Flame controle burner

Thread Starter

Ivan Vanherpe

Joined Apr 25, 2018
I'm looking for info concerning ionisation flame controle.
Can I build a circuit that controles some relays to open valves for the gas?
I read that the probe in heating devices output some millivolts, would that be enough
to conduct a small transistor?


Joined Jan 15, 2015
Thermocouple or thermopile outputs as used for flame detection in residential furnace applications to detect flame typically output between 28 to 32 mV and no current to speak of. They can hold open the gas valves they are designed to work with but less additional circuitry for amplification they are a very small signal device. The furnaces they work with are also generally Off or ON devices using a low pressure valve and the burner ignited by a pilot flame which the thermopile or thermocouple detects. When heat is called for the pilot flame ignites a main burner when heat is called for by a thermostat. Newer residential furnaces get more complex but the older ones work pretty much as I mentioned.

The choice of gas valve you plan to use will determine the best method for control. Do you plan an ON / Off valve or a valve for flame which can be throttled? This all starts with the choice of valve, gas pressure and type of gas be it NG (Natural Gas), LPG (Liquid Propane Gas) or other gas of your choosing. Aside from using a thermocouple for flame detection, there is flame or not, there are also Flame Rods and UV (Ultra Violet) flame detection methods. All coming back to exactly what you want to accomplish.



Joined Mar 19, 2019
I'm not sure where you live or precisely what you are trying to do but when dealing with gas burners you run into regulatory and insurance issues. Be very careful!


Joined Jan 15, 2015
@SamR, I'm shocked you necroposted, thread has been dead since 2018.
I think what happens is an old thread gets a spam post. That spam moves it to the top of the pile. Moderation removes the spam post but the old thread remains at the top of the pile. I have been guilty of not catching the date. :( In this case since a few years ago I replied to the thread I got an email notification which got me here (again). :) Anyway I think that is what happens. I have been guilty of not catching a date also.



Joined Jan 11, 2015
Thermocouple or thermopile outputs as used for flame detection in residential furnace applications to detect flame typically output between 28 to 32 mV and no current to speak of.

No, a standard thermocouple can output around 30 mV but it's a very low source impedance. It takes a lot of current
to hold the "push to light" button in. Electromagnets strength depends on current * turns. Due to the low voltage
it's not a lot of power though.

The one valve I measured dropped out around 136mA. so the thermocouple valve combination in operation has
a higher current flowing than this.

In my case, the hold in coil connections inside the gas valve were just push on and after 6 years they went high
resistance enough that the current wasn't enough to hold the gas on so the pilot wouldn't stay lit. This is the "other"
failure mode of that value model, the first failure mode (which didn't hit me, but was well documented on the web)
was the 24 volt connections going bad, again push type contacts in the electrical path. Since these are outside
the valve there are several videos on bypassing them with soldered wires.

So much for "ease of manufacture" vs reliability.

I figured once I opened the sealed valve it was toast even though the hold in coil resistance dropped once
I remove and reinstalled it in the gas valve. It would have just failed again anyway. Got a new gas valve
made by a different company. The new one's been working for 9 years now.


Joined Mar 19, 2019
That's odd... Yeah musta been a spam post revived it because I responded to a post in the recent tab. Weird... I sure don't go looking for old posts to respond to! There was also someone else that responded right after me and that post is gone now?


Joined Jan 23, 2018
As far as gas valves and other things being powered by thermo couples, there is the "power-pile" system that does use that.
And MANY long years ago my grandfather got a thermopile conversion for his coal furnace that used a huge array of thermocouples in the heat exchanger to power both a gas valve and a blower. It was a natural gas conversion from coal and was independent of the AC mains. It work fairly well. But that was about 1955, I think. Not anything new.