Setback thermostat to control gas fireplace?

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aaronml

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I have a natural gas fireplace with a standing pilot and a thermocouple in series with a normal SPST 120 volt switch. The switch is definitely connected only to the thermocouple circuit and not to any 120V source. From what I can determine, closing the switch allows current to flow to open a gas valve and the pilot ignites the gas. I want to be able able to use an electronic setback thermostat to ignite and turn off the gas burner at given times. It seems to me that I should just be able to take the the wires that are currently attached to the SPST switch and attach them to the RH and W terminals of an extra setback thermostat I have. When the thermostat calls for heat it will close the circuit and ignite the burner. When the set point has been reached, it will open the circuit and cut off the gas. In other words, it will function just as the SPST switch, but I will be able to control it by time and temperature. Does anyone have an opinion on this ideas?

Thanks, Mark
 
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Although there appears to be a safety margin built in (valve powered by thermocouple) are you sure you want it to be turning on when nobody is present?
 
Thank you for your input. It would be used in very limited circumstances, and only when someone was in the house. There seems to be a question, however, as to whether the milivolt output of the thermocouple would be sufficient to complete the circuit. The gas valve is a Honeywell VS8421 and a response from a tech there said I would probably have to have a previoulsy unused thermostat since the normal make-break of the contacts in it causes some arcing (which I presume creates resistance in the circuit).
 
The Honeywell VS8421 uses a thermopile that generates hundreds of millivolts, so it might work with your thermostat. Is it that difficult to try it? What harm can it do if it doesn't work?
 
Thanks for the comment. It should be easy to try so I will give it a shot within the next week and post the result.
 
Be careful when playing around with gas powered appliances. In many countries you need a certificate to prove you've undergone specific training before you can legally perform work on a gas appliance.

I must warn you that you might be breaking the law.
 
Thanks for the warning. In essence all I did was to replace one wall switch with another, but your point is well taken.
 

Not illegal in English Canada. Hardware stores sell gas fittings here.
 
They sell them here too but it isn't legal to fit them yourself.

You have to be CORGI registered to work on appliances.

You might be able to do something simple like connect and disconnect an appliance (I'll have to check on that) but beyond that you're breaking the law if you do.

As I said, I can't speak for other countries though, you'll have to sheck the laws where you live before you do it.
 
Hero999 said:
They sell them here too but it isn't legal to fit them yourself.

You have to be CORGI registered to work on appliances.
Click to expand...

I thought CORGI made toys.

Maybe I should surrender to the local RCMP. "Corporal, I'm guilty of rewiring my electrical panel, and I changed the gas hot water heater."

I might be executed in Quebec.
 
If you came to England, serviced someone's gas boiler and it blew up and killed someone you would go to prision for a long time.
 
Hero999 said:
If you came to England, serviced someone's gas boiler and it blew up and killed someone you would go to prision for a long time.
Click to expand...

A licence is required to service other peoples electrical and gas here too, but we can legally install and repair anything in our own house.
 
They've taken it a step further, here in the UK, it's actually illegal to service your own gas appliances.

Suppose you service your own gas boiler and it blows up and takes out the house next door (this could easilly happen as semi-detached houses are pretty common here then the UK) you could go to prision.
 
And yet the people who come to change your gas meter aren't CORGI registered - and are completely incompetent, they have tried to change mine three times so far, and not even come close
 
I have reason to suspect that you might not need to be CORGI registered for some things.

For example, in science class we connected up Bunsen burners without being CORGI registered.
 
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