Heating stopped working

Users who are viewing this thread

Edgar Miller

New Member
Messages
13
Reaction score
0
Points
1
Location
Texas
Hi,
New at this forum.
My heating stopped working over night, I'm suspecting the thermostat.
Is there any way to test it before springing for a new one?
Thanks,
Ed
 

DIYorBust

Active Member
Messages
745
Reaction score
94
Points
28
Location
Long Island, New York
Sure, pull the thermostat and jump the the tt wires. If the heat goes on, but the thermostat itself fails to make that connection when it should, you have identified the problem.
 

WorthFlorida

Clinical Trail on a Cancer Drug Started 1/31/24. ☹
Messages
5,727
Solutions
1
Reaction score
982
Points
113
Location
Orlando, Florida
What type of heat source? Most likely the 24v transformer went bad but if your thermostat works on batteries, you need the 24 v AC for relays to start motors and valves.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
A thermostat is just a smart switch...it connects power to the required device (heater in the winter, a/c in the summer, if you have it, and often, a way to bypass the auto fan, and turn it on constantly).

So, as was said, the easiest way to verify it's the thermostat is to just jump the R and W wires together (note, they may not be those colors, but they should be installed on those terminals of the thermostat).

If you thermostat DOES use batteries, replace them first!
 

Reach4

Well-Known Member
Messages
38,796
Reaction score
4,412
Points
113
Location
IL
OK, guys,
I have jumped the R and W wires but, alas, nothing happens.
What now?

Transformer could have failed. Control board could have failed. Gas valve could have failed. Maybe you left an interlocked cover off (guess how I thought that was worth mentioning).

I would start by checking the voltage across the output of the transformer. About 24 vac is normal. If there is an interlock switch on a cover you remove, you can often bypass that by pulling the plunger.
 

Reach4

Well-Known Member
Messages
38,796
Reaction score
4,412
Points
113
Location
IL
Can that voltage be measured at the thermostat?
What's an interlock cover/switch?
Voltage could maybe be measured at the thermostat, but you would need a C (common) wire. Most installs don't have the C wire run unless it was an electronic thermostat.

Interlock switch would be under a big cover at the furnace. It would shut things down when the cover is open.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
If you do not have the instructions for your heater, see if you can find it on the manufacturer's website. That will often have a troubleshooting guide in it.

Usually, inside of the unit, there will be a block diagram. If you're not familiar with them, they can be a bit intimidating to read and decipher. But, functionally, there's a transformer in the unit that produces 24vac (approximately). The R wire has one side of that output. When the thermostat closes, it applies that 24vac to the device(s) in the heater to make it turn on. There's usually a relay involved, and the relay could be bad. Some of them are plug-in, so easy to replace. There may be some switches along the way, often called interlock or safety switches, to make sure things don't turn on when a cover is open, or some other problem might have occured, like overheating. If the system is a more modern one, and has a sealed combustion system, there will be safety interlocks that verify that the circulating fan for the burner is working and there's no holes or blockages in the air intake and exhaust.

The instructions often will also have a theory of operation in it, that can help you understand YOUR heater.
 

Edgar Miller

New Member
Messages
13
Reaction score
0
Points
1
Location
Texas
Well, it's an older system but has an electronic thermostat.
I'll check up in the attic over the weekend to see if there's anything I can do.
Thanks so far for the input.
Ed
 

Edgar Miller

New Member
Messages
13
Reaction score
0
Points
1
Location
Texas
OK, will do.
Just a quick question: Does the fact that the fan does not run when I turn it on has something to do with the transformer or the thermostat?
 

Fitter30

Well-Known Member
Messages
4,290
Reaction score
777
Points
113
Location
Peace valley missouri
R to W with wires separated should have 24 vac. If you have a C wire R to C 24vac. If it's a gas furnace could have a kill switch that the blower door has to be on.
 

WorthFlorida

Clinical Trail on a Cancer Drug Started 1/31/24. ☹
Messages
5,727
Solutions
1
Reaction score
982
Points
113
Location
Orlando, Florida
You need a voltmeter to check what's wrong. #1 do you have 24 volts AC? Thermostats rarely go bad. You didn't state what type of unit you have. Being TX, I'm assuming you have an A/C - heat pump and electric elements for the AUX heat. If you outside compressor and fan run but the fan in the air handler does not, its a bad fan motor. If both don't work at all, it's the transformer (90% sure).
 

Edgar Miller

New Member
Messages
13
Reaction score
0
Points
1
Location
Texas
My unit is a regular A/C and a gas heater.
Just checked the voltage between R and W and it's 24 V AC!
 

Edgar Miller

New Member
Messages
13
Reaction score
0
Points
1
Location
Texas
Yes, the AC works.
No pilot light. Electronic ignition.
One other thing: suddenly the fan works again when I switch from Auto to On.
Also, I can hear the normal clicking sound in the thermostat when I manually raise the temp setting and I hear a hum up in the attic but the heating, usually kicking in after 15 or so seconds, does not start.
 
Last edited:

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
Gas devices typically have a flame sensor. The typical sequence is:
- call for heat
- if there's an exhaust fan, turn that on to purge any combustible gasses
- open gas valve and turn on ignitor (might be simultaneous, or a slight delay between)
- sense if the burner lit
- if not, purge the burner of combustible gasses
- try again. If after (depends on the system) X tries, and no flames sensed, lockout the system

Your system may slightly differ, but that's the general idea.

So, if you hear things trying to turn on (it may have started to make those sounds if you shut off power to it, resetting the lockout), and it can't sense the flame, it will eventually lock out again.

The flame sensor can vary depending on the system. It could be a microcurrent probe that is shot, dirty, or bent so it's not directly in the flame. Some use a hot surface ignitor, and that can also act as the flame sensor. WHen it's trying to light, are there clicks, or just that hum? If there are clicks, it's probably a piezoelectric sparker. If it just hums, it's probably a hot surface ignitor. All of this would be discussed if you found the operator/installation manual. FWIW, sometimes, they stuff it inside the unit. Have you looked? THey usually often glue on a copy of the functional diagram inside the door. IF you find that, take a picture and post it.
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks