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Old 02-28-2020, 04:59 PM   #1
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Rheem Gas Furnace Board Question


My RHEEM Furnace's Gas valve is not turning on. I checked the voltage across the Valve and it reads 13.9 V immediately after I turn the unit on and once the igniter ignites it still reads at 13.9V . The valve requires 24V to open. Question should the voltage across the valve not be 0.0 V immediately after the unit is turned but before the igniter comes on?

Question: Is this sufficient information to replace the control board or is there something else I should check before replacing the IFC?
Appreciate any advice.
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Old 02-28-2020, 06:06 PM   #2
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Re: Rheem Gas Furnace Board Question


There should be no voltage at the gas valve when it starts.

The pressure switch needs to close to prove there is sufficient draft and air for combustion B4 it energizes the igniter. Then it waits for 30 seconds to warm up the igniter or whatever timing they have programmed. Then the gas valve gets powered up.

It could be the gas valve relay on the board is sticking closed a tiny bit and allowing some voltage across.

If you can read the wiring diagram and it shows you the GV, gas valve relay and you can find it then try tapping it gently and if the voltage goes away the the relay and board are shot.
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Old 02-28-2020, 06:06 PM   #3
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Re: Rheem Gas Furnace Board Question


There shouldn't be voltage at the gas valve until the igniter is warmed up and it should be around 24v.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.
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Old 02-28-2020, 06:42 PM   #4
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Re: Rheem Gas Furnace Board Question


What is the furnace model number?
Is there a condensate pressure switch on this unit? Over to the left of the gas valve?
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Old 02-28-2020, 08:23 PM   #5
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Re: Rheem Gas Furnace Board Question


Yuri
Couldn't find the GV Relay attached is the wiring dia. if you can spot it using the coordinates I should be able to find it on the board.

user_12345a
I started it again and it goes to 13.9 V and stays there throughout the Ignition sequence until the system shuts down when it returns to 0.0V

roughneck
Model # RGRA-12ERAJS
there are 2 pressure switches see below
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Old 02-28-2020, 08:41 PM   #6
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Re: Rheem Gas Furnace Board Question


Test that one. Probably where your problem is. I have the exact same furnace in my house, and had the exact same problem.
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Old 02-29-2020, 12:20 AM   #7
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Re: Rheem Gas Furnace Board Question


I checked the pressure switch and on one terminal it shows 14V on the other it shows 0.0V which I think means the switch is open
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Old 02-29-2020, 12:43 AM   #8
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Re: Rheem Gas Furnace Board Question


Don’t check to ground. Check across the switch.
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Old 02-29-2020, 05:38 AM   #9
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Re: Rheem Gas Furnace Board Question


Normally you would have a error code if it has diagnostic lights showing pressure switch stuck closed.

That is a 1992 model and if it does not have error codes then likely the problem is the switch.

On most Lennox circuit boards they have markings like GV etc on/near the relays on the board and if you can read the small lettering you may be able to find which one. Not easy to do.

I suspect the pressure switch is the problem.
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Old 02-29-2020, 07:06 AM   #10
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Re: Rheem Gas Furnace Board Question


Quote:
Originally Posted by yuri View Post
Normally you would have a error code if it has diagnostic lights showing pressure switch stuck closed.

That is a 1992 model and if it does not have error codes then likely the problem is the switch.

On most Lennox circuit boards they have markings like GV etc on/near the relays on the board and if you can read the small lettering you may be able to find which one. Not easy to do.

I suspect the pressure switch is the problem.
Yuri, the furnace will have a diagnostic code for the pressure switch. However, itís for the other pressure switch in the system, the conventional one to prove draft through the heat exchanger. The one I referenced, is for the condensate system and is wired inline with the gas valve. Iíve highlighted the photo to reference.
Itís a bit of a odd design that Rheem did. The big problem is if that pressure switch starts to fail, and doesnít 100% stick open, it can deliver low voltage to the gas valve and burn the coil out quickly.
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Old 02-29-2020, 07:19 AM   #11
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Re: Rheem Gas Furnace Board Question


OK, that is a specific Rheem feature and problem.

Good to know. I wonder if the new units have it also as my Sister has a 2 stage unit that is around 5 yrs old. Now that I know it may be there I can be aware of it.

Older Ducanes have a weird 3 function pressure switch which monitors the drain/condensate trap for blockage also and if you don't understand it can be confusing.
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Old 02-29-2020, 07:24 AM   #12
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Re: Rheem Gas Furnace Board Question


Quote:
Originally Posted by yuri View Post
OK, that is a specific Rheem feature and problem.

Good to know. I wonder if the new units have it also as my Sister has a 2 stage unit that is around 5 yrs old. Now that I know it may be there I can be aware of it.

Older Ducanes have a weird 3 function pressure switch which monitors the drain/condensate trap for blockage also and if you don't understand it can be confusing.
I know they still make this furnace, but I’ve not seen a brand new one. So I’m not certain they still have this switch. The way I learned of this issue was via my own furnace when it had this problem.
I don’t see a lot of residential equipment at work, I’m primarily commercial and industrial.
Rheem is very popular around this area for home builders.
Mine is a bit different and I believe newer then the OPs furnace. The one I have is actually spark ignition to main flame. There is no intermittent pilot. It’s a very reliable and robust system.

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Old 02-29-2020, 07:30 AM   #13
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Re: Rheem Gas Furnace Board Question


I had a fluttering pressure switch on a Goodman and it damn near drove me nuts.

Caused the circuit board to chatter like crazy. It would not stick off long enough to give a open pressure switch code and would reset so fast the board did not know what to do. Keep starting the inducer on/off forever. Had the symptoms of a faulty inducer fan relay.

Rheem is a good meat and potatoes unit in their basic units and their circuit boards are almost bulletproof. Very rare I have problems with those boards.

Like the Energizer bunny they keep going and going and going.

They are a HUGE pain in the arze to remove the blower on as the trap is in the way etc but other than that they are a very reliable unit.
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Old 02-29-2020, 11:26 AM   #14
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Re: Rheem Gas Furnace Board Question


I checked for voltage across the Pressure switch in question and it reads 0.0V (open).

I also started up the furnace and noticed that this time the igniter did not come on. It had been turning on previously with the GV not energizing and then shutting down. This time it went through the whole sequence (a little over a minute) but the igniter did not light up and then it shut down.
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Old 02-29-2020, 11:42 AM   #15
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Re: Rheem Gas Furnace Board Question


Does it give you a error code with the flashing lights?

The legend of what they mean may be in fine print on the side of the wiring diagram and is hard to find.
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