gas fireplace not igniting

[kwired]

I write this to (hopefully) add a little bit of information. I am a retired Mechanical Engineer and not an Electrician or Plumber). Our fireplace had the same intermittent problem. The fireplace has a standing (lit all of the time) pilot. The flame from the pilot light heats a thermocouple that provides millivolt power to the switch on the wall. Of course, this millivolt power opens the gas valve when the switch closes. I recently learned are while this switch LOOKS like a $4.00 rectangular decora switch it actually should be a more costly $20 millivolt switch probably with less resistance. Skytech sells this millivolt switch, a timer switch, or a remotely controlled switch for this application. Now, to be perfectly honest, i bought the special millivolt timer and the measured resistance across its internal switch but it didn't seem much different than a regular $4.00 decora switch. Still, I can imagine the Skytech switch has a genuine purpose beyond my understanding. While installing the timer, I decided to lightly sand the 15-year accumulation of residue (carbon?) off of the thermocouple. Since I replaced the switch and cleaned t
he thermocouple, the fireplace always starts. I suspect that my repair had more to do with cleaning the thermocouple off than changing the switch. Most repair guys (as noted by a previous writer) would replace the thermocouple (which is reasonable). This is is more of an a/c or fireplace repair guy's area of expertise than an electrician's.

That millivolt rated switch probably has different contact material and/or has a "wiping" action to help ensure a low resistance connection is made. Though the cheaper general use switch has a low resistance now, it may not keep as low of a resistance over time compared to one designed for millivolt operation.

 

[GoldDigger]

That millivolt rated switch probably has different contact material and/or has a "wiping" action to help ensure a low resistance connection is made. Though the cheaper general use switch has a low resistance now, it may not keep as low of a resistance over time compared to one designed for millivolt operation.

A millivolt circuit will, naturally, have both a low voltage and a correspondingly low current. It is much more sensitive to circuit resistance than a control or power voltage circuit.
But, AFAIK, the biggest reason for a special switch is the have a contact design (wiping) and material (silver or gold) which will not develop a resistive or semiconducting surface corrosion layer that would break down when subjected to several volts or more but would block a millivolt circuit completely.
Reed relays for low voltage applications typically use gold plated contacts even though that increases the likelihood of contacts welding/sticking on closure.

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[retirede]

I write this to (hopefully) add a little bit of information. I am a retired Mechanical Engineer and not an Electrician or Plumber). Our fireplace had the same intermittent problem. The fireplace has a standing (lit all of the time) pilot. The flame from the pilot light heats a thermocouple that provides millivolt power to the switch on the wall. Of course, this millivolt power opens the gas valve when the switch closes. I recently learned are while this switch LOOKS like a $4.00 rectangular decora switch it actually should be a more costly $20 millivolt switch probably with less resistance. Skytech sells this millivolt switch, a timer switch, or a remotely controlled switch for this application. Now, to be perfectly honest, i bought the special millivolt timer and the measured resistance across its internal switch but it didn't seem much different than a regular $4.00 decora switch. Still, I can imagine the Skytech switch has a genuine purpose beyond my understanding. While installing the timer, I decided to lightly sand the 15-year accumulation of residue (carbon?) off of the thermocouple. Since I replaced the switch and cleaned t
he thermocouple, the fireplace always starts. I suspect that my repair had more to do with cleaning the thermocouple off than changing the switch. Most repair guys (as noted by a previous writer) would replace the thermocouple (which is reasonable). This is is more of an a/c or fireplace repair guy's area of expertise than an electrician's.

I also have a standing pilot fireplace (Heat N Glow). When the thermocouple has issues, the pilot will not stay lit. I’d say your new switch fixed the problem!

 

[GoldDigger]

I also have a standing pilot fireplace (Heat N Glow). When the thermocouple has issues, the pilot will not stay lit. I’d say your new switch fixed the problem!

FWIW, my free standing propane heating stove has two metal elements in the pilot flame. One is a thermocouple for the pilot safety on the gas valve, the other, larger, one is a thermopile that runs the main burner valve through the millivolt thermostat.

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[GoRaWa1]

You are probably right...

You are probably right...

I also have a standing pilot fireplace (Heat N Glow). When the thermocouple has issues, the pilot will not stay lit. I’d say your new switch fixed the problem!

Could be. One thing I always try to do when I repair something is to avoid replacing two parts at the same time. As it is, having replaced the switch and cleaned the thermocouple, I have no idea which solved the problem.