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Phil
Phil, Mechanical Engineer
Category: HVAC
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Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a wall heater that has had the thermal coupler

Customer Question

Hi,
I have a wall heater that has had the thermal coupler transducer ( pilot light burns fine), thermal over heat cut out sensor, gas valve and thermostat replaced professionally. The pilot light stays lit. The furnace burns to a set temperature on the thermostat, shuts down and will not relight when the temp drops below desired level. Sometimes it will light off and burn for a few minutes and then cut out.
The gas company comes out and "fixes" the problem. Tehn it comes back within the hour.
Submitted: 1 year ago.
Category: HVAC
Expert:  Tim H. replied 1 year ago.

Hi, I'm Tim and I'm an HVAC expert here to assist you with your questions. Please provide all information I request as it makes it more difficult to troubleshoot something when I'm not there onsite. Essentially, you become my eyes and ears. Thanks for the opportunity to help. At the end of our dialog, you will have a chance to rate my service, but please wait until we are finished and you're satisfied you have all the information you need.

This sounds very much like a bad thermostat. Have you tried replacing it?

Customer: replied 1 year ago.
I have replaced the thermostat four times, the last one by a professional.
Expert:  Tim H. replied 1 year ago.

When this happens, are you getting 24 volts AC to the gas valve terminals?

Customer: replied 1 year ago.
As I have said ui have replaced all components. It is a old property. Three units and recently PG&E ( local gass company did a main line maintenance repair, put bottlred gas on the property temporarily. Prior to their work the three heaters, 3 stoves, 2 dryers and 2 H20 heaters worked fine. I think they changed the gas pressure provided. Could this effect the wall heater a the end of the line?I figure the draw on the line is 300,00 btu.
Customer: replied 1 year ago.
that is 300,000 BTU through one meter. All I know is everything worked before and now it does not.
Customer: replied 1 year ago.
I also had the line between the buildings , about 50 feet increased to 1 1/4 inch.
Expert:  Tim H. replied 1 year ago.

Of course, it could if it's getting insufficient gas. It may not be sufficient to allow the burners to light. Most heaters, water heaters, and furnaces operate on 3.5 inches water column gas pressure and need at least 6-7 inches WC at inlet.

Customer: replied 1 year ago.
PGE said it was adequate yesterday but he did not give me number. It is supposed to be 7 inches but can very +/- 2
Expert:  Tim H. replied 1 year ago.

I would not go as low as 5 inches water column at inlet.

Expert:  Tim H. replied 1 year ago.

But I would still need to know if the gas valve is getting 24 volts on a call for heat. Just because you replaced all the parts does not mean there could be some bad wiring, etc on an old unit like this.

Customer: replied 1 year ago.
24 volts AC from a thermal couple? I thought the current generated from the pilot light was DC???? I can check that but when I by pass the wall thermometer by connecting the two wires the furnace may or may not work. OHM's resistance checks goo on the wires.
Expert:  Tim H. replied 1 year ago.

Is this a 24 volt heater or a millivolt system?

Customer: replied 1 year ago.
That is to say the wall thermosat seems to not effect the heater staying lit or firing off. Would a higher pressure from the gas company maybe fix the problem. I could turn off all demand on the line other than the wall furnace to see if it would work alone. Or, if PGE show up tonight, I could turn all appliances and see if wht the pressure is then. does that sound like a good idea???
Expert:  Tim H. replied 1 year ago.

Yes, I would try that.

Customer: replied 1 year ago.
This is not going well so please do not charge my card unless you have an answer. Any more ideas???
Expert:  Tim H. replied 1 year ago.

I had asked you a question which I need an answer to in order to proceed. Is this a 24 volt system (power to it) or a millivolt system?

Customer: replied 1 year ago.
It is a millivolt system.
Expert:  Tim H. replied 1 year ago.

OK, and has the millivolt section been tested? These usually require a 750 millivolt thermopile but there needs to be enough power left over to power the valve.

Customer: replied 1 year ago.
The professional matched the thermostat and thermal couple, generator
Expert:  Tim H. replied 1 year ago.

OK, gonna open this up to other experts. Please do not respond until another expert picks this up.

Customer: replied 1 year ago.
The system works fine until there is a really cold day when everyone is demanding gas....And I have not tested fro 750 MV, but since it works sometimes I think I have the minimum, right?
Customer: replied 1 year ago.
OK, I will work on it alone tonight and see if I have any more success. My tenant is a good one and I am just trying to keep her happy.
Customer: replied 1 year ago.
As I said, please do not charge my card the $45.00, I do not feel that I got any new information. Thank you, ***** ***** Happy New Year.

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