the entire call for heat.
There are however different ways it is applied from Intermittent Pilot Application to Direct Spark Ignition and including Hot Surface Ignition. Each has its own distinct advantages and problems.
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Universal Gas Valves Systems
In this section we are going to discuss Universal Gas Valves systems—in particular the Robertshaw 720-079 Gas Valve Uni-Kit. We will present some of the basics initially, then the installation and finally get into troubleshooting.
The 720-079 is designed to work equally well on Intermittent Pilot, Direct Spark and Hot Surface Ignition. Its features include:
• Dual automatic valves (redundant)
• Wiring connections, manual selector and adjustments on top of the valve
• Meets all codes requiring dual valve safety shutoff
• Reduces service truck stock-freeing up valuable space and saving money.
The 720-079 shown in Figure 1 features a manual selector, wiring connections and adjustments that are all easily accessible on the top of the valve. The 720-079 Uni-Kit has 1/2″ side outlets and a pressure regulator that is preset at 3.5″ water column (WC) for natural gas. For liquefied petroleum (LP) gas applications, the valve can be converted by using a pressure regulator conversion kit that is included.
The 720-079 incorporates a manual valve, pilot valve (dual automatic valves) and a main gas pressure regulator all in a small compact valve with a swing radius of 3–9/16″. The small size of the 720-079 lends itself to replacing many original equipment manufacturer (OEM) and competitive valves. The 720-079 is multiposition and can be mounted in any position, except upside-down.
The red manual selector arm easily identifies the 720-079 shown in Figure 2. In fact, all Robertshaw 7200 series valves with this arm are dual seated valves. The valve also features side outlets, which can be plugged if not used. Also illustrated are the inlet and outlet pressure taps and the regulator adjustment cap.
Figure 3 illustrates the capacities for different inlet and outlet sizes for the 7200 series. The 720-079 as shown in the ordering data is ½” x ¾” valve with a capacity of 150,000 BTUs. The cross-references in Table 1 cover White-Rodgers and Honeywell Valves.
Intermittent Pilot: Pilot Tubing
Figure 4 illustrates the installation instructions and wiring for intermittent pilot applications. The instructions for pilot tubing are as follows:
1. Make sure tubing is free of burns and dirt.
2. The pilot orifice must be checked and cleaned if necessary at this time.
3. Connect pilot tubing into the control using fitting provided and tighten for a gas tight seal.
Wiring
DO NOT short gas valve terminals. This will damage the wall thermostat and void the warranty.
1. Check the system for the proper transformer by comparing the volt amps (VA) ratings of the transformer and the system. The system rating is determined by multiplying the voltage draw x the amp draw. Normally 20VA transformers are sufficient for heating only applications and 40VA for heating/cooling applications. (Note: improper transformer VA rating will cause erratic system operation).
2. Connect the wire from the “MV” terminal on the ignition control unit to the “M” terminal on the gas valve.
3. Connect the wire from the “PV” terminal on the ignition control unit to the “P” terminal on the gas valve.
4. Connect the wire from the “MV/PV” terminal on the ignition control unit to the “C” terminal on the gas valve.
Hot Surface Applications: Pilot Tubing
Figure 5 shows the wiring and instructions for hot surface ignition (HSI) applications. The pilot plug must be in place as there is no need for a pilot. It can be either local sense using the igniter as a sensor or remote sense with a separate flame sensor. The instructions for pilot tubing are as follows:
Note: Some hot surface applications use the pilot outlet. If so, proceed as directed below. If not used, install pilot outlet tubing plug packaged in the 720-079 gas valve kit, then proceed to “wiring.”
1. Make sure tubing is free of burns and dirt.
2. We strongly recommend that the pilot orifice be checked and cleaned if necessary at this time.
3. Connect pilot tubing into the control using fitting provided, and tighten for a gas-tight seal.
Wiring
DO NOT short gas valve terminals. This will damage the wall thermostat and void the warranty.
1. Check the system for the proper transformer by comparing the VA ratings of the transformer and the system. The system rating is determined by multiplying the voltage draw x the amp draw. Normally 20VA transformers are sufficient for heating only applications and 40VA for heating /cooling applications. (Note: improper transformer VA ratings will cause erratic system operation).
2. Locate the white wire terminal adaptor (jumper) that is included with the 720-079 gas valve.
3. Connect the white wire terminal adaptor to the “M” and “P” terminals on the 720-079 gas valve.
4. Determine which wire was connected to the “main valve” terminal on the original valve. Connect this wire to the remaining terminal on the white wire terminal adaptor.
Direct Spark: Pilot Outlet
Figure 6 illustrates the wiring and instructions for direct spark ignition either remote or direct sense. The installation instructions for the pilot outlet are:
First, install the pilot tubing plug packaged in the 720-079 gas valve.
Wiring
DO NOT short gas valve terminals. This will damage the wall thermostat and void the warranty.
1. Check the system for the proper transformer by comparing the VA ratings of the transformer and the system. The system rating is determined by multiplying the voltage draw x the amp draw. Normally, 20VA transformers are sufficient for heating-only applications and 40VA for heating/cooling applications. (Note: improper transformer VA rating will cause erratic system operation).
2. Locate the white wire terminal adaptor (jumper) that is included with the 720-079 gas valve.
3. Connect the white wire terminal adaptor to the “M” and “P” terminals on the 720-079 gas valve.
4. Determine which wire was connected to the “main valve” terminal on the original valve. Connect this wire to the remaining terminal on the white wire terminal adaptor.
5. Connect the remaining wire from the original gas valve to the “C” terminal on the 720-079 gas valve.
Operating Instructions
The operating instructions are as follows:
1. Turn the wall thermostat to its lowest setting. Remove burner access panel(s). Selector arm must only be operated by hand (see illustration in Figure 7). Do not use pliers, wrenches or other tools to operate the arm. The selector arm cannot be moved into the Off position without first depressing the lever in the On position.
2. Move selector arm to the Off position.
3. WARNING: Wait at least five minutes to allow any gas in the combustion chamber to vent; if you then smell gas in the appliance area or near the floor, STOP and follow warning instructions. Failure to do so may result in fire or explosion.
4. Move selector arm to the On position, and set room thermostat to desired temperature. Allow burner to cycle on and off.
5. Leak test with a soap solution after installing with main burner on. Coat pipe and tubing joints, gasket, etc. with soap solution. Bubbles indicate leaks.
6. Replace burner access panel.
Pressure Regulator Adjustments
An adjustment of the pressure regulator is not normally necessary since it is preset at the factory. It may need to be adjusted, however, to maximize the combustion efficiency of the equipment. (Note: manometer attachment may be accomplished at pressure tap plug, above control outlet, as shown in Figure 7. It is also a good idea to measure inlet pressure at the tap on the other side of the valve). Field adjustment may be accomplished as follows:
1. Manometer or gauge attachments may be accomplished at pressure tap plug (D).
2. Remove regulator adjustment screw cap (top of control-A).
3. With small screwdriver, rotate adjustment screw (B) clockwise to increase, or counterclockwise to decrease, pressure to comply with manufacturer’s specifications.
4. Replace regulator adjustment screw cap (A).
Regulator Conversion
Caution: main burner and pilot orifices must be changed when regulator is converted from one type of gas to another.
1. Turn off gas and electricity to appliance.
2. Remove slotted cap (A), adjusting screw (B) and natural gas spring—not color-coded— (C), from control.
3. Install new LP spring—color-coded with a black strip.
4. Install new adjusting screw (B).
5. Attach manometer or pressure gauge at the outlet pressure tap (D).
6. Turn gas and electricity on.
7. Turn room thermostat to call for heat.
8. With burner on, adjust screw (B) to supply LP gas to pressure as recommended by the appliance manufacturer.
9. If adjusting screw (B) reaches its maximum depth (bottoms out) before recommended pressure setting is reached, turn screw counterclockwise until pressure drops slightly (approximately 0.1″ WC)
WARNING: do not stretch or alter spring.
10. Turn off gas and electricity to appliance.
11. Remove manometer or pressure gauge and reinstall pressure tap outlet plug (D).
12. Install new red slotted cap in place of (A).
13. Turn gas and electricity On.
14. With burner operating, immediately check all fittings for leaks with soap solution. Bubbles indicate leaks that must be corrected.
15. Conduct a combustion test on the equipment to determine safe and efficient firing of the equipment.
16. Attach label to show control has been converted to LP.
17. Set room thermostat to desired temperature.
In this article, we will discuss Universal Gas Valves systems, in particular the White Rodgers 36E98-304 Universal Slow Opening Gas Valve. We will present some of the basics initially, then the installation and finally get into troubleshooting.
The 36E98-304-combination gas valve illustrated in Figure 1 is designed for intermittent ignition, direct spark ignition and hot surface ignition system applications. The valve is equipped with a redundant solenoid valve that controls gas flow to the pilot and main burners, a main valve that controls gas flow to the main burner, a pressure regulator to maintain a constant outlet pressure and a two-position gas cock knob for manual gas shut off. It uses a 0.008 internal orifice to cause the slow opening affect. Do not use this valve to replace White-Rodgers 36E02, 36E37 or 36E97 slow opening valves; they use different orifices and have different opening characteristics.
The pipe size/capacities are from 3/8″x 3/8″ giving 75,000 British Thermal Units (BTUs) for natural gas and 121,500 for liquefied petroleum (LP) gas to 0.5″x 0.75″ giving 125,000 for natural gas and 202,800 for LP gas.
The valves are shipped with the pilot outlet plugged. For installations requiring a pilot, remove the plug and use the fitting supplied separately with the control. Figure 2 shows location of pilot outlet along with outlet pressure tap. Features include:
1. Poppet style manual valve (capable of withstanding high inlet pressures)
2. Conical inlet and outlet screens protected from pipe damage
3. High sealing force direct acting solenoid valves
4. Controlled gasket clinch between castings to with stand high inlet pressures
5. Tamper-resistant screws
6. Automated assembly and test
Figure 3 shows the correct piping procedures. Figure 4 illustrates the internal operation of the gas valve.
Valve features and performance
The next few sections illustrate some of the features and options on this valve series. Not all of these features may be found on equipment you run into but they are options that Original Equipment Manufacturers (OEMs) can choose for their particular equipment.
Table 1 illustrates the particulars of the one-inch pressure drop capacity.
Features & Options: Fast Open Control
This control has a fast-opening characteristic that is suitable for a wide range of applications. It provides a fast rise to full pressure upon energizing the main and redundant solenoid. Regulator spring conversion kits are available for this control.
Features & Options:
Slow Open Control
This control has slow opening characteristic. It provides a slow increase of gas to full pressure for smoother ignition, as may be required by some applications. There is also a 36E36-304 fast opening valve from White-Rodgers that is classified a universal replacement valve. There is a cross-reference for this valve in Table 2. It has all the other wiring and other characteristics of the 36E98-304 other than its opening time.
Intermittent Pilot Directions
See Figure 5.
1. Connect wire from “MV” terminal on ignition control unit to “M” terminal on gas valve.
2. Connect wire from “PV” terminal on ignition control unit to “P” terminal on gas valve.
3. Connect wire from “MV/PV” terminal on ignition control unit to “C” terminal on gas valve.
Hot Surface Ignition Directions
See Figure 6.
1. Connect tan wire terminal adaptor (jumper) included with gas valve to “M” and “P” terminals on gas valve.
2. Determine which wire was connected to “Main Valve” terminal on original valve. Connect this wire to remaining terminal of the tan terminal adaptor.
Features & Options: Delay Open Control
This control has delayed opening characteristic. This control is equipped with a pressure switch system.
Features & Options: Step Open Control
This control has a step open feature that regluates the outlet to an initial step pressure (non-field adjustable) and rises to the full outlet pressure. It provides a step pressure for softer ignition.
Features & Options: Two-Stage Control
This control has a two-stage feature. Upon energizing the main and redundant control valves, the servo regulator will provide a low fire rate until the high fire solenoid is energized. (Field adjustable on both low fire and high fire).
Features & Options: Convertible Control
The convertible regulator option for this control involves a construction that permits easy Natural to LP conversion (or vice versa) by removing, inverting and replacing the regluator cap screw. This feature easily lends inself to use in mobile homes and infra-red applications.
Table 2 gives the cross-reference for the 36E98-304-gas valve and includes the OEMs that use the old part listed.
Adjustment
Hot Surface or Direct Burner Ignition Systems Install jumper wire across M-1 and P-3 terminals. This will open both redundant valves together.
Manual Valve Knob
The manual valve knob is a two-position (on-off) type. To turn manual valve On, rotate knob clockwise or counterclockwise to line up the word On on the knob with the indicator on the cover casting as shown in Figure 7. To turn manual valve Off, rotate knob clockwise or counter-clockwise to line up the word Off on the knob with the indicator on the cover casting.
Pilot Gas Adjustment
These controls are factory preset to provide proper pilot gas flow for most applications. If additional adjustment is necessary, follow these steps:
1. Turn off all electrical power to the system before making any adjustments.
2. Remove the pilot adjust cover screw and gasket to expose the adjusting screw shown in Figure 8. Turn the adjusting screw clockwise to reduce pilot flame, or counterclockwise to increase pilot flame.
3. Replace the gasket and pilot adjust cover screw and tighten securely.
4. Restore power to the system.
Pressure Regulator Adjustment
These controls are shipped from the factory with the regulator set for 3.5″ W.C. (natural gas, full flow). Consult the appliance rating plate to ensure burner manifold pressure; it should be 3.5″ W.C. If another outlet pressure is required, follow these steps. If a valve has been factory-adjusted for the 2.5″–5″ W.C. range, it cannot be field-adjusted outside that range. This is also true for valves adjusted to the 7.5″–12″ W.C. range for LP gas.
1. Turn off all electrical power to the system.
2. Attach a manometer to the outlet pressure tap of the valve.
3. Turn on system power and energize valve.
4. Remove regulator cover screw and turn regulator adjust screw clockwise to increase pressure, or counterclockwise to decrease pressure, as shown in Figure 8. Always adjust regulator to provide the correct pressure according to the OEM’s specifications listed on the appliance rating plate.
5. Replace regulator cover screw and tighten securely.
Figure 9 is a top view of the valve showing the manual knob, pilot solenoid, electrical panel, main solenoid and the adjustment for the regulator. On all gas valves, only main gas is regulated. The pilot is on line pressure and is adjusted by the screw, to the right of the manual knob.
Please contact assteditor@indoorcomfortmarketing.com for graphs concerning the 1″ pressure drop capacity and performance curves for the gas valves in this series, as well as an overview of gas valve models available. ICM
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Timmie M. McElwain is President of Gas Appliance Service, which provides training for those servicing gas combustion equipment. He is a certified instructor and test proctor for the Propane Gas Association in their CETP program.
