Help on a Lennox Superior Fireplace needed

[Ken H.]

I am working on a Lennox Superior gas fireplace, similar to their model SDV35 (although the exact model on the fireplace I can’t find online anywhere: DT-400CMN-J)

It has the millivolt system (manual spark pilot igniter with a ‘three flame’ pilot, a thermocouple and a thermopile). Looks like this:

It has a SIT Millivolt gas valve.

I know the purpose of the thermocouple and thermopile (to generate electricity when heated), more by the thermopile, but I’m not totally clear on what affect each has on the gas valve’s operation.

Symptoms are the pilot will light and stay lit as long as the gas valve knob is in the Pilot position, even when not held in/down. However as soon as it is turned to the On position the pilot goes out (and of course the burners do not come on).

In that the pilot stays on in the Pilot position, I’m thinking that this is not a thermocouple issue, but rather may be a thermopile issue? But does the gas valve need electricity from the thermopile just to keep the pilot on when in the On position? Conversely, is it possible for the thermocouple to be not working such that it will keep the pilot on when in the Pilot position but not when in the On position?

The pilot flame seems well wrapped (say 60%) around the thermocouple and thermopile and I cleaned off both. Although I didn’t measure the voltage coming from the thermopile (understand it should be somewhere around 400 millivolts) as I didn’t have the appropriate meter with me when I was there. I will do that when I go back.

Any advice on how to troubleshoot this problem is appreciated.

 

[Heatsource]

B-vent unit? bypass spill switch for testing if so (always re-connect later!)

what is the t-couple putting out? they are a cheap and easy part to "throw" at this problem
also, you could call lennox 800-655-2008

http://woodheatstoves.com/images/Nova SIT 820 troubleshooting guide.pdf
here is one of my old uploads that might be helpful,
dave

 

[Ken H.]

Yes it is B-vent. (I am in Canada where vent-free is not allowed.)

Thanks for the .pdf, it sheds some more light.

Where is the bypass spill switch typically located?

I will measure the thermocouple output too when I go back to the site.

Tks.

 

[Heatsource]

the spill switch will typically be hung under the rear draft hood, where exhaust could "spill" into the home if the venting isn't working....

 

[Ken H.]

In the end it was the thermocouple. Surprising that it would produce enough voltage to kept the pilot on in the gas valve’s knob Pilot position, but not in the knob On position, but there you have it. Replaced it and away it went.

Tks for all the tips, I learned a lot.