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Paul, HVAC Technical Trainer
Category: HVAC
Satisfied Customers: 112
Experience:  15 years experience with all phases of HVAC from OEM design through installation and service.
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My heater works intermittently. What could be wrong?

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My heater works intermittently. I think its the thermostat, a really old one (Robertshaw). I turned down the thermostat, shut off power to the furnace at the circuit breaker, pulled the panel off the furnace, shut off the red on/off button, waited 5 minutes, no gas smell, turned everything back on, no heat. The fan works though. Yesterday I turned on the heat, no go. I turned on the fan and after a while pushed the switch to heat and it worked. Heat all day/night. Then I turned it off this morning. Big mistake. I had HVAC service last Friday. The first thing they said was (after looking at the furnace for, oh, 30 seconds), lady you need a new furnace. I said thanks but no thanks,  and  what  is the problem, no answer. As they left, the heat came on. Its been okay since using the fan first then switching to heat. Any help other than a new furnace would be greatly appreciated. I am not savvy about the programmable thermostat. Not sure if offset one/two are the issue.

There are several things we can do to decide if your problem is with the thermostat or the furnace.
You said it was a Rheem furnace. They usually have a model number inside the furnace - can you try to find that for me as it would help to know what type of furnace and controls it has...
The first real test would be to find where the red and white thermostat wires connect at the unit - the simple test is to temporarily connect them together with a jumper (or just a wire nut if they are wire pigtails). The furnace should start and run constantly with these jumped. Are you comfortable in doing that? and does it run?
I look forward to your response.

Customer: replied 7 years ago.

Will go out to the garage and find model # if I can. Gotta find a flashlight. I assume you are calling the furnace the unit, re: wire connection. Each color is screwed on and has a letter beside it. I will double check that too. Do I need to turn the red switch off first before connecting the wires? I'll take my laptop out there with me.

Yes, I was calling the "furnace" the "unit"...
The wire colors red and white should be attached at the letters R and W... if you have a scrap piece of wire you can use that to jumper the terminals...
It is best to turn off the power when you are connecting jumpers etc that way; nothing can get shorted out - it is just 24 v control power but we don't want you to have to replace the transformer.

Customer: replied 7 years ago.

I am in garage now. Model number is upside down but looks like 34NL-306A which is on a small gray box with a Robertshaw label on it. Oh found another model number on the inside wall of the furnace: RGDO-08NE-GR I have to go around to the other side of the house to get to the circuit breaker to turn off power. Sorry for all the time delays. There is also a green wire connected to G and two with no wire, C & Y. I'll be back.

Judging by your model number your furnace is about 25 years old and has a very basic control system - this shouldn't be too hard to figure out...
Also that model has a cleaner burning flame option in it so you must be in California?
Let me know what happens with the test jumper on R & W.

Customer: replied 7 years ago.

I turned off the main power source (circuit breaker). When I first unscrewed the white wire it sparked and there was a click inside, so I turned off the red control switch. Then I unscrewed the red one and touched the two together, nothing. Should I turn the red power switch back on? and touch them again? Did I screw up with the white wire???? OMG. Yes, I'm in California.

It should not have sparked with the power off.
You said that on the terminal strip you had just red, green and white - correct?
If so please disconnect them all and put the wires aside for now. You will have effectively disconnected the thermostat.
What I want you to do (with the power still off) is to get a paper clip, straighten it out and to put it onto the terminals so that R and G are connected.
Turn on the power and see if the fan starts.

Customer: replied 7 years ago.

It sparked with the main power off as I was removing the white wire from its screw, but the red switch on the furnace unit was still in the on position; but I think that is just the automatic ignition switch. So I then turned it off, no spark. Yes I have red white and green in descending order. Initially I unscrewed white first, then red. I will go get a paperclip. The furnace is on the opposite side of the house from the circuit breaker. Just to be clear, by the terminals you mean the actual screws, not the wires, yes?

Yes, please connect to the actual screws - not the wires.
Here is a really bad drawing to illustrate where the paperclip goes...

Customer: replied 7 years ago.

Should the red ignition switch be on????

Once the paper clip jumper is in place turn everything on - the fan should start and stay on... this tells us that the transformer is Ok and that your spark was nothing.

Customer: replied 7 years ago.

The paper clip worked, yeah. The glow of the burner appeared. So I twist it to stay on while I turn everything back on??? Red switch and circuit breaker??

Let me back up for a second. I want to be certain that we are on the same page:
Is the paper clip on R and G? If so only the fan should come on - not the burner.
Or is the paper clip on R & W? In which case the furnace should start and run.

Customer: replied 7 years ago.

The paper clip is on R and W, sorry I didn't see your diagram till now. I turned on the red switch and the circuit breaker came upstairs and the push buttons are on auto and heat. Nothing. I'll go back turn off the circuit breaker and attach red and green, okay.

I don't think you need to - we just jumped ahead a step.
When it was connected to R & W the furnace should have come on and fired up, you can watch it go through the full sequence and the blower should start shortly after the flame is lit.
Was that all correct?
At this point we are just testing the furnace - your thermostat is disconnected.

Customer: replied 7 years ago.

Fan works great. So do I reattach the wires or do I override the thermostat till I can get a new one tomorrow?

OK, what this tells me is that all the functions of the furnace are working correctly.
You will have to remove the paper clip jumper as it was just for testing purposes. Then reconnect your red, white and green wires. Right back to how it was before we started.
I can't do much temporarily for tonight to keep the heat on, but at least we know for sure that all you need is a new thermostat. You should be able to get one locally and put it on quite easily in the morning.

Customer: replied 7 years ago.

Sorry I know its late your time. When I reattached the paper clip to red and white screws the glow just comes on, then after a few seconds fades and goes off. Nothing else happens. No blower.

Ok, now that's different... the ignitor should stay energized for up to 30 seconds.
If its not doing that then you would either have a bad ignitor (not likely if it glows at all) or a bad ignition control module.

Customer: replied 7 years ago.

Could be thirty seconds. Should I time it? What about the blower not engaging? Is that significant? I'll buy a new thermostat anyway, I hate this one. Can I install the ignition control module myself??

The blower won't engage until after the gas has lit so that's not a concern yet. Timing it would be good. Do you have a voltmeter that you can use? For another test. I don't like to condemn parts until I am sure...

Customer: replied 7 years ago.

I have a battery tester that only goes to 22.5 V. Is that okay? Why didn't the gas light? I turned everything back on - the red ignition button and the circuit breaker?

A battery tester won't work as we need to test a 24 V AC signal.
The ignition module follows a very definite sequence of operation for safety... it controls almost everything inside your furnace...
Paraphrasing from the tech manual for that control module...
First there is the call for heat (which we have manually controlled with the paper clip).
Then the ignitor will glow for 40 seconds.
The gas valve should open at 35 seconds (just before the ignitor goes off)
The flame should light
Then 15-45 seconds later the blower will start...
If you have timed the glow ignitor it sounds like we are going through the sequence to where the gas valve should open. We would need to check for that voltage to determine if the valve is bad or the control is not telling the valve to open...

Customer: replied 7 years ago.

There is a tool lending library open tomorrow. I can get a volt meter there. If not I'll buy one when I get the new thermostat. Do you think you could follow up with me tomorrow and can I replace the ignition control module myself? With your help, of course. I hope so.

Absolutely, I will be glad to continue working with you...
Don't buy the thermostat yet, after what you have told me about the ignition sequence I don't think that it is having any affect on the problem... you might as well save the money...

Customer: replied 7 years ago.

I was just going to get a cheap on-off, not programmable, thermostat.

Ok. Thank you, I will make sure that I am available. Have a good night.

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