New to the forum, old forced air furnace problem

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Old 02-28-08, 08:38 AM
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New to the forum, old forced air furnace problem

I will try to make this as detailed and brief as possible. Yesterday, the my old forced air gas furnace kicked off and didn't come back on.

I went downstairs to check it out, and noticed that the pilot was lit. So I thought there was a problem with the fuse box or thermostat. The fuses are fine. I replaced the batteries in the thermostat, it is set to "heat", to "auto", and I hear a click when it makes the connection to tell the furnace to turn on.

So now I'm thinking there is a problem with the blower, but I stuck my hand inside and it spins freely. I took the cover off the combination temperature control, turned the dial clockwise slightly, and the blower kicks on, and will stay on only blowing cold air unless I manually move it back.

So, it doesn't seem to be a electrical, or thermostat problem. It appears that the furnace is ignoring the thermostat and not firing.

As I mentioned, the pilot is lit and seems to be burning strongly. It is not a weak flame. The flame is touching the thermal couple properly, the top 1/2"-3/8" of it. So I'm wondering if I should try to replace it... There is a hardware store close by that has them, and it's the only thing I can think of.

If replacing the thermal couple doesn't solve the problem, can anyone suggest anything else to check? If the pilot is burning, what would cause the furnace to not fire up and get hot?
 
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Old 02-28-08, 08:50 AM
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I'm not a furnace expert, but if the pilot light is on, the thermocouple is okay. It's job is to insure that you have a flame at the pilot and are not dumping raw gas into the furnace. I would suspect the control box where the thermocouple and pilot light lines originate. Sounds like it is getting a signal to turn on the main burners but is not doing so. There is a different sensor for the blower and it tells the blower when the furnace temp is high enough to start moving air over the plenum and thru the house.
 
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Old 02-28-08, 09:00 AM
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Originally Posted by goldstar View Post
I'm not a furnace expert, but if the pilot light is on, the thermocouple is okay. It's job is to insure that you have a flame at the pilot and are not dumping raw gas into the furnace. I would suspect the control box where the thermocouple and pilot light lines originate. Sounds like it is getting a signal to turn on the main burners but is not doing so. There is a different sensor for the blower and it tells the blower when the furnace temp is high enough to start moving air over the plenum and thru the house.
Yeah, I just read somewhere else that if the pilot is lit, the thermocouple is OK.
 
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Old 02-28-08, 09:12 AM
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You'll have to use a volt meter to see if you're getting 24 volts ac at the gas valve during a call for heat.
 
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Old 02-28-08, 10:02 AM
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OK. So other research I've done conflicts and makes it sound like it could still be a problem with the thermocouple. I may just pick up a new one just in case.

As far as using a volt meter to check the 24 volts AC... I have one that I bought for my car, but I'm not experienced with using it. It has two settings for AC marked 750 and 200. It also has three places to plug the red & black wires into. The options are:

10ADC (10A MAX UNFUSED)
VOmA (750VAC, 1000VDC, 200mA MAX)
COM (500V MAX)

It is made by Actron. Any suggestions or links on how to use this thing?

EDIT: Found a .pdf manual for the volt meter, going to test it now.
 

Last edited by djpeckonbox; 02-28-08 at 10:15 AM. Reason: clarification
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Old 02-28-08, 12:06 PM
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I believe I did this correctly. The furnace has what I think is called a transformer on the inside wall. Wires leave the transformer going to the gas valve and the CTC. The CTC works, and when I test the AC at the transformer terminals, I get a reading. When I try to test the connection where the wires go to from the transformer, to the gas valve, I get nothing. Now, mattison said to test the gas valve during a call for heat, since it is so cold in here, wouldn't the thermostat always be calling for heat? My point is, I think I did it properly, and it seems to not be responding...

So if we are to assume that this is the case, I need a new gas valve. Searching around, I'm finding that they're pretty pricey, and my new problem is that I can't find any information on a replacement.

The model number on the valve currently installed is:

V 810 A 1012

Is there a generic 24 volt thermocouple standing pilot gas valve I can replace this with? Or any other crucial info I need to consider?
 
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Old 02-28-08, 12:25 PM
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Another question...

Something I keep seeing mentioned is 1/2" inlet x 3/4" outlet. What I've noticed about mine is that the line coming out is actually smaller than the line going in. Am I confusing terminology here? Or actually looking at replacement parts that are not compatible?
 
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Old 02-28-08, 02:43 PM
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AC @ 200.

Hookup up the 2 leads in the 3 holes by choosing the bottom two and red lead goes in middle one and black in bottom one.
 
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Old 02-28-08, 03:00 PM
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Originally Posted by ecman51` View Post
AC @ 200.

Hookup up the 2 leads in the 3 holes by choosing the bottom two and red lead goes in middle one and black in bottom one.
Cool. That's what I did and the meter showed nothing, just a bunch of zeros. But I did test other things to make sure the meter was working properly since it has been sitting around for a while. So... The gas valve could be the culprit...
 
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Old 02-28-08, 03:11 PM
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If no 24 volts AC there, then you are not getting 24 volts out of your control board or module. And you can't condemn THAT yet either until you make sure that 24 volts is making it's way through every safety switch and entering the control board or module, before suspecting the control board or module.

A further definitve test would be to disconnect the two wires from the gas valve that you are testing and then put your leads on those wires, while the inducer has been running, and then see if you get voltage.

If you HAD 24 volts at the gas valve, and yet no burner came on, THEN it could be the gas valve.

Have you stuck the leads into an outlet to make sure you get 110-125 volts from one, at your 200 setting? You should.
 
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Old 02-28-08, 03:21 PM
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Originally Posted by ecman51` View Post
If no 24 volts AC there, then you are not getting 24 volts out of your control board or module. And you can't condemn THAT yet either until you make sure that 24 volts is making it's way through every safety switch and entering the control board or module, before suspecting the control board or module.

A further definitve test would be to disconnect the two wires from the gas valve that you are testing and then put your leads on those wires, while the inducer has been running, and then see if you get voltage.

If you HAD 24 volts at the gas valve, and yet no burner came on, THEN it could be the gas valve.

Have you stuck the leads into an outlet to make sure you get 110-125 volts from one, at your 200 setting? You should.
Well, The gas valve looks like its supposed to be getting its electricity from something called a transformer. I'm not sure if the transformer, a control board, and a module can be considered the same thing or not...

What I do know is that the transformer also has wires going to the CTC, and the blower, and those two things work, and show AC in the volt meter, while the gas valve shows nothing....
 
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Old 02-29-08, 05:49 AM
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If you don't have 24 volts at the gas valve then you need to trace backwards through any safeties until you find where you're losing the 24 volts.

PS. I mentioned on a call for heat because some people may not even turn the stat on before trying to troubleshoot.
 
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Old 02-29-08, 05:53 AM
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Gotcha. I think I'm going to just replace the thermocouple, transformer and the gas valve and see what happens. The only thing I can think of is mabe the transformer has a fuses inside of it? The transformer is really the only thing I haven't taken apart yet..
 

Last edited by djpeckonbox; 02-29-08 at 05:55 AM. Reason: to add
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Old 03-01-08, 03:39 PM
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Go to "HVAC Fan Always ON" thread to see picture of a board.

Modules are encased and are for starting the ignition.

Transformers are one square piece about 2 or 2 1/2 inches and have 2 wires for 110 AC going in, and two wires for 24 volts AC coming out. These can be mounted anywhere but often right near line entrance to furnace.

I surely would not be just replacing parts, unless you are financially well off and feel throwing parts at this is cheaper than a house call by a pro, or you are that deperate and feel you cannot find the problem the proper way. But that is why us posters, like me and the other guys, post, to try to help guide one, like yourself, through this.

You need to find out if you have 24 volts, anywhere. If you do, your transformer is good. You can cross that off your list.

Then you have to make sure 24 volts is making it's way to a control board or module (during a call for heat and after the inducer motor comes on, as that is what allows the 24 volts AC to progress through the chain of safety devices on it's way to the control board or module.) If it is NOT, you have one of the 24 volt safety items telling the current to not go through any further, for some reason.
 
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Old 03-01-08, 03:57 PM
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I had the same problem and the heating guy said it was a switch in the unit that got stuck, he fixed it and now it works fine
 
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Old 03-02-08, 10:39 AM
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Click?

If you have pilot & the gas valve clicks when the thermostat calls for heat, the gas valve is getting power & not opening. You would need a new gas valve.
 
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Old 03-02-08, 02:36 PM
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Thanks for the replies everyone...

I just picked up a new thermocouple & transformer. I figure, even if they're not the problem, they can't hurt to have around just in case. I will be home tomorrow to see what I can figure out, and I will post any new info I find.

I hear the thermostat click when there's a call for heat, but I've never heard the gas valve click ever. Maybe that's because I'm never able to be in two places at one time? The furnace is in the basement.
 
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Old 03-02-08, 05:39 PM
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Gas Valve

If you remove one wire from the gas valve, turn the thermostat up, then reconnect the wire, you will be able to tell if the valve clicks or not.
 
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Old 03-02-08, 05:51 PM
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Ahhhh... Cool. Thanks a lot Grady
 
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Old 03-05-08, 03:48 PM
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Just a little update for anyone who's interested...

Thermocouple is replaced. Pilot is burning a strong blue flame. Transformer is replaced & volt meter is showing the proper reading. Gas valve is replaced. Fuses are fine. Thermostat is replaced. Burners are still not firing...

We give up, and are calling a repair tech, or installing a new furnace... Over a week now with no heat. Thanks for all of the advice though, we really tried to DIY... I feel like I lost a war
 
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Old 03-05-08, 05:04 PM
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Originally Posted by djpeckonbox View Post
Thermocouple is replaced. Pilot is burning a strong blue flame. Transformer is replaced & volt meter is showing the proper reading. Gas valve is replaced. Fuses are fine. Thermostat is replaced. Burners are still not firing...
Reminds me, years ago, where me and this guy are waiting in the mechanics garage and I ask the guy why HE is there, and he tells me he replaced almost everything under the car's hood for about $350, in quest of his elusive problem, only later to find out it was the $2.99 condenser.

Let us know what your problem was.
 
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Old 03-05-08, 05:10 PM
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Originally Posted by ecman51` View Post
Reminds me, years ago, where me and this guy are waiting in the mechanics garage and I ask the guy why HE is there, and he tells me he replaced almost everything under the car's hood for about $350, in quest of his elusive problem, only later to find out it was the $2.99 condenser.

Let us know what your problem was.
Yeah...it probably is something like that. So far, about $150 has been spent...so not too bad.
 
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Old 03-05-08, 05:28 PM
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No Heat

Bummer. I too want to know what the problem turns out to be. Thanks for hanging in there with us.
 
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