Pilot Light Circuit Issue/Question

[cyclone76]

I have a KIT Model 5GN36HAPR gas fireplace installed in the early 1990's. It was manufactured by Security Chimneys LTD. in Canada. They're out of the fireplace business.
Anyway, it's worked fine but this fall the pilot light was out. It will not stay lit. The gas valve is a Robertshaw.
The thermocouple puts out 0.5 volts. What I don't understand, the pilot light circuit includes a Robertshaw mercury switch in series with the thermocouple. I've attached a schematic. It appears this mercury switch is bad. If I short across the mercury switch terminals the pilot light stays on and the fireplace lights and works fine. Why is this switch there? Seems like it isn't necessary. Is it really needed? The switch probe is next to the thermocouple probe at the pilot light. Mercury switches are hard to get. From what I can find, Robertshaw does not make them anymore. The other switch in the schematic is just an on/off switch to light the burner.
Thanks for the help!

 

[gasser9]

Im not a fireplace guy or a rocket scientist. But since you've got no other replies, I will offer my $.02. Mercury switch is probably used as a safety to shut it down if it tips over. I've noticed with portable elec heaters, they shut off when kicked over. Not sure whether or why this would be an issue in a fixed, permanent installation.

 

[Install fire 1]

The mercury switch acts as a thermocouple. Its the pilot safety. The thermopile controls the main burner side and the switch controls the pilot side, like modern day thermocouple/thermopile systems. The mercury expands down the tube and makes contact in the ceramic portion of the switch to keep the pilot lit. Has nothing to do with tipping over.

They work when they are jumped, but the problem with it is that if the unit gets blown out, (pilot) the pilot side will continue to put raw gas into the unit. This is because the time it takes for the thermopile to cool (drop out) takes minutes compared to seconds with the mercury switch or a thermocouple.

typical drop out times are 15-30 seconds on a thermocouple or merc switch. When you light the pilot, blow it out and listen for the pilot gas to stop flowing. you can time it. Glass off obviously.

This means you notice your unit has gone out for some reason, and you push the piezo to re-light it and the box is full of gas, and the glass comes out in your face. Dangerous situation.


They are very hard to get and are worth over 200 bucks here in ontario if you can get one.

Target wholesale out of toronto sells them, but they dont ship well, and usually fail out of the package.

Typically anything of your vintage that i have come acrossed in my years in the industry are in need of replacing.


HTH
Adam

EDIT: Looks like the thermopile puts out 500 milivolts which is in range with the switch off.

 

[jtp10181]

There are lots of fireplaces out there with ONLY a thermopile, and then an EPU wire running to the pilot solenoid.
This looks like a step between the thermopile only and the current setup of thermopile and thermocouple.

The thermopile is there to supply the 1/2 a volt to the burner solenoid, and the mercury switch is a quick drop out for the valve if the flame goes out and gas is still flowing.

The old units with only a thermopile are referred to as a "2-minute drop out". That is, gas will flow to the pilot for up to 2 minutes after the flame goes out.

I would not condone rigging your fireplace to work in this manner. If there was a fire or explosion and it was discovered the fireplace safety system was bypassed you would not get insurance coverage.