Furnace comes on Lo but no shift to High. Fan does not come on.
I have a Rheem Furnace RGPK series. Turn Heat on, set thermostat differential 10degrees or more) Stage one begins and the furnace ignites. However when stage two (hi burner) is called for the unit shuts down, ie, the burners go completely out. The thermostat begins flashing and 'call for service' comes on. The burners never get the chance to go 'hi' and the main fan never comes on (doesn't even try). There is no restrictions in 'return' air flow. I removed the lower panel and the fan area looks clean (system about 18mos old, 2nd winter). I have inspected the circuit board and found no appearance of damage, ie, burned wires/components, fuses, etc. Any help, advice is appreciated. Al
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Re: Furnace comes on Lo but no shift to High. Fan does...
There will be a separate pressure switch for hi fire, make sure all hoses are connected and free of moisture and debris. You can temporally jump the switch once the inducer fan speeds up, not before. If the unit fires on high, you have a pressure switch issue. If the fan does not speed up, you may have a bad relay on the control board. If the unit is just being put into winter service, it may have a cob web restricting gas flow to the burner. You will need to check the air inlet as well as the exhaust pipe for restriction. Check on the wiring diagram to make sure that the DIP switches are in the proper position, you never know. Hope this helps.
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Since you have two thermostats that operate the heating unit and the fan, and the fact that neither will turn the blower on, I would suspect the furnace/ air handler. Turn the thermostat to heat and see if the heat and the fan comes on, if it does operate normally in heat, the fan relay or the high speed on the blower is most likely the culprit. (If the furnace is equipped with a circuit board, the fan relay is integral to the board, if it an older furnace it has a separate fan relay). If it does not operate normally in the heating mode, verify power to the furnace, see if the flame comes on, if it does: the fan motor, circuit board or the fan relay are most likely the cause. hope this helps!
Sounds like the differential control (combustion air fan thermostat) that cycles the combustion air fan is stuck closed and the safety circuit opens at 212 degrees to shut off the combustion air fan due to overtemp conditions as the two are probably wired in series.
Hi Dave. I'm Roger. Its normal for a furnace to shut down completely when the set point on the thermostat is reached. The first and second stages are used when the temp. gets close to set point. The heat output drops to its low stage at around 3 degrees before set point. Check your owners manual for specifics on the fan though.
On properly sized furnaces there are a few controls to look at. Check the settings of the fan switch. If these are unchanged then there may be a reason the furnace is shutting off due to poor air flow and resulting high temperature. Check filter and verify air flow is good. It may be possible that a safety switch is failing and intermittently on and off. Are all the covers securely in place and hitting any safety switches? If not seated properly the cover may loosen when the fan runs and the switch shut off the unit. There is often a flame roll out switch that could have been strained. Lastly, has the thermostat been working long? Thermostats have internal heaters to stop the furnace which is called a heating anticipator. If the thermostat is near a heat source (heat duct, refrigerator, oven, person, hot water pipe), worn out, anticipator is set wrong (older electric thermostats have adjustable settings) or covered (yes, I have seen some covered) then the free room air cannot measure correctly and the thermostat cannot control what it cannot accurately measure. Of all these possibilities I would start with the thermostat and proper furnace air flow, second check the fan/high limit switch and third go component by component through the wiring diagram looking for defects in each component. If available follow the manufacturer's troubleshooting guide.
My guess is that you have gas heat. If this is the case, sounds to me like your main control board or depending on age of furnace, fan controller is not working. With gas furnaces, it utilizes 2 fan speeds. High for cooling, & Low for heating. Your thermostat only controls the High speed - switches when you turn your fan on & when there is a call for cooling. The only other thing I can think of is it is possible that the Low speed windings on the blower motor are open causing it to not turn on. Most motors have 3 to 4 speeds & only 2 are used. Switch the heat motor wires in the furnace (on the control board) with one that is on a spare terminal.
All you need is 2 fan speeds, 1 for cool and 1 for heat. Leave the low speed disconnected. Normally we have high speed for cooling, and med. or low for heat. So either speed connected to the heating circuit is fine.
Is there a setting on the thermostat for Auto or Fan? If so, make sure it is in the Auto setting. You will have a contactor that turns on the elements and a relay for the fan. Follow the wiring diagram to see which relay energizes the fan then unplug it to see if the fan stops.