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millivolt floor furnace trouble shooting (pilot lit but gas valve will not open)

millivolt floor furnace trouble shooting (pilot lit but gas valve will not open)


  #1  
Old 01-03-13, 11:15 PM
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millivolt floor furnace trouble shooting (pilot lit but gas valve will not open)

Hi All,

First off I am new to this forum and new to Home Heating Furnaces. This summer I purchased my first home, a southern California home (built in 1951). It has a millivolt controlled floor furnace. For the past three weeks the furnace has been working without issue (after we cleaned it up).

Now the furnace will not turn on when I set the thermostat contacts to the closed position, I do not hear a sound when this happens. The pilot light is still burning.

I measured the voltage at the contacts on the thermostat. I get 450 MV when the thermostat is not closed. As I understand it I need at least 325 MV so I believe I am in good shape and that the thermopile is doing its job. When I close the circuit by moving the temperature control stick all the way up. The voltage drops to 0. I was expecting a drop of about 50%. I don't know if the drop to 0 indicates a problem or not.

Once I get a jump suit and other equipment (I need to buy) I will get into the crawl space and try to connect TH/TP & TP to see if this turns on the gas valve. If the valve turns on then I will think that I have an electrical issue and try to rewire the thermostat. If the valve does not turn on then I will assume the valve is bad and I will replace it.

Would someone mind validating my line of thinking and providing some insight on why my voltage drops to 0? I am hoping your feedback will give me direction on how to proceed.

Thanks!
 
  #2  
Old 01-04-13, 12:28 AM
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Thinking about it more, it probably makes sense that the voltage drops to 0 because at the closed thermostat because I am not measuring the entire circuit at that point.
 
  #3  
Old 01-04-13, 05:02 AM
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Try removing the two wires going to the thermostat and touch them together. If the furnace works it's probably dirty or corroded contacts in the thermostat or just a bad thermostat. It does not take much to stop that low voltage thermopile voltage.
 
  #4  
Old 01-04-13, 06:02 AM
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Also measure directly at the power pile. You said you cleaned it up. Apparently you did something when cleaning.

Not to be funny but is the gas valve to on and not pilot? Its always the simple things......
 
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Old 01-04-13, 04:31 PM
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Taking an open read such as you did really proves nothing except your meter works. Reads should be taken with the thermostat closed (calling for heat) and at the gas valve. Read A- across the pilot generator terminals B- across the gas valve operator C-across the thermostat terminals
Typically you need around 120 millivolts to open the gas valve, yes, that is the low end of the scale. Make your first read across the PG terminals, record that read, second read across the thermostat terminals, record that read. Now subtract the tstat read from the PG read and that is the voltage going to open the valve. You indicated you had a read of 450mv so that means you have a PG-9 style PG which factory fresh open read is 750mv. If you could provide a picture of the controls and a picture of the inside of the thermostat would be nice.
 
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Old 01-05-13, 10:57 PM
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Thanks for the replies so far. It looks like I need to get under the house and test. Once I get a jump suit I will get down there and figure out what is going on. I will let everyone know what I find.

Here is a picture of the thermostat

Attachment 7539
 
  #7  
Old 01-06-13, 12:45 AM
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Your picture did not attach correctly.
 
  #8  
Old 01-06-13, 07:38 AM
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Hello: adseligman

Suggestion:
You live in an area where in home natural gas appliance service is available free. Providing the heater is using natural gas as it's fuel type. (???) If so, all you need to due at this point is make a phone call to southern cal gas company. Oops! That's the old company name... New name used for many years now is just referred to as "The Gas Company."...

Service rep will determine what exactly is the cause of the problem. If T-Stat is the original old type, it will have snap together contact points. Rep should or will and/or might simply snap contacts together using the lever or dial. Furnace may or may not fire up. If furnace does not fire up then likely it's because the contacts get dirty or slightly pitted.

In which case can be cleaned using a pipe cleaner to remove dust or residues. In more difficult cases, an emery finger nail filling board can be used between the contacts to gently clean up the contact surfaces.

If T-Stat is the liquid filled (silver fluid, which is liquid mercury) float bulb type, physical contact between contacts not the cause of the problem. Usual cause is a weak millivolt system. Usual cause is a improperly burning (weak and/or yellowed) pilot flame or a worn-out PG.

(PG refers to and/or means 'Pilot Generator.' As accurately described and referred to by mbk3... )

Bottom Line. Call the gas company and request an in home free inspection service request. Let the company rep due the work and determine the cause. Also what needs to be done if furnace does need a part. Which company does not replace nor sell such parts.

A PG will be available locally at any retail appliance parts stores in your area. If you need to replace the PB, remove the old one and take it with you. They come in different sizes based upon millivolt output needed. Usually needed and used for a floor furnace is a PG-9 as noted and mentioned by mbk3. Thanks.

Side Note:
To all members whom are reading this thread and others like it.
Part terminology for clarification and knowledge base. Humor added too!...

The millivolt part is called a Pilot Generator. (PG)

NOT a thermo "PILE."...:NO NO NO: Not any kind of a 'PILE.'

A PILE is what you find on your front lawn or in the grassy area of the parkway between the front lawn and gutter left by a stray animal...... Hopefully you "C" the PILE before stepping into or onto the PILE....
 
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Old 01-06-13, 10:05 AM
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Thanks for the info. I enjoy trying to figure this out and learning through the process. I have the non-mercury Thermostat. I will hop under the house and took a look myself then work with the "Gas Company" to do a full analysis. Thanks.

 
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Old 01-14-13, 07:44 PM
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Update

Hi All,

Did not crawl under the house yet. I noticed that could get the furnace to turn on if I tapped the pilot. I cleaned the furnace from the house. After a good cleaning I could get the furnace to turn on (we had it cleaned after the renovations but I guess they didn't do a good job).

I also had a friend look at it and he said that I had the wrong pilot, it looks like I should have a PG9 pilot with a crown style generator.

Everything is working right now. Once it gets hotter outside I will crawl under the house and replace the pilot. If other things break then I feel that I understand enough to try to fix the problem, or call the gas company.

Thanks!
 
  #11  
Old 01-15-13, 09:12 AM
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The "crown" style PG-9 won't be a PG-9. That would be a PG-1 and the PG-9 is the appropriate replacement for a PG-1.

Go to Ebay and type in PG-1 pilot generator and you will see the difference between a PG-9 and PG-1. The PG-9 is rated 750 millivolts and the PG-1 is rated 320 millivolts. Remember these reads are: factory fresh and open read.
 
 

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