furnace cycles once, stops

[zwright]

Hello, I have a Lennox Pulse gas furnace with a programmable Honeywell digital thermostat. Whenever there's a difference of more than a few degrees between current and desired temperature, the furnace will cycle once and then stop. The thermostat shows a "system on" message for the entire time but the furnace will not cycle back on. If I reduce the temperature setting below the current temp, thus turning the system off, and then turn it back up, the furnace will light. I may have to do this a couple of times to get the room back to desired temperature. Once the furnace is near the desired temperature it has no problem keeping it there. What could be the problem? Your help is appreciated.

--Zach

[Jay11J]

In the main furnace area, on the left hand side is the ignitor modular.. Is there any flashing error code on that when the furnace stops?

If there is an error code flashing, what does it say for the code on the control?

[zwright]

Opened it up and found the ignitor module. I didn't see any kind of display and/or lights that would display an error message. (The furnace is from around 1989, I think.) The ignitor module DID seem to be humming loudly, though.

[Jay11J]

Are you the orginal owner of the furnace, or know of or see a sticker on the furnace that it has been inspected/pressure tested?

Also, on the exhaust pipe that comes out the side of the furnace, can you tell if the lower drip leg has been cut?

[zwright]

No, I'm not the original owner but there is an inspection sticker on the furnace from 1989. The furnace was also inspected when I bought the house and I remember the inspector looking closely at the furnace and saying everything looked fine, including the exhaust. On the other hand, there was no pressure test and I'm not sure the inspector was an HVAC specialist.

I'm not at home right now but I think the drip leg goes into a pump that periodically empties into the washtub?

[Jay11J]

The leg I'm talking about is the pipe on the left of this furnace. It below the "T" and reduce from 2" down to 1 1/2".

[zwright]

Ah, I see. I'm about 90% sure it looks exactly like that but I'll take a photo when I get home.

[ecman51`]

Is the furnace filter clean so that high limiting is not your problem?

[zwright]

I replace the filter every few months and this problem happens whether it's new or old. I use the 3M Filtrete filters and not the washable filter that the furnace came with. I'm pretty sure this problem happened even with the original filter.

Here's a photo of the furnace with the cover removed:

[Jay11J]

Ok, don't look like the pipe has been serviced.

It's the pipe on the right, just below the "T".

If you want, you can cut this pipe, but before you do that, get a coupling, it can be either PVC or rubber.. I would suggest the rubber.

I would advise you to get a Lennox Dealer who has a pressure test kit to test the furnace to be sure all is sound, and also flappers may need to be replaced .

[zwright]

Thanks Jay! This seems like a simple fix that will save me some money if it works.

So I'm guessing that dirt/sediment gets trapped in the drip leg and needs to be removed? I just need to cut the pipe, clean it out, and then reattach it with a rubber coupling. That sounds pretty straightforward.

I'll get the furnace inspected and tested by a professional too.

--Zach

[Jay11J]

Yeah, over time things can get built up in that trap, and may of built up to the top to slow down the drainage.

With the drainage slowed down, it may cause the furnace to lock out on you like you've been having, and when you turn down the stat and reset the temp, the furnace unlocks.

[zwright]

I thought I'd come back to this thread to report my results. I was too worried about damaging the furnace when I needed heat to do this last fall, but last night I went ahead with cutting that drip leg and now I wish I had done this a year ago! It's like a completely different furnace. It now stays on long enough to actually put out hot air instead of the slightly warm air it used to put out. Thanks for the help!

[Jay11J]

Thanks for the update, I m sure it will feel better when winter comes now and have heat!

If you need anything else, let us know.

Also just want to check with you.. On the t-stat.. the Cycle Per hour (CPH) make sure that is set to 3.. (if book don't say that, then "System Type" to HE furnace/boiler setting.)

[zwright]

Thanks. I'll check that setting.