How to test a wall thermostat to see if it's actually working?

This morning, I pulled the plug, and then, one by one, disconnected *every* wire I could, and cleaned the contact surfaces, and reconnected them.

I also blew compressed air over the boards, and tapped on each solenoid of the gas valve and relays in the fan-control PCB.

This took about an hour. When I powered it back up, I heard a click click click, and then a small whoosh, and then after a while, a bigger whoosh, and finally the blower.

The house is toasty right now - but it has only been working for a couple of hours. Thanks for all the help & advice!

I'll report back, to let you know how it progresses along over the next day or three ...

I see it. Thanks. This is the backside of the thing:

So that makes four safety switches to debug next time:

1. The fusible link (also visible in that picture above)
2. The vent safety shutoff switch (also in that picture above)
3. The lockout timer (also in that picture above)
4. The limit switch (visible in the picture below)
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The only one of those four that I don't understand is the lockout timer. Googling for the part number 990-141-1, I find it's a $200 24VAC 60HZ SPST switch:

This PDF purports to explain what it does:

It seems to have a "pickup time" of 7 seconds, and a "dropout time" of 5 minutes.

Since it's hooked in series with the pilot light and spark igniter, it has something do do with stopping that circuitry after about five minutes. But, I'm not sure what controls it, unless it's a pure timer.

Is that what it is? A timer to interrupt current to the pilot switch after five minutes?

Just as an aside, the $279 lockout timer appears to be a normally open switch, based on these two descriptions:

It is called a "Combustion Safety Control Lockout Timer" (whatever that is supposed to mean) over here, also sold for $279:

So, it seems to be an expensive normally opened timed switch.

It seems to have a timer of about five minutes. After five minutes, presumably it closes the SPST switch. I'm still trying to figure out what it's supposed to do.

It's hooked up *before* the pilot, so, being normally open is odd.

I must apologize for having never even looked before this morning to see how to turn off the furnace (I just hit the breakers up until now).

Following the power cord with a flashlight in the dark mouse-infested recesses, I see it terminates on an inaccessible back wall, in a normal 120V three-pronged power cord:

I pulled the plug this morning, and wished that I had not, since it's a bear to get back on the wall, because of the ductwork in the crawl space.

So, next time, unless it's an emergency, I'm gonna flip the breaker.

If I ever get a round to it, I'll see if I can hook up a switch on the outside of the furnace, where that wire enters at the door switch.

Well, she's running hot now:

Thanks. I've identified four safeties:

1. The lockout timer (still not sure what its specific role is)
2. The fusible link
3. The vent safety shutoff switch
4. The limit switch

Luckily, all four must be working fine, since the heat is on! Woo hoo!

I don't understand *what* it does yet. Sure, it's called a "lockout timer", so it locks out the gas, I guess. But, how does it figure out when/if to lock out?

All I can tell, so far, is that it's a normally open SPST switch.

I apologize for not having looked far enough into the mouse-infested recesses of the ductwork to provide the answer sooner...

There is no power switch; but, there is a power cord, which is plugged directly into a wall outlet, *behind* the furnace (and to the side):

I made the mistake today of unplugging it, and then regretted that since I had to get covered in insulation and mouse poop in order to plug it back in.

Next time, I'll just hit the circuit breaker!

No shutoff switch. And the breaker is on another floor. So, most of the time, I leave it powered up.

I made the mistake of unplugging, this morning, and wished that I hadn't (since it's really not easy to put the plug back):

So, when I have the inclination, I'll see if I can put a normal 120V switch mounted on the side of the furnace.

I hate it when installers save a buck ... (their buck) ... as I agree; it should have a switch mounted on the side of the furnace.

Mine is in California. Built in the 1980s. It's plugged into the wall ...

As far as I can tell, there is no switch. Just a power cord plugged into the wall.

That could be the butt of jokes.

Looking up why a propane furnace needs a lockout timer, I find that they want the gas to be cleared from the pipes *before* igniting the pilot (hence why the lockout timer is a normally open switch).

It seems to say so, on the last sentence, of this document:

It seems that only the propane retrofits have this lockout timer. Apparently, the heaters are originally made for LPG. When they're converted to Propane, they apparently add the lockout timer.

Apparently propane gas has qualities that require that lockout timer so that the gas is evacuated from the passageways.

That's all I know, so far (or think I know).

Yeah. They call taxes "fees" out here so that they don't need a super majority to pass them. The people out here are too stupid (politically) to even notice.

They outlaw basically everything; again, the people here are too stupid to notice.

It's a weird state - but - the weather is excellent - if we could just get rid of the politically dumb inhabitants.... :)

Hmmm... I have no idea what an efficient flame looks like. Am I supposed to adjust the flame somehow?

It's interesting that you noticed that ... I will take a look to see what's going on. I personally don't have a clue as to why, or, if it is indicative of something ...

I thought LPG (liquid petroleum gas) and propane were the same?

Are illegal Mexicans a protected class, there?