Help! My thermostat only works sometimes?

KatW

Hi all,
We live in a small, older home with a single wall heater. It's always worked just fine, but the past few days the thermostat will just randomly stop working. I assume it is the thermostat because the heater seems to be fine, the pilot stays lit. But we'll turn up the thermostat and no matter high we set it the heater just doesn't kick on. We'll turn it all the way off, and then try again later. Sometimes it kicks on, sometimes not. It seems to be that it is turning on in the mornings only, but it hasn't been happening very long so I can't totally confirm that pattern.
My landlord is willing to send someone out to look at it, but we live in a Covid hotspot and I'm not anxious to have people coming in and out of the house.
Is there any sort of solution to this that I can take care of without outside help? I've attached a picture of our thermostat if that helps.
Thank you for any help you can give me!!

mattmia2

I think that is a mercury switch thermostat so the chances that it is the problem are relatively low. You could jump the thermostat contacts on the heater (I assume it is a wall furnace?) and see if it comes on. Some pictures if the furnace with the cover off would help us tell you what to look at.

KatW

Here are both sides of the furnace with the covers off! I also attached a picture of the thermostat with its cover off if that helps at all!
Thank you!!

Jamie Hall

That is the mercury switch type, and they very rarely fail. That said, with the cover off like that they are also very easy to troubleshoot. When you turn the thermostat all the way up for heat, make sure that there is nothing obstructing the bulb and that it tips so the mercury is in the end with the contacts. That should fire the boiler.

No joy? carefully take a short wire (insulated!) and bridge from the terminal strip on the far left (by where the control is in the picture) to the similar strip on the far right. If that fires the boiler, but tipping the bulb does not, try a similar trick but now bridge the two terminals of the bulb. If that doesn't fire the boiler, inspect those two flexible wires from the bulb to the terminal strips. It's possible that one of them may have broken.

Now. No joy with firing the boiler with the jumper between the two terminal strips? Your problem is either in the wires between the thermostat or in the burner control. You will need to play with the furnace to check this, and you might want to have someone else do it -- it's a little scary. The idea is to find the two terminals to which the thermostat wires connect, and jumper those two. If the boiler fires there, but not with junpering the two terminals at the thermostat (step one in the second paragraph) the problem is in the wires. If it is still obstinate... probably in the burner control.

EdTheHeaterMan

STOP Don't touch that thermostat

That is a millivolt system. Time for a new flame sensor. The flame sensor is called a power pile generator. The flame from the pilot heats up the bulb at the end of the wires and generates 750 Millivolts of electricity. That is enough power to maintain the pilot valve and open the main valve on a call for heat from the thermostat. As the power pile generator ages, the amount of electricity generated diminishes. Enough to keep the pilot valve open but not enough to open the main valve

https://www.supplyhouse.com/White-Rodgers-G01A-502-Mv-Power-Pile-Generator-w-36-Fiberglass-Leads-and-armor-sleeve

This could be a DIY if you are handy and not afraid of taking apart gas piping to the pilot. Or you can call your plumber or HVAC service tech.

Good thing about this heater is that when the power goes out, you still have heat! No external electricity needed for the flame to operate. You still need electricity if there is an optional fan.

If you do call a Pro, please remember that you are paying for his knowledge (the labor portion of his price be it hourly of Flat Rate) The cost of getting his technician to your home (Diagonist fee, Trip Charge, or minimum first 1/4 or 1/2 hour) and the part, which he is entitled to have a reasonable markup. The hardware store might charge you double what the Supply House charges for that part.

So be understanding, the cost of operating a business is not just the service man's hourly wage and the parts price given to you at Cost.

KatW

Oh wow! Thank you all so much! Of course when I went to test these out, the heater started right up, but this is all SO helpful.
Thank you Thank you Thank you!!!

JUGHNE

Because the system runs on only .75 volts generated by the power pile, all connections must be clean and tight.
I would try to loosen and clean all connections one at a time, that may fix things. Check the wiring connections on the tstat also.

Then go for a new power pile generator.

Jamie Hall

And why not touch the thermostat, might I ask, @EdTheHeaterMan ? Millivolt or not, if there is something amiss with the thermostat and wiring, it isn't going to work right. Jumpering the contacts on the thermostat should not cause a problem with the thermopile, seems to me?

I wait to be enlightened...

EdTheHeaterMan

So your pilot generator may still be good enough to give you heat. Try tightening the screws on the connections to the gas valve and the thermostat. (only 5) That may get you thru the winter.


Then get maintenance in the spring or fall during "Lower Price Service call" season. If you have the part on hand during a Lower price Tune-Up special. You might be able to talk the tech into replacing it for you. That is a rare part for some companies.

JUGHNE

The few millivolt systems I have encountered have use a fairly heavy (for tstats) click switch. It seems the very thin flexible wires on the HG tstat might create some voltage drop.
The current flow for this could have been marginal and now some more resistance added with corroded connections may have taken it over the edge???

EdTheHeaterMan

I saw the thermostat without a heat anticipator dial. Dead give away for Millivolt. The old Honeywell T87F mercury thermostat has a little brother, the TS86A for millivolt operation. Looks naked inside the without the Heat Anticipator.

EBEBRATT-Ed

@KatW
at the very least you can give the service tech that is coming a heads up that you think the powerpile is bad. That way he can bring one with him. many won't have one on the truck they are not used as often anymore

Intplm.

I sure do miss the old thermocouple.

Jamie Hall

I still want to know why not touch the thermostat?

Intplm.

Jamie Hall said:

I still want to know why not touch the thermostat?

I agree. It's easy enough to disconnect the wires and twist them together if there's no wire to jump it with.

mattmia2

My grandma has had the honeywell branded version of that t-stat on a millivolt furnace for about 40 years without a problem. Those flex wires are so short that the voltage drop over them is trivial even in a millivolt system.

EdTheHeaterMan

Know your audience. At the time I posted "Stop Don't touch your thermostat" @KatW has only one post. Clearly not an expert in all things HVAC. By manipulating the thermostat with a jumper and the like, the problem does not go away it is only delayed. After the thermostat is replaced (for no reason) the problem will present in a month or so as the thermocouple pile deteriorates further.

By then, they have a new thermostat (which may not be needed) and another trip to HH next month for more advice. (The OP may even get instructions to get a Nest Thermostat LOL) The warning was for the novice troubleshooter, not the experienced technician But you,@Jamie Hall, already knew that the warning was not for the experienced, Hence your query!

Looking at the Bold Letters, I may have overreacted. Just trying to save time and $ for the OP.

As we all know 750 MV is not even going to electrocute a flea, but any reduction of the resistance in the circuit will increase the chance of a weak power source not completing its purpose. That initial surge of electrons sufficient to move the solenoid to open the main valve. Once open, a lesser amount of electron movement will maintain the valve position. So as the power pile gets progressively weaker over time, the problem frequency will increase until the power source is no longer sufficient to "Get 'er Done"

Those are my reasons, and I take full responsibility for my actions.
"
THE BUCK STOPS before it ever gets to my wallet

Jamie Hall

Fair enough, @EdTheHeaterMan -- and I have a sneaking suspicion that you may be spot on -- an elderly thermopile and an arthritic solenoid may well be the problem here. I just wanted the OP to find out that they shouldn't blame the thermostat...

HomerJSmith

Looks like William's furnace.

OK, I breezed the comments, you want the 411? I can't see too clearly the open thermostat, so I have to ask. Does the thermostat lever when turned all the way to the off position separate the contacts, turning off the power to the gas valve?

Ed's picture of the thermostat shows two red arrows pointing to two screws. The red arrow on the left is where the switch is if you have one and the lever may separates the points, severing continuity to the gas valve. Is this true for your thermostat? If so, sometime the lever separates the points where they hardly make contact when the lever is set to heat. You would need to squeeze the contacts together with a pair or needle nose pliers to restore good contact.

Your wall furnace has a Thermopile in pilot flame that generates the electricity to operate the gas valve sending voltage thru the thermostat. .300 volts to .750 volts is the measured range of the Thermopile. The higher the voltage the better it functions. Thermopiles do wear out and need to be replaced. Anything in the Thermopile circuit that creates resistance lowers the voltage at the gas valve.

There are also safety devices in the Thermopile circuit, usually Thermodiscs. They cut the voltage to the gas valve when they open, shutting down the burner flame. They are usually located on the heat exchanger and sometime just outside the draft hood.

If you jump the two screws that Ed's red arrows point to, the furnace should kick on. Jump them repeatedly and if the furnace kick on each time, the Thermopile is probably ok. You can leave the jumper on for a period of time and see if the higher temperature causes the flame to cut out. Don't over do this as excessive temperature to the heat exchanger isn't good.

Does this wall furnace have a blower on it?

HomerJSmith

Jamie Hall, The thermostat is probably radioactive. hmmm

HomerJSmith

Or, one may not want to break the mercury bulb. Oh, hell, here comes the hazmat team. Big Buckos!

Jamie Hall

That's a mercury thermostat, @HomerJSmith -- there are no moving contacts inside it.

KatW

Wow! Thank you again for all of these amazing responses!
No, I am most definitely NOT an expert, thank you for explaining things so clearly!
To answer some of the questions... Yes, when you turn the thermostat all the way down, a little piece of plastic on the lever separates the connection! But, it looks like a very solid connection when the thermostat is turned up.
Does this wall furnace have a blower on it? Honestly, I don't know. I don't think so? It's very tall and it seems to me the heat just radiates out of it.
And, of course, everything has been working normally since my reply yesterday, but when it inevitably goes out again I will try some of the suggestions here and call my landlord if needed.
Thank you all again! I was not expecting this level of response so THANK YOU!

HomerJSmith

Jamie Hall, It took me a while to figure out what you meant in your last post, but I think I got it.

There were mercury thermostats that when the lever was moved to the left (the off position), the lever slipped between two contacts breaking the electrical path thru the thermostat.

Sometimes, the part of the lever that separated the contacts would spread them to the point where the lever when moved to the on position, the contacts wouldn't close completely and one might experience intermittent operation. That's why I asked that question.

KatW, If it had a blower you would know it. Kat, get a Carbon Monoxide detector, it saves lives.

Jamie Hall

And I had forgotten that there were mercury thermostats which had that off switch on them -- so my apologies on that and thank you, @HomerJSmith !