19. Combustion Air Proving Switch
The combustion air proving switch is a pressure sensitive switch that monitors air pressure to ensure that proper combustion air flow is avail- able. The switch is a single pole - normally open - device which closes when a decreasing pressure is sensed in the outlet duct of the flue gas collection box.
On start-up when the heater is cold, the sensing pressure is at the most negative level, and as the heater and flue system warm up, the sensing pressure becomes less negative. After the system has reached equilib- rium (about 20 minutes), the sensing pressure levels off.
If a restriction or excessive flue length or turns cause the sensing pres- sure to become less than the switch setpoint, the pressure switch will function to shut off the main burners. The main burners will remain off until the system has cooled and/or the flue system resistance is reduced. The Table on the right lists the approximate water column negative pres- sure readings and switch setpoints for sea level operating conditions.
Model | Start-Up | Equilibrium | Set Point | Set Point |
Size | Cold | | "OFF" | "ON" |
25-400 | -1.0" w.c. | -0.60" w.c. | -0.47" w.c. | -0.64"w.c. |
| | | | |
| | | | |
DANGER: Safe operation of this unit requires proper venting flow. NEVER bypass combustion air proving switch or attempt to operate the unit without the venter running and the proper flow in the vent system. Hazardous conditions could result. See Hazard Levels, page 2.
20. Gas Valve
Main operating valve is powered by the 24-volt control circuit through thermostat and safety controls. The main control valve is of the dia- phragm type with magnetic pilot servo bleed operators, providing regu- lated gas flow preset at the factory. The valve body also incorporates a magnetic valve providing pilot gas control for the electronic ignitor sys- tem and redundant or dual valve safety shutoff function.
WARNING: The operating valve is the prime safety shutoff. All gas supply lines must be free of dirt or scale before connecting the unit to ensure positive closure. See Hazard Levels, page 2.
21. Pilot and Ignition System
These unit heaters are equipped with a spark ignited intermittent safety pilot system that shuts off the pilot gas flow between heat cycles. In addition, propane units are equipped with a spark pilot system that in- corporates a lockout device that stops the gas flow to the pilot if the pilot fails to light in 120 seconds. The spark pilot with 100% lockout requires manual reset by interruption of the thermostat circuit. Propane units require the lockout; natural gas units may be equipped with either standard spark pilot or spark pilot with lockout (Option AH3). Refer to the wiring diagram with your heater for pilot system identification and proper wiring.
The ignition controller in the spark pilot system provides the high volt- age spark to ignite the pilot gas and also acts as the flame safety device. After ignition of the pilot gas, the control electronically senses the pilot flame. (A separate solid metal probe in the pilot burner assembly is employed for the flame sensing function. A low voltage electrical signal
is imposed on that metal probe which is electrically isolated from ground. When the pilot flame impinges on the flame sensing probe, the flame acts as a conduction path to ground. The pilot flame recti- fies and completes the DC circuit. The ignition controller acknowl- edges the flame and energizes the main gas valve.)
22. Burners
These unit heaters have individually formed steel burners with accu- rately die-formed ports to give controlled flame stability without lift- ing or flashback with either natural or propane gas. The burners are lightweight and factory mounted in an assembly which permits them to be removed as a unit for inspection or service.
23. Burner Air Adjustment
All sizes of these unit heaters that are equipped with standard alumi- nized burners are designed to operate without burner air shutters when fueled with either natural or propane gas. However, Sizes 165 through 400 equipped with optional stainless steel burners (Option AD2) re- quire air shutters (Option AE1) when used with propane gas (Option AA2).
Optional air shutters, either factory or field installed, are available for any size model for use where unusual conditions cause excess pri- mary aeration.
Before making any adjustments to the air shutters, allow the heater to operate for about fifteen minutes. The air shutter adjustment screws can be reached by opening the bottom panel. (Remove the two screws located at the rear of the bottom panel and allow the panel to hinge down from the front.) The adjustment screws for the air shutters are visible at the rear of the burner rack . See Figure 14.
When making the adjustment, close the air shutters no more than is necessary to eliminate the problem condition.
Observe the flame for yellow-tipping. A limited amount of yellow- tipping is permissible for liquefied petroleum gases. Other fuels should not display any yellow-tipping.
Two adjustment screws are used (See Figure 14). Rotating the screws clockwise closes the shutters, reducing the primary air supply. Coun- terclockwise rotation opens the shutters, increasing the primary air supply. The two adjustment screws should be rotated alternately to open or close the shutters. Attempting to gain adjustment by not alter- nating between the two screws may cause the shutters to bind.
After proper adjustment has been completed, eliminating the problem condition, close the bottom panel and replace the retaining screws.
Figure 14 - Air Shutter Adjustment Screws -- Alternate Turning Screws When Adjusting Shutter
DANGER: Failure to install and/or adjust air shutters according to directions could cause property damage, personal injury, and or death.
