Furnace cycles on and off w/o heating adequately

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When heating, my furnace turns on, heat and blower, for a short time, then stops. After about 10-15 minutes, it goes on again but the same thing happens. As a result, the temperature never approaches the thermostat setting. The furnace people came and replaced the ignition module and gas valve to the tune of $500. But this did not work. Now they say it's the limit switch. That sounds plausible -- but I'm in the dark here. Any thoughts? JM

-- John McQuaid (jmcquaid1@aol.com), November 11, 2002


Find the web site:"How Stuff Works". Find "Furnaces". Educate yourself. Replace the limit switch yourself for maybe $20 (just guessing). Call your repair people and thank them for the anal sex.

-- Zen Clown (MartyS@iland.net), November 11, 2002.

Oh, be sure to turn off the power and make sure you specify that you want a FAN limit switch. Go to the aforementioned 'site first so you'll know what you're talking about. My e-add is valid if I can help. Marty Stoetzle, Montrose, Mo.

-- Zen Clown (MartyS@iland.net), November 11, 2002.

I've been looking. Seems "How stuff Works" doesn't have the info. Try this: http://www.toad.net/~jsmeenen/furnace.html. I went to Google and looked at "furnace repair". THE TRUTH IS OUT THERE. :-)) Marty

-- Zen Clown (MartyS@iland.net), November 11, 2002.

John! Everytime I hear of folks bein "taken" by service people it makes me irate. They should have found the problem the FIRST time they came to your house. And $500 for a gas valve and ignition Module? Boy, I could've gotten rich doin that!! I can sleep tho!

The problem COULD be in the limit switch but it also could be in the anticipator of the thermostat. The way to find out is "jumper" the limit switch with a short piece of insulated wire. If the limit switch is bad it will shut down the furnace before it reaches the desired tempature. Jumping it will bypass the limit and let the furnace continue with it's heating cycle. Replace the limit switch and DONT' leave the jumper on.

The anitcipator is a little adjustment in the thermostat. On a mechanical or mercury t-stat there will be little pointer. It is is set at different amp settings. Look at the gas valve and see what the amp draw is. A little tag will note something like .04 or some such number. Make sure the little pointer in the thermostat is pointing at that particular number.

On Electronic t-stats it may be a little different depending upon what type/brand and furnace type it's hooked to. Pull the cover off and have a "looksee".

Sometimes the thermostat will become unlevel [mercury type only] and that will throw off the temp.

Let me know what you find. old hoot. Matt.24:44

-- old hoot (hoot@pcinetwork.com), November 11, 2002.

could it be the main motor--in that it maybe has some accumulated junk (dust, etc) in it and that it needs to draw more power to keep it running (thereby causing possible burnout of ignition module)?

-- Jaspar Collins (coldinco@aol.com), December 15, 2002.

Hello every one. No one believes in using test lamps. You can get 1819 bulbs from Radio Shack and monitor any 24 volt section of the system. The indicator lamp is better because it puts a reasonable load on the controls and allows you to see what is happening. Meters are cute but you have to watch them, with a test lamp you can see the whole operation and put the crooks out of bussiness.

Scott Meenen N3SJH Laurel Md http://toad.net/~jsmeenen/gasform.html

-- Scott Meenen (jsmeenen@toad.net), December 31, 2002.

Having been a service tech for over a decede I can tell you that whoever did your work initially was a beginner. As for all of us being crooks, sorry to hear you think that. But I will say that armed with a good meter you can pretty much check out most functions of your furnace, but be careful there is voltage running through there and you don't want to meet it with your body.

Does sound like a bad limit switch or dirty filters/ bad airflow to me. I know I found this thread late, but just wanted to contribute.

-- Fred DeMeuse (opusd2@yahoo.com), December 14, 2003.

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