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Tim H.
Tim H., HVAC Professional
Category: HVAC
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Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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PILOT WONT LIGHT HELP ! Hydrotherm Natural Gas Furnace HC-125

Customer Question

Hydrotherm Natural Gas Furnace HC-125 (Cast iron Boiler, base board heat)
Residential furnace/boiler with a Honeywell Powerpile (thermopile) gas valve.
Already replaced the thermopile, but valve won't reset or some other malfunction.
I can light the main flame while holding in the pilot control knob, but when I release the knob
to turn gas valve to on position pilot is not lit.
Submitted: 6 years ago.
Category: HVAC
Expert:  Tim H. replied 6 years ago.
Thanks for giving me the opportunity to help! If you have replaced the thermopile and it has good output with pilot flame hitting it correctly, then you most likely have a bad gas valve/gas control. Is this a very old unit?
Customer: replied 6 years ago.

Yes, this is a 45 year old, but still very dependable and functional boiler type furnace.

The gas valve is a Honeywell VS8279A1034 1/2" X3/4" RATED 1/2 PSI FOR G-LP. With the standard Honeywell VS8002A Control.


The four contact wiring on it has only a very simple 2-wire aquastat connected to the two top contacts, a two -wire thermopile wired to the two bottom contacts, and a two wire thermostat wired to the two left most contacts (one of the top and one of the bottom contacts).

It's hard to believe that anything would go wrong with this essentially untouched gas valve. I believe that it is just failing to easily reset after the pilot blew out from a backdraft or similar cause. I had the flames on the burner turned way down to just above the lowest setting , just a notch above where the pilot actually lights the burners. This is in a dry basement of approximately 1200 sq. ft., with a 50 gallon gas fired hot water heater about two feet away. Adequate ventillation, good oxygenated air,and no breezes. The water heater is burning fine just two feet away. So good gas pressure. The igniter flame lights easily and has a good color flame and the flame touches the top 3/8" of the thermopile like it should. It just seems like the valve won't activate and allow gas through the 1/8" pilot tubing. And I have already cleaned and checked the pilot tubing for any obstructions, of which there were none. Know of any TRICKS to get the millivolt Honeywell gas valve to reset/open to allow gas through the 1/8" pilot tube ? The valve already lets gas through the 1/4" igniter tube right next to the pilot.

Expert:  Tim H. replied 6 years ago.
If the thermopile is putting out the proper millivolts, then the valve must be bad. A good thermopile will put out around 750 mV open reading. This is with leads disconnected and holding down pilot knob so pilot stays on.