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rhamann387 02-07-2006 11:02 PM

Furnace with gremlins.....
We have a Heil furnace in our house. For the most part it has been a good furnace. The last month it has been driving me nuts. The thermostat will call for heat...the exhaust/combustion blower will come on....the gas valve will click...the ignitor will fire....and the main burner lights. So far, so good. It heats up and then the main blower comes on. It will run for approx. seven minutes. Then the main burner goes out, and then the exhaust/combustion blower will shut off. After about 30-45 seconds the exhaust/combustion blower comes back on....the gas valve clicks...the ignitor fires...and the main burner lights up again. The main blower is still running the whole time.

It does this over, and over until the thermostat quits calling for heat...then it runs through the normal cycle and shuts off. I have put a new ignitor/flame sensor on it, plus a new control box, and have traced wires looking for loose connections, or a bad ground. All the sensors seem to be working from what I can tell. I cleaned the drain part for the exhaust to make sure it was draining okay. I can't afford to replace the furnace, as my wife lost her job at the first of the year, but I can't keep throwing money at it either. I need some direction as to which way to go.

If anybody has heard of this, or has any suggestions...I would be very grateful.

allan 02-09-2006 07:32 AM

Furnace cleaning
It sounds like your problen lies in furnace cleaning .
First make sure that furnace is clean and the furnace filter is clean
Then remove the blower from the furnace.
Crawl in side the furnace and look at the heat exchanger.
It will look much like a rad in a car get a brush and vacum the whole coil
Clean the blower blades.
After that replace the blower and turn it on.
Make sure that your blower is running properly.
Also make sure that you do not have return airs blocked and that all dampers are fully opened.

Here is what is happening.
You are not getting enough air through the furnace.
The furnace is lockng out on high limit.
That is why the furnace blower continues run.
When it shuts off next time
Get an ohm meter and test for continuty across the limit swith.
you should find that it is open.
when the limit opens the blowert motor continues to run to cool off the furnace,

Let me know how you made out

Allan Thorne
Clarkson Comfort Zone Heating and Air conditioning
Mississauga Ontario

Aceinstaller 02-11-2006 04:57 AM

allan summed it up well.:)

after his suggestions, if the problem still persists, have a contractor take a look at your sheetmetal. This is a common problem due to furnace oversizing. YES you heard me right furnaces can be OVERSIZED. Or the sheetmetal can be undersized.

If the furnace is oversized, then there wont be enough air supplied through the sheetmetal to efficently cool the heat echanger to its range of operation, therefore a safety fan limit control will tell the gas valve to shut down. when the heat echanger reaches a cooler temp. then the fan limit will tell the valve to turn back on.

If you are not planning any additions to hour living space soon, and would like to save money on your high gas bills(due to very large burners firing and purging all of the time), I would have the furnace properly sized. Also, Air conditioning will become a problem on an improperly sized furnace, because an air conditioner sized to the furnace will freeze up(for the same reason the furnace gets too hot during heating cycles). And a properly sized air conditioner will allow even less air to pass over the heat ecanger during the heating system.(making matters worse.)

If the sheetmetal is undersized, it will pose the exact sme problems. That is why I suggest a local Contractor should take a look at it.:cool:

oddjob 02-14-2006 12:36 AM

Make sure you haven't blocked any supplys or returns.

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