Pilot stays lit, but no heat?

Old 12-12-09, 05:38 AM
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Pilot stays lit, but no heat?

Hi, I have an old HBSMITH furnace made back in 1992 or so. It has a honeywell VR8300a continous pliot gas control and a johnson automatic damper.

The problem: pilot stays on but flame thus no heat.

What I observe prior the issue: pilot is on, thermostat turned on, furnance makes a few noises (24v transformer turning on, automatic damper moving with green led on), a few seconds later, flame on.

What has happen: The furnance turns on until it reach the desire temperature set by the thermostat, and then the furnance stops however a few moments later the pilot goes out. Temperature drops, furnace does not start pressumably because the pilot is not lit. Turning off the thermostat and relit the pilot works but it will soon stop after desire temperature is reach and the pilot goes out.

Now: The automatic damper no longer lit green to show that the vent is open.

What I have done: Clean the pilot and replaced the thermocoupler. The pilot now stays on.

I have very limited knowledge about how this whole thing should work. My guess on how this gas furnace works is:

1) Lit pilot, thermocouple sends out small volt to open small gas valve so the pilot stays on.
2) turn on the thermostat which turns on 24v transformer
3) The automatic damper opens the vent, send signal to 24v transformer which in turns engerized the main gas valve to open.
3)House has heat.

So the pilot stays on, but the automatic damper does not have green light on which means the vents are closed. There is an emergency process to open the vent manual. It basically disable motor and allow the vent to be manually adjusted. It also states that when the vents are open, the green lit should be on. I try the manual process but the light does not come on. If I put the vent on open position and put it in automatic, the motor will turn the vent to closed position.

I can only think of 3 things that went wrong in order listed:

1) damper is defective hence no green light which won't send signal to 24 volt transformer to turn on the main valve.

2) 24 volt transformer is defective.

3) Honeywell pilot gas control main valve is defective.

Before yesterday, I have zero knowledge about how a furnace works except that when you turn on the thermostat it turns on the furnace which produce heat. Please help.
Old 12-12-09, 08:45 AM
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Well, your theories of how the furnace works aren't quite right.

Let's co nsider the pilot safety system, since that's where you are having trouble.

This system operates on the basic idea that you don't want the main burner gas to turn on unless there is a pilot light lit to light the gas.

The thermocouple you replaced generates a small volatage --- about 15 millivolts (15/1000 volt) if 1/4" of the tip is engulfed by the pilot flame.

When you push down on the gas valve to get gas for the pilot, you are pushing down on a safety valve that snaps closed if the pilot is out. An electromagnet holds the safety valve open when the pilot is lit, and the electromagnet is energized by the 15 millivolts from the thermocouple.

If the pilot light goes out, the millivolts fade away and the pilot safety snaps closed, preventing the gas from flowing to the main burner and pilot burner.

Now. You are observing that the pilot light stays lit when the burner lights, but goes out when the burner shuts off.

What does the pilot flame look like? It should look like a small blowtorch. If it looks has traces of white flame, or it is easily blown around like a candle flame, the pilot burner needs to be disassembled and cleaned and the pilot orifice needs to be taken out and either cleaned or replaced. Some pilot orifices can't be adequately cleaned and need to be replaced.
Old 12-12-09, 10:35 AM
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Thanks for the reply.

Originally the pilot would go out after the burner stop. I had since replace the thermocoupler and use can air compressor to clean out the pilot orfice. It seems to be blowing a small blue torch and it's covering about 1/4 inch of the thermocoupler and it does stay on now.

I will have to check with a voltage meter to make sure there voltage coming from the thermocoupler. Would it be safe to say if the pilot does stay on, that the thermocoupler is working correctly?
Old 12-12-09, 11:24 AM
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Try bypassing the stat by connecting the red and white wires (they may be different colors but that is the most common) together and see if the main burner will light. If the stat is digital replace the batteries.

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