gas fireplace main burner will not light

[black alder]

I have a gas fireplace which utilizes a milli volt control system.
The main burner will not light- no problems with the pilot light.

I have checked the millivolt readings on the thermopile with the following results:

Gas valve ON, Pilot light ON, Wall switch OFF thermopile = 650mv
Gas valve ON, Pilot light ON, Wall switch ON thermopile = 3mv

All wiring connections check good, Wiring and switch resistance good.
Switch connections jumpered at gas valve- no main burner light up

With gas valve On, Pilot light On, and wall switch On reading at gas valve accross thermopile connections is supposed to be 100mv min.

It is obvious that with a reading of 3mv the valve will not open.

Is it possible to have a bad thermopile that reads normal ( 650mv ) with no load and then have it drop to 3mv when heat is called for, or is it likely that there is a fault within the gas valve which is drawing down the voltage to the point that it will not open the valve.
Thanks

[HayZee518]

the millivolt generator actuates a small electromagnet within the main gas valve. if you are reading 650mv then it should be enough to keep the main valve open. change the main valve.

[black alder]

Burner will not light

Thanks for the reply
I only have 650mv on the valve untill I switch on the wall switch - at this point the voltage drops to only 3mv, The instructions state that with the wall switch selected the voltage on the valve from the thermopile should be in excess of 100mv

[HayZee518]

what is the wall switch for? the thermocouple output should be fed directly to the gas valve. if ANY of your connections are loose or have a high resistance then the millivolt output will be compromised.

[HayZee518]

OK, the resistance of #14 ga copper wire is .00297 ohms per foot and #12 is .00187 ohms per foot. say you have a run of 60 feet to your boiler from the wall switch. multiply this by two because you're going from the furnace to the switch and back to the furnace, so your gonna have 120 feet of copper to deal with. for #14 you'll have .35646 ohms in series with the generator and .22440 ohms with the #12. if you know the current of the gas valve then you can calculate the voltage drop of the switch leg. E=IR
fOR ac SYSTEMS CODE SAYS NO MORE THAN 3% VOLTAGE DROP PER 100 FEET BUT THIS DOESN'T APPLY HERE. i JUST THOUGHT i'D THROW THAT IN.

[black alder]

gas burner does not lighr

Hi Again,
I have bypassed all the wiring and wall switch by removing the switch wiring from the valve and jumpered the "switch wiring" directly on the valve, at this point when I jumper the switch contacts the millivolt voltage drops to 3mv from 650mv and the burner does not light

[HayZee518]

can you send me some kind of schematic diagram. well if the voltage goes to 3mv I'd say the valve solenoid is shorted out.

[black alder]

gas burner not lighting

I think I am coming to the same conclusion as you- I think the valve is toast.
The fireplace has worked well for 10 years and just recently started not to light on 2 prior occassions.
I guess things are not made to last forever anymore.
I am attaching the wiring diagram- I hope
Very straight forward not complicated
Thanks

[HayZee518]

gas heater

ok, thanks for the diagram. now we're on the SAME page.
The thermocouple is in the flame all the time and its screwed in connection goes into the main valve. its function is to keep the pilot flame valve open. your previous posts didn't mention the THERMOPILE which is a horse of a different color. In similarities it is almost like the thermocouple only instead of actuating a pilot valve, the thermopile actually generates a dc voltage depending on flame intensity. the hotter the flame the more voltage it produces. you also show a rocker switch connected to the main valve. this is your on-off connection - as you stated and is very close to the unit. one thermopile wire goes to the valve, the other goes to a dummy terminal. another terminal goes from the dummy terminal to the other side of the valve. total of four wires. the back of the switch shows pigtails to a remote thermostat. on the back is a pair of resistors which I guess is current limiting resistors to make up for the copper wire losses. to a standard mercury thermostat. an electronic one won't work in this application as you need 16-24 volts for operation. AND as you specified I'd look into another valve.
Found in: Most modern VENTED gas stoves, fireplaces and fireplace inserts as well as vented gas log sets with thermostat or remote control.

This valve is similar to the thermocouple only valve, however has a pilot which heats up BOTH a thermocouple and a thermopile. The thermocouple still acts to prove that the pilot flame is on and allows this flame to continue after startup. The thermopile is used to power a second circuit which is used to open the main valve. This second circuit is powerful enough (300-600 millivolts) to allow the use of a thermostat, wall switch or control switch to operate the main valve. Control of the valve is obtained simply by hooking a pair of wires to two terminals located on the valve.

Startup is similar to the thermocouple-only valve. A piezo spark ignition is used to ignite the pilot after the gas knob is turned to the “pilot” position and depressed. Once the pilot is lit, the knob is held in for 30 seconds to “prove” the heat and then released and turned from the “pilot” to the “on” position. The main burner will then respond to the switch, thermostat or remote control.

[sunmtnforge]

Did this problem get fixed? I have the same issue, pilot on:750mv, then turn the toggle switch on to activate the burner and it reads 3 mv. EXCEPT: I replaced the valve and the same thing happens with this valve-no gas to the burners-Any suggestions?

[HayZee518]

read the post before yours it explains how this furnace functions.

[black alder]

Fireplace main burner not lighting

Did this problem get fixed? I have the same issue, pilot on:750mv, then turn the toggle switch on to activate the burner and it reads 3 mv. EXCEPT: I replaced the valve and the same thing happens with this valve-no gas to the burners-Any suggestions?

Problem was fixed by replacing the thermopile.( not the thermocouple , as the pilot worked just fine)
I guess after 10 years the thermopiles can degrade.
The reading with no load was in excess of 650mv when switch turned ON to light main burner mv dropped to 3mv.
Thermopile failed under load

[HayZee518]

the other guy changed out the thermopile and this fixed his problem. if you know anything about thermopiles, they are made up of dissimilar metal wires for instance iron constantan and copper form what is called a j-k thermocouple. joined at one end one metal gives up electrons when it's heated forming a dc current. these are connected in series adding up their voltages to something usable.

[sunmtnforge]

I must be buying the wrong thermopile, I have the third one that I have bought installed. This one only lasted a few months. They have come from two different stores. I will look for a bigger supplier. Thank you for your help.

[HayZee518]

check out this NEXTAG site for thermopiles.
Thermopile - Home & Garden - Compare Prices, Reviews and Buy at NexTag - Price - Review