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Posted: 11/28/2009 8:30:49 AM EDT
[#1]
Quoted:
I'm trying to help my dad get some heat going in his add on, any help you guys could offer would be great. He has an older (30 years? maybe more) warm morning gas wall furnace. The pilot light has remained on but neither the burner nor fan come on when you turn the thermostat. He's already replaced the thermocouple himself. I've taken a look at it, there's power on both sides of the transformer. I took off the thermostat and jumped the 2 leads at the wall and got nothing. With the thermostat wire shorted I tested for voltage across the TH and TR terminals of the gas valve and got nothing. Do I need to start looking into limit switches or something? Yes start checking thru your saftys your open somewhere and for a reason.....also with age of your unit check the combustion chamber for cracks with a flashlight.. |
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Posted: 11/28/2009 8:31:38 AM EDT
[#2]
Do you have an electrical schematic?
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Posted: 11/28/2009 8:42:59 AM EDT
[#3]
Sadly no diagram, can't find really anything on the web about it either.
There are 3 limit switches inside as far as I can find, 1 up near the very top of the unit above the fan, and two spaced just a few inches height apart near the heat exchanger. How do you test them? Just short across the terminals of the switch? There is also another transformer near the top of the furnace near the fan. Why are there two? |
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Posted: 11/28/2009 9:02:34 AM EDT
[#4]
I'd say the Main gas valve
Pilot stays lit Power in and out Main gas valve OR the wrong thermocoupler |
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Posted: 11/28/2009 9:05:30 AM EDT
[#5]
Quoted:
I'd say the Main gas valve Pilot stays lit Power in and out Main gas valve OR the wrong thermocoupler Wouldn't I see 24v across the gas valve terminals then with the termostat calling for heat though? |
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Posted: 11/28/2009 9:11:42 AM EDT
[#6]
Set the thermostat to a lower setting,,,,,,,wait a couple of minutes..........get beside the unit and have somone else turn the thermostat to a higher setting,,,,,,,,,,,,,,you should be able to hear a "click"
That "click" would be the stat sending voltagem to the main gas valve,,,,,,,,,,,,,,if you hear the click and nothing happens,,,,,,,,,,then it's the MGV if you don't hear a click,,,,,,us a "jumper" wire and bypass the thermostat if it lights,,,,,,,,,,,replace the thermostat,,,,,,,,,,,,,if not replace the MGV |
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Posted: 11/28/2009 9:13:07 AM EDT
[#7]
Quoted:
Quoted:
I'd say the Main gas valve Pilot stays lit Power in and out Main gas valve OR the wrong thermocoupler Wouldn't I see 24v across the gas valve terminals then with the termostat calling for heat though? Yes you should the thermocouple produces its own power to open the pilot and don't start short'n across safteys take your meter and read across it if you get 24 or a volt reading it's open and should have a reset on it if no reset then you'll have to replace it...don't understand why you have 2 transformers??? need a better discription... |
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Posted: 11/28/2009 9:38:17 AM EDT
[#8]
Try jumping the valve directly if your sure it’s 24V and not millavolts, disconnect the T-stat first. Just to see if it’s not the valve. IMHO |
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Posted: 11/28/2009 9:38:28 AM EDT
[#9]
Quoted: Sadly no diagram, can't find really anything on the web about it either. Most times there's a diagram on the inside of the device - usually on one of the removable panels. |
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Posted: 11/28/2009 9:40:50 AM EDT
[#10]
Some pics
I short these two terminals together when I'm testing: 
Putting the volt meter across the terminals at thegas valve or at any of the limit switch shows no voltage when the thermostat is calling for heat. Also figured out 2 transformer mystery, the transformer near the gas valve is the doorbell. The one at the top is putting out 24v |
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Posted: 11/28/2009 3:39:28 PM EDT
[#11]
You said you checked for voltage. I'm not trying to be smart but, did you check for continuity at the furnace through the thermostat? If the thermostat show that you have continuity then I'd look at the gas valve considering the thermocouple has been replaced.
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Posted: 11/29/2009 10:42:28 AM EDT
[#12]
Set the thermostat to call for heat and check across the limit switches with your volt meter. When you see 24 VAC, that's the switch that's open.
Limit switches are usually just a bimetal that will reset once they cool down. "Rollout" switches are either one-shot burning types (like a fuse) or are manually resettable usually with a small button directly between the two wire connectors on the back side. |
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Posted: 11/29/2009 10:48:29 AM EDT
[#13]
Check for 120 V into the transformer high side and 24V out on the low side. If you have that then jump the red and white thermostat wires, see what that does. Then check to see if, when the thermostat calls for heat, that there is 24V at the gas valve. if there is, unhook the wires and OHM the gas valve out. See if you get OL or a reading.
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