Mechanical thermostat on propane fireplace

[Snagglefoot]

I have this White Rodgers mechanical thermostat controlling a propane fireplace. The first and second ones have lasted about four years each. I still have them in a box. When the first one failed I cleaned it’s contact but it didn’t help. Basically moving the thermostat control arm makes the contact close but it would not let the firelplace start. The pilot light works, the gas control is switched to ON, and I can override the thermostat by moving the electrical switch to on at the fireplace. Then the fireplace works fine.

I bought a new one on Amazon and installed it. It works fine. Anyone know what the problem is with this type of thermostat? Thanks.

 

[Oildudeny]

Is it a millivolt thermostat?

 

[Snagglefoot]

Is it a millivolt thermostat?

I believe so.

 

[Kawiguy454]

the lack of wires does look like a millivolt system. If you look at the pilot you should see a thermopile in the flame. It is like a cigarette diameter and a will have 2 wires that go to the gas combo valve. The pilot flame induces a tiny millivolt charge from thermopile to the gas valve and the thermostat just interrupts that voltage so controls the main gas valve.

Check the thermopile that it is in the pilot flame and fully enveloped in flame to maximize its output. Also make sure the flame is full and clean. the pilot orifice can become clogged and flame will be small and weak or uneven. This safety device proves there is a good pilot and powers the main gas valve to open.

If you have a good pilot flame and you jumper the thermostat the gas valve should open. This indicates the thermostat contacts are oxidized and you need to clean them with DeOxit or emery cloth. If jumpering the thermostat does not open the valve you may have a bad thermopile. typically the thermopile will be distorted from heat and i have seen them with holes in them. Put new thermopile and it'll usually work. usually a very simple reliable system.

 

[Snagglefoot]

The wiring diagram shows the switch is good for 24 volts coming from a transformer. I’ll pull the sub mounting plate and check the wires and voltages.

 

[Snagglefoot]

One observation with the last thermostat is that I needed the contacts to be closed when I used the manual ON button to get the fireplace going and when I moved the thermostat to break the contact, the fireplace turned off, however it would not start back up when I closed the contact again.

 

[Kawiguy454]

that sounds like a weak thermopile output and/or dirty contacts. I explained how to test for both conditions. Or just call a heating guy to do it,

How to Test your Thermopile - www.mygasfireplacerepair.com

Before beginning this section make sure your pilot light is running, and you have read the Main Burner Troubleshooting Section so we can rule out faulty wall switch and ... Read More

www.mygasfireplacerepair.com

 

[Snagglefoot]

OK, I have one black wire and one red. The voltage across them is 0.42 V. When I short our between them the fireplace starts up.

 

[mk378]

0.4 volts is good. That's the typical open circuit voltage of a millivolt system.

Check all wiring and connections carefully, you can't afford to lose any voltage.

The anticipator coil means that thermostat is for 24 volt systems. It should not be used on millivolt.

Certain electronic thermostats are suitable for millivolt but they will all need batteries in the thermostat.

 

[Kawiguy454]

420mv sounds low should be more like 750mv. If the output is below spec for that thermopile then it is either not getting proper flame from the pilot light. or it is old with low output. The output voltage/current drives the valve to open so this leaves you 2 ways to fix it. Increase output voltage from the thermopile. OR reduce the resistance of the thermostat contacts or any associated wiring/connections.

 

[Snagglefoot]

Is it possible that the thermostat is wrong for a millvolt system? The fireplace and wall thermostat was installed 8 years ago. Thanks.

Here are some random photos. The propane fireplace has a fan, hence the plug in.
Here we go.

 

[Snagglefoot]

Thanks guys. I learned a lot.

 

[Snagglefoot]

I finally read the fine print at the end and it mentioned where to set the heat anticipator for millivolt operation.

 

[Snagglefoot]

0.4 volts is good. That's the typical open circuit voltage of a millivolt system.

Check all wiring and connections carefully, you can't afford to lose any voltage.

The anticipator coil means that thermostat is for 24 volt systems. It should not be used on millivolt.

Certain electronic thermostats are suitable for millivolt but they will all need batteries in the thermostat.

Thanks. I read the owners manual and found how to adjust the anticipator for millivolt operation.

 

[y_p_w]

That's a bimetal thermostat with a mechanical switch. Years ago they would be almost like that, but with a mercury switch inside a capsule. I remember when they turned in, there would typically be a couple of sounds coming from the furnace as the mercury bounced in the capsule. I also saw a spark in the capsule.

It could be a poor contact.

 

[Snagglefoot]

Here’s the part on adjusting the anticipator for millivolt operation.

 

[Snagglefoot]

Well, latest news is I tried to turn the fireplace on. It had been working every time I tried. This time, no start. I jumped the red and black wires and it started ( with the contacts closed). When I turn back the thermostat and open the contact it stops.

Whatever resistance is in the anticipator must be too much for the 420 mV I guess. I’ll check all the connections and look for a new thermopile.