Hi, I'm having trouble with my gas furnace short cycling. I changed out the old thermostat and that didn't make any difference so I was going to clean off the thermocouple and realized that my pilot light looks nothing like the pictures that I've seen. It's BIG and at the end of the pilot light tube is a Y. One end burns on the thermocouple and the other end seemingly goes no where. A

t first glance, when I saw the big flame, I was going to try and adjust the pilot but now I am really curious if I am overlooking something else to try and why does this pilot tube split at the end.

Any other tips for fixing the short cycle? Cleaned burners, fan runs great, the cycle runs for 5 minutes or more, pilot light stays lit...furnace is really old but can't afford to replace it so I have to make it work.

Thanks in advance for your help.

 

The side of the pilot flame not going onto the thermocouple probably is what lights the main burner.

What happens if you turn the thermostat all the way up?

 

Thank you Grady for your reply. Yes, you are right the other side lights the main burners. I just turned the thermostat all the way up to see what happens and is still short cycles. I turn it on, I can hear the the burners light up, seconds later, the blower starts and then it turns off. I was wrong when I said it ran for 5 minutes. It turns on and off in one minute and back on in one minute intervals.

So far, I've replaced the filters, cleaned the blower and burners and took steel wool to the thermocouple as well as replaced the thermostat. So far, nothing has stopped the short cycling. As soon as it turns off, it starts the cycle again and turns off.

Do you think I should try and replace the thermocouple myself? Since the pilot light stays lit, is there even a shot that a faulty thermocouple is causing the short cycle? I thought all that did was cause the pilot to not light? I see they are about $20. **How do I know what kind to buy? I'm going to run out and get one today but looking on line it seems that length is the only difference? What's the worst that can happen if I can't get it back together if I'm going to have to call someone anyway? It looks simple, is it? I am going to try and put it in without removing anything else, just loosen the bracket and slide it in.

No one is going to work on this almost 50 year old furnace and they are going to try to sell me a new one which I can't afford so any other ideas of things for me to try would be much appreciated!

 

Yes.... the thermocouples are installed by length of the line. It has a nut attached to the end that screws into the gas valve.

It could be weak and not have enough voltage to keep the main gas valve open.

However... your problem could also be a high heat limit problem although it usually takes several minutes to go into a high heat issue.

 

Thank you PJmax, Is there anything "tricky" about replacing it myself? It looks easy and as long as I get the proper length, I should be good to go? I'm going to get one today and give it a shot. I can't blow up the house, can I?

 

Thermocouple length isn't that important. If yours is 18 or 24" & all you can find is 30", just coil the excess without kinking. It'll work just fine.

 

If the pilot stays lit there is nothing wrong with the thermocouple.

However, if you have a thermopile then it is possible that it is not generating enough voltage to actually open the main gas valve. A thermocouple has a single copper tube that screws into a special opening on the gas valve. A thermopile has two wires that connect to the gas valve in the same area where the room thermostat connects.

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(Image of thermopile courtesy of Thermometrics corp.)

 

Yes, thank you Furd, The pilot light stays lit so I shouldn't bother changing the thermocouple out? There is no thermopile. When I turn on the unit, I can heat the pilot light burning on the thermocouple, then burners turn on, the blower starts and then it shuts off. Can you tell me what else to check then if the furnace short cycles every minute? It's getting really cold here...

 

I am going to change it out anyway just to see so do I have to turn the gas line valve off or is it enough to just have the pilot light turned off to change out the thermocouple?

I can't turn the gas shut off valve off because it's frozen so is it safe to change out the thermocouple if I just have the pilot light turned off?

 

Well, as Furd predicted, it's not the thermocouple. I was able to get the valve unstuck and changed it out and it still short cycles. **Anything else I can try to do myself?** Thanks again for your help.

By the way, that thermocouple was TRICKY. First of all, I couldn't get the bracket off as the screw was stuck so I tried to do it while it was attached. Couldn't just squeeze in the wings of the flange to release the old thermocouple. I had to physically rip it to shreds just to get the flange off to allow the thermocouple to slide out of the holder. Then, the new one would not go through the hole with the flange around it. I had to put the flange in and push it through the hole, then slide the thermocouple in after opening up the flange and then bend it back in place. I had to buy 2 just to replace the first flange that I ruined and then I was finally able to get the screw off to release the bracket but it was all hard and took me 2 hours. It does NOT work like the videos I found show-not a 15 minute job. Oh, and I had to get someone to turn the stuck gas valve off and then back on because it takes brute strength-yikes.

After all of this, still wearing my coat in the house so any tips of what else to try would be greatly appreciated. Thanks in advance for any more replies.

 

Final check in=had service guy out and it was the high limit switch. It wasn't broken but somehow? it had been adjusted so that the first two pins were right next to each other causing it to cut out before the cycle was complete.

Best news of all-he fixed it for NO charge as he just adjusted the pins and it worked perfectly. PS, he also fixed my gas fireplace for NO charge. It took him all of 15 minutes. I wish all service people were so wonderful. This has made me so, so happy!

Worst part, I was so excited, I forgot to tip him-ack. By the time I realized it, he had already pulled away and didn't answer his phone so guys if you are leaving someone's house and you know they were happy and your phone rings, ANSWER it because they just realized they forgot to tip you!

 

My first thought had been high limit but the timing was wrong. Misadjusted pins explain that.

There are more service guys out there like that....... trust me

 

He's a keeper! Definitely send him something in the mail if you have an address. If he works for a company then send a thank you letter to the company. Recommend him to all of your friends as well.

 

He does work for a company and they always call to ask how their guys did so I will give him glowing reviews but geez, I'm so sorry that I didn't tip him to let him know how wonderful it was of him to do all that for me. I feel terrible.

I think I called that the wrong thing and found a video about it-grrr-could have done it myself it I had realized to pull off the cover. It's called the fan and limit switch, not sure if that is the same as the high limit but just thought I'd clarify in case someone searches down the road. The switch is Honeywell and the furnace is 47 year old-a Rheem. He said they still make that part which is amazing to me.

Thanks again to everyone here for your help along the way!

 

In addition to the glowing verbal report, a letter to his company is always appreciated. As PJmax said,
there are more service guys like yours out there.

 

I am thrilled to be able to report that we have HEAT this morning! It turned on as programmed and there was a brief time of kicking on and off, maybe 2 or 3 times, until now it's been steady for the past hour.


I realized that I made a stupid mistake in the programming-lol. For the away cycle, I forgot to adjust the temp all the way down low. At first, I couldn't understand why it kept coming on at the away part-ugh. I have it now though and it seems to be working perfectly.

Thanks for all of your suggestions. I called the company and told them how nice and most of all, competent, their guy was and will also write a note so they can keep it in his file. I do know that if I have anymore trouble, I'll call and request this guy

**Do you agree that even though this furnace is working today that I should probably start thinking about saving up to replace it? It's 47 years old, how much longer can I expect it to live? Any tips about shopping and pricing out a new one? I forgot to mention that when I initially started to try and fix this, I couldn't understand why there was so much sand under the burners. I vacuumed it all out and it wasn't sand, it was rust and about 1/4 inch deep!

Thanks again!

 

At 47 years old, that furnace has outlived 2 normal life expectancies. Most certainly start saving for a new one.
As far as shopping, check with friends, neighbors, co-workers, etc about whom they've had do similar work & how well satisfied they were with the contractor. If your service company also installs, they are certainly a place to start.

 

At 47 years old you should definitely consider an upgrade. Since it's working now.... you have some time but it never hurts to start shopping for a new furnace.

Check with your local gas provider for energy/rebate options. Some gas providers will not only give you a rebate but you can purchase it thru them on a payment plan. Never hurts to explore all options.

Get several estimates before committing to any contract. Ask friends and neighbors for referred contractors.