millivolt gas fireplace trouble igniting

[geddy]

I have an old Buck Stove brand millivolt gas fireplace. Recently it has developed a weird problem. Pilot light works fine. When the wall thermostat tries to turn on the fireplace, nothing happens. Thinking it was a wring problem, I temporarily connected a jumper between the TH terminals on the fireplace and sure enough it ignited. However, when I removed the jumper, the fireplace stayed lit! To turn off the fireplace, I have to turn down the wall thermostat (or wait for the room to heat up to the temp on the wall thermostat). Further investigation has revealed that with the jumper disconnected, resistance across the TH terminals drops to zero when the wall thermostat tries to turn on the fireplace. Why then does it not ignite until I short out the TH terminals (even though there is already zero resistance between them)? I tested the electric outlet the fireplace is plugged into and it appears to be properly grounded.

Thanks for any help!

[HayZee518]

Only thing I can contribute to your problem is the heat anticipator within the thermostat. It is a resistance device that's supposed to anticipate the rising temperature in the room it's controlling as the temp approaches the setpoint.
The "Stat" has a set of mercury contacts within a glass bulb that's supposed to close when the bridge across them is completed via the mercury which is highly conductive.
The anticipator is also within the temp sense circuit.

[imeduc]

You need power for sure and that old T/C dont have it any more. When you jump it and the burner comes on that puts out more heat to the T/C
thats why it stays on. A millivolt tstat with a fixed anticipation is 1.5 . One that you can play with is 0.15 to 1.2A. Id say take out the T/C you have there and take it with you so you get the right one to put in. They run from 250mV to the big ones are 750mV. make sure and clean out the pilot light real good up in its venturi also.

ED

My mistakes dont define me they inform me.

[HayZee518]

Robert=Shaw and Honeywell make excellant stats for nillivolt generators and the anticipator is variable over a large range.

[Mike3]

the fact that you can jump tstat and burner stays on indicates resistance somewhere in the system. dirty pilot not generating enough m.v's--loose wire terminal--wrong tstat or one that is not properly adjusted for millivolt system--poor splice these are the most common problems and are usually quite easy to correct

[imeduc]

He said he jumped them at the fireplace on the valve Thats why Id say it came on. didnt have the wire or the tstat in there.

My mistakes dont define me they inform me.

[Mike3]

Would you please supply us with the millivolt reads. A-across the pilot generator terminals C-across the t'stat terminals All reads with t'stat calling for heat.(if you haven't already corrected problem) Thank you

[mjohnson125]

quote:Originally posted by Mike3

Would you please supply us with the millivolt reads. A-across the pilot generator terminals C-across the t'stat terminals All reads with t'stat calling for heat.(if you haven't already corrected problem) Thank you

Mark Johnson

[mjohnson125]

quote:Originally posted by mjohnson125

quote:Originally posted by Mike3

Would you please supply us with the millivolt reads. A-across the pilot generator terminals C-across the t'stat terminals All reads with t'stat calling for heat.(if you haven't already corrected problem) Thank you

Mark Johnson

What should the reading be at A)across the pilot terminals? And at the T/stat?

Mark Johnson

[Mike3]

Well depending upon what type of generator you have. Lets do a 750 mv With t'stat or switch calling for heat should be around 350-400mv
with a 250 mv around 150mv. It is difficult to give decent numbers without knowing more info. Lets do this - Calling for heat: Reads at generator terminals 350-- reads at t'stat terminals 50 - that leaves 300mv at operator to open gas valve Remember A minus c equals b -depending on valve type may take around 120mv to open valve- If read is low we have to determine why-- That usually is determined by other 2 reads Whew sorry for the lenght of reply

[Dean]

One thing I learned years ago working on many old Millivolt furnaces is to keep in mind that since the voltage is so low to begin with (750/1000 of one volt) the slightest voltage loss is very critical.
Wires splices really should be either soldered or secured under a terminal screw rather than twisted in a wire nut. Even though a wire nutted splice may work OK for quite some time, it usually eventually will be a voltage loss source.

Also I have found that the best operating MV thermostats have no heat anticipator to rob voltage.

[mjohnson125]

mjohnson125

quote:Originally posted by Mike3

Well depending upon what type of generator you have. Lets do a 750 mv With t'stat or switch calling for heat should be around 350-400mv
with a 250 mv around 150mv. It is difficult to give decent numbers without knowing more info. Lets do this - Calling for heat: Reads at generator terminals 350-- reads at t'stat terminals 50 - that leaves 300mv at operator to open gas valve Remember A minus c equals b -depending on valve type may take around 120mv to open valve- If read is low we have to determine why-- That usually is determined by other 2 reads Whew sorry for the lenght of reply

Thanks, Im only geting 250 and its a 750mv powerpile system. What about flame adjustment, could that be why its not putting out right voltage? How often does a gas valve go bad?

[Mike3]

You are saying with the t'stat jumped the burner comes on- That would indicate to me you have either a bad splice-loose wire or t'stat problem-- Maybe you have a heat anticipated t'stat needing adjust or replace-- need read at t'stat terminals on valve please