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Old 02-13-2020, 07:41 PM   #1
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Little help figuring out this gas valve on furnace.


Have a 100kbtu input furnace in the shop. We usually just keep it about 50F. I cranked it up today as I needed to work and realized the gas valve was cycling. It will go through a normal warm up, gas valve is getting a full 24.4V, it will then kick the blower and running normally for about 5min, then gas valve will then just kick out.

This one has a pilot and pilot stays lit. Because Tstat is still calling for heat, the blower will eventually kick down after cooling down enough, then after a few min, the gas valve fire up again and the process will continue.


It is cold here. I appears the gas valve is pulling about 100mA. While running, the voltage to the valve stays constant and voltage stays hot so this is not a thermal issue or anything that is shutting off the gas valve. The valve itself is kicking out for some reason.

Is it possible that gas pressure is low? Solenoid toast? How can I trouble shoot this? We need heat in that shop!
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Old 02-13-2020, 07:56 PM   #2
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Re: Little help figuring out this gas valve on furnace.


What’s the model number of the furnace?
Is the unit cycling on the limit? I assume coil voltage is dropping out to the valve when it fails?
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Old 02-13-2020, 07:58 PM   #3
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Re: Little help figuring out this gas valve on furnace.


No, I thought I made that clear, 24V is present when it shuts down. Not sure on model yet, but its 30yo or so. A Bryant I believe.
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Old 02-13-2020, 08:07 PM   #4
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Re: Little help figuring out this gas valve on furnace.


Is it standing pilot or intermittent - lit with spark?

need full model number.
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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 02-13-2020, 08:53 PM   #5
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Re: Little help figuring out this gas valve on furnace.


The plate on furnace is super hard to read but....
Appears to be RGMA-100C then CN3D203 Its a Rheem, my bad.


This is standing pilot. I pulled the burner inspection plate and did a burn. Everything appears normal and even got a 10min burn out of it. When the gas valve kicks on, it has a nice "tick" to it that it has opened. When it shuts down, it just sort of "rolls off".

At the time of this burn out, I did not have any other gas devices running and other furnace runs fine. Also been in place many years and never had this trouble.

The gas valve is a Unitrol, but I cannot determine the model just yet. I think it was 70001 or something like that. hard to see in there and my access is a bit restricted.

I did confirm the gas valve coil is rated at .1A at 24V so its about right. Not sure how sensitive they are, but wondering if that coil is heating, then resistance is going up just a touch to a point it won't hold any more. How/why it would reboot on its own is beyond me. this thing is stupid simple and the call for heat is constant to the gas valve.

Last edited by viper; 02-13-2020 at 09:01 PM.
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Old 02-13-2020, 09:46 PM   #6
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Re: Little help figuring out this gas valve on furnace.


marginal connection inside maybe?

tap the valve with the burner on and see if it cuts out in response to that.
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Old 02-13-2020, 09:56 PM   #7
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Re: Little help figuring out this gas valve on furnace.


I have a couple theories by now so I am headed out to do the typical cleaning of terminals and see what happens.

However, what I suspect is a pinhole in the main diaphragm which would cause gas pressure to eventually equalize on both sides so it can no long hold the "door" open. Its just a guess.

I am going to put my precision meter on the coil to see exactly what the current is. A clamp meter just doesn't cut it at 100mA.

If my other theory is right on the coil, which is it heats up and looses contact somewhere, I will see the current drop out but voltage to stay present.

Of course it would be one of the coldest days and I need that heater to protect stuff so I guess I am "in it" tonight.
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Old 02-13-2020, 10:16 PM   #8
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Re: Little help figuring out this gas valve on furnace.


I think the diaphragm is not doing its job but may not be defective itself. That wouldn't ordinarily be intermittent as described. The solenoid could have an intermittent connection that responds to the temperature change when energized for long periods of time. If that be true, subsequent run cycles would be shorter than the initial one.

Either way, you need a new valve.

If you have a 56 ohm 1 watt resistor lying around, you could put it in series with the 24 volts and see if the valve solenoid stayed cooler and ran without dropping out. That would drop the voltage down to a marginally low value. No repair but might give some temporary warmth until the new valve comes. Might need an even higher value (68 next norm size)... you could jumper it at pull-in and then remove the jump and it would probably hold at much lower voltage. Worth a shot if you are in dire need. Any TV repair shop will have that resistor.

Last edited by surferdude2; 02-13-2020 at 10:39 PM.
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Old 02-13-2020, 10:39 PM   #9
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Re: Little help figuring out this gas valve on furnace.


Quote:
Either way, you need a new valve.
...or just find a scrap furnace on kijiji/craigslist, 30 year old unit isn't worth putting a new gas valve into.
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Old 02-13-2020, 11:19 PM   #10
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Re: Little help figuring out this gas valve on furnace.


Probably a $250 valve, like most. It would be nice to cannibalize one from a junker if it came to that. Sometimes e-bay has the right valve... some people work at junk yards and grab that kind of stuff and make more money than their day job pays.

Good luck... it's hitting 11 F. here tonight and my Trane is still on the track!

edit: I see you mentioned it may be a Unitrol valve... that a lot cheaper valve, under $100 most models.

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Old 02-13-2020, 11:26 PM   #11
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Re: Little help figuring out this gas valve on furnace.


VERDICT!
Bad coil. First indication was though the old fluke clamp meter wanted to tell me .1A, my Fluke analyzer painted a different picture at .235A, which is more than double the rated current. Once I saw that, I figured I was onto something. Just FYI for the pros, use the right meter. The current looked very stable, then dropped out as predicted, still full voltage. IMPO, that did not act like a broken wire, that acted like a auto reset breaker built into the coil to prevent the magic smoke.

I decided if I could roll back the amperage, I might be able to sneak by for a bit if I could stay in the window where the valve would open. Wanted a 100ohm resistor but only had a 50, which got me to .15A. It worked and is holding the line for now.

User is right though, that thing won't get a new valve. If I can't find something used and free to throw at it, it will just get replaced. I think vintage is closer to 40yrs.

Will post a pic in a few.
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Old 02-13-2020, 11:30 PM   #12
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All I needed was some duct tape to make this repair official!
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Old 02-13-2020, 11:46 PM   #13
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Re: Little help figuring out this gas valve on furnace.


Quote:
Originally Posted by viper View Post
All I needed was some duct tape to make this repair official!
*
*
I hereby award you a full 10 points for style! I would have used a clothes pin but that just me... I'm a country boy so there's that to consider.

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