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DoAsFan 10-18-2007 10:48 AM

Direct vent wall furnace not heating up
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Hello all,

Looks like the attached image. Tried to turn on the heater in anticipation of the coming winter, but it won't heat up. The pilot is lit and stays lit, but when I slide the thermostat temperature setting up, it doesn't turn on.

I've replaced the thermostat with a new one and still the same result. Only two wires go into the thermostat. I've inspected the 3 wires going into the furnace and they appear to be fairly secure.

I've read posts about replacing the thermocouple, but not sure if I'm looking in the right direction. Also not sure if its safe to disconnect the wires going in to the furnace and reattach. Are these low/milli volt, or 110V?

Any suggestions? Thanks!

47_47 10-19-2007 08:45 AM

I would go to the manufactuters web site and download a installation manual. This will tell you what the other three wires are used for and if you have a millivolt system. I have a millivolt direct vent stove and it has a standing pilot light and uses AA batteries to operate the gas valve. Make sure your new thermosat is also rated for a millivolt system if this is the case, some only operate 24 volt appliances.

DoAsFan 10-19-2007 10:56 AM

Thanks for the reply. Even when I touch the two wires in the thermostat together, the heater doesn't heat up (gas doesn't turn on).

Is the thermocouple a possible issue if the pilot is lit and stays lit? Everything I'm reading is telling me that the thermocouple is a safety to shutoff gas if the pilot isn't lit. If the pilot is still going, that leads me to be believe that the thermocouple is working and it detects a flame, so gas is flowing.

Could cleaning the thermocouple help?


47_47 10-19-2007 11:29 AM

You are correct, the thermocouple senses the heat of the pilot. This is a safety feature, so the gas valve will not open without the pilot light on. This is why you hold the button in for 30 seconds when lighting the pilot, to supply gas to the pilot to heat the thermocouple.
Do you know the make and model?

DoAsFan 10-19-2007 11:36 AM

Unfortunately not. Heater is at my sister's rental location. I'll try to make note of it and see if I can find a manual. Thanks!

gregzoll 10-21-2007 02:53 PM

A lot of times, there is a switch for the fan to turn it on, and this may be a interlock with not allowing the burners to kick on. I had an old furnace that when you turned off the Blower, no matter what you did at the Thermostat, the Pilot Light continued to be the only thing burning gas.

DoAsFan 10-23-2007 09:43 AM

Make/Model and Millivolt CONFIRMED!
Its a "Perfection Schwank," model appears to be PDW850-SEM-1.

There is a label on it that says "Millivolt Only." The $10thermostat I picked up at Orchard Supply Hardware says it works for millivolt heat-only systems.

I vacuumed throughly, removed the thermocouple, cleaned it off a little, and reinserted. Lit the pilot per instructions with no trouble. It still stays lit, but no heat!

I guess the next step is replacing the thermocouple, since I've been hearing/reading different reports on how that might affect gas flow. If that doesn't work, then the gas valve, but that might be a little more advanced than I'm ready for.

Haven't been able to locate a manual via Google yet. Did Perfection get absorbed into Schwank Group?

DoAsFan 10-23-2007 10:38 AM

Perfection Schwank now defunct
Schwank Group 1-877-446-3727 pointed me over to Kent H. over at Texas Infrared 1-281-445-1700, which bought up all the surplus parts when the plant closed down six years ago.

Kent was very helpful and suggested that with my symptoms, I either had a bad pilot generator (which I now understand is a thermocouple) or a bad millivolt gas valve.

To test this I've been instructed to try and determine the voltage across the thermocouple. He said that the pilot generator needs to be able to generate 250-300mV to get the gas valve to open. If I'm under 250, then I'll need a new pilot generator (~$30) and if I'm in or over that range, I'll likely need a new millivolt gas valve (~$100+).

Any further suggestions before I go blow myself up?

47_47 10-23-2007 11:17 AM

Did they mention where the pilot generator gets its source power from? It must be connected to either a AC or DC (battery) source. I have also re-read my manual and as gregzoll has said mine also has a switch, however this switch is on for the for using the internal thermostat and off for using a remote.
With an accurate DVOM you could check the pilot generator input and output power. With all surplus repair parts bought up 6 years ago, after the plant closed is a red flag. I would also consider having a new higher energy efficient professionally installed.
If you feel that you could possibly blow yourself up, I would stop and call a pro.

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