Thermostat won't turn furnace on

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  #1  
Old 10-15-04, 09:36 PM
hhsieh
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Thermostat won't turn furnace on

Hello. I bought an old house and one of its rooms is equiped with a direct-vent gas furnace (Perfection-Schwank, Model ASV712RTN Version T). The house is built in 1950 but I suspect this furnace is a lot newer judging from its look.

Last summer when I receive the house, the previous owner turn on the furnace to show me it was in working condition. But when I tried to turn it on yesterday (first time since then), it did not work. This was what's happened: I first lighted the pilot from through a small view hole, held the pilot button for one minute and the pilot stayed on all right. I then turned the valve control knob to "on" and replaced the furnace housing. The problem came when I turn the thermostat knob on the housing and nothing came on . The room temperature was around 60F.

I re-read the lighting instruction on the inner side of the furnace front cover. One thing I could not figure out was that, as a last step of the pilot-lighting procedure, it says something like "turn the power back on". I cannot find a power switch anywhere on the unit. In fact, I'm not even sure this furnace is connected to any power source!

The bottom line is that the pilot light stays on but the thermostat won't turn on the furnace. Could someone please give me some potential root causes for this problem. Thanks in advance!
 
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Old 10-16-04, 12:07 PM
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Hello: hhsieh Welcome to the Do It Yourself Web Site.

Thermostat could be faulty. If there are wires going to it, disconnect them and reconnect them together. If furnace fires up, T-Stat defective. Replace it. If furnace does not fire up, may be a weak T-Couple (T-C) or Pilot Generator (PG) which allows pilot to remain on but not activate burner gas control valve.

With either a T-Couple or PG, pilot flame has to be all blue. No yellow. Yellow indicates dust in the pilot assembly. Yellow pilot flames do not provide enough heat to either a T-C or a PG. Both must be very hot and have a cheery glow red on the tip to be producing full current.

If pilot flame not clean or any doubts exist, blow out flame, blast entire assembly with canned air use to clean computer keyboards and relight. Might resolve the problem.
 
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Old 10-16-04, 04:53 PM
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Wink

You say you cant find any power to this heater and you have a tstat . Then it must be powered by a Millivolt thermopile of some kind . Take and pull the thermocouple looking thing its the generator and take it with you to get a new one just like it. They come from 250 to 750 millivolts.

ED
 
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Old 10-17-04, 01:14 PM
hhsieh
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Re: Your suggested diagonostic procedure

"... If there are wires going to it, disconnect them and reconnect them together. If furnace fires up, T-Stat defective. Replace it. ..."

Dear Sharp Advice,

Thank you for your advice! Regarding your suggestion quoted in above, are you suggesting

1. disconnecting & reconnecting the wire(s) to check for loose connections?

or

2. disconnecting & reconnecting (shorting out) certain wires to bypass something (what?)?

If it is case 1 is correct, how do you conclude T-Stat is defective? If it is case 2 you are suggesting, what wire(s) are to be disconnected and which are to be reconnected? (I do have fair electrical knowledge and have replaced my hot water heater TC a couple times, I think your clarification will help me understand the rationale behind this suggestion. Thanks again!
 
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Old 10-19-04, 02:12 PM
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Wink

If you jump the R and W at the tstat and it come on then it is the tstat .If it dont, get a new T/C for it . That is what makes the electric for it to work.

ED
 
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Old 10-26-04, 11:04 AM
hhsieh
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Thank you, Sharp and Ed for all your valueable insights.

I got the problem resolved and this was what happened:

- The original pilot light was indeed weak (but it was blue & not yellow )
- I pulled out the thermopile and heated it in my kitchen while measuring the voltage output. In 1/2 minutes it reached 600 mV. Although it has a dark burnt look, I deemed that it is operational.
- I removed the Alum feed tube (from the gas valve to the nozzle assembly) and found it was ok (not blocked)
- I tried to use a thin wire to clean out potential debris in the pilot nozzle (from the flame end)
- I also used a 1.5V battery to excite the gas valve (by touching the TH and THTP screws)

After I did all the above and put everything back, I got:
- a 4-time bigger pilot flame than before.
- burner worked under the command of the thermostat.

My guess is that the most crucial step from the above actions was the attempt of cleaning out the pilot nozzle. The blocked nozzle did not produce enough flame (heat) to the thermopile and hence not enough mV to power open the gas valve. Any other insights?

I have another question for the expert...

When the leads of the thermopile are connected to the TP and TPTH terminal, does it matter which lead connected to which terminal? If so, how do you distinguish the leads? Is it DC current that flows in the thermopile? What would happen if one connects it in the "opposite" way? Thanks.

hsieh
 
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Old 10-26-04, 01:28 PM
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You got me there . You know over the years I have never looked what wire went on what off the millivolt generator. I have put a lot in. Keep and eye on that one they put out 250mV and 750 mV. SO 600 mV can be low. In the pilot just take some caned air and bend the straw to blow out the pilot venturi there.

ED
 
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