Furnace shuts off

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Old 12-29-04, 12:11 PM
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Talking Furnace shuts off

I have an intermittent problem on my gas furnace
. Its a Coleman DES 80 - HSI
. 10 years old - never had problem before
Description of problem: When the stat calls for heat:
- the flue vent comes on for 5 secs, stays on for 50 secs
after which the ignitor will begin to glow
- at the end of that period the gas valve opens and an orange flame comes on, turns to blue with an orange portion below the deflector.
- after about one minute the furnace blower comes on. Up to there: no problem.
The problem: When the furnace blower comes on, some times the flame shuts off, the vent blower stays on but the furnace blower turns off,as well as a faint smel of natural gaz.
The electronic filters are clean, the floor vents and return grilles are opened and unobstructed. The furnace is located in a large area in a heated basement. I have a 3 tons Colemam T.H.E. 11 HEAT PUMP 3736, which is turned off most of the winter, however it defrosts and blows cold air 50F (9C). The defrost is "as needed", but there is a fixed cycle of 9 to 12 mins. Since this "problem" seems to hapen when erything seems to work ok, cud it be that the cold prevents the vent blower in doing its job.
Must tell you that when I wanted to have the whole unit cleaned I was told that its a tight closed furnace and since its gaz, it very seldom needs to be cleaned??
Appreciate any suggestion, since most of the time this problem comes on during the night; have to shut down the "off" switch on the stat, wait a minute or so, and turn it on again=works ok until next time...
Thanks in advance P.A.T.
 
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Old 12-29-04, 09:11 PM
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I htink it would be best to get a pro out and take a look at your system..

Your heat pump should work well in the winter months, unless the temp drops below 32.
 
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Old 12-30-04, 07:49 AM
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Talking Furnace shuts off

Jay11j

Tks for fast response. However my problem is "Furnace shuts off".

The reference to the Heat Pump was in case is could have an incidence of the problem.

The original quote from my post is: "The problem: When the furnace blower comes on, some times the flame shuts off, the vent blower stays on but the furnace blower turns off,as well as a faint smell of natural gaz".

Since its very hard to get a competent HVAC tech in these parts of the woods, would appreciate to get a hint where he could start looking. Incidently I was in the electronic technical field for 35 years and can handle the testing of different components. Familiar with VOM, Ampprobe, etc. But wont meddle with gaz, not my bag.

This problem can wait until after NewYear (have to get up at nights to put the on/off stat switch to get heat - dont want to pay double and triple rates. Tks in advance.
P.A.T. (Senior)
 
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Old 12-30-04, 08:01 AM
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The problem: When the furnace blower comes on, some times the flame shuts off, the vent blower stays on but the furnace blower turns off,as well as a faint smell of natural gaz
Now that I am more awake now, it sounds like you may have a crack in the heat exchanger.

Reason I say this is.. The burners are running, pressure in the heat exchanger is ok.. Now the main blower kicks in, with a crack, the pressure changes in the heat exchanger, and the pressure safety switch is dropping out and cause the flame to go out. Now the flame is out, there is no heat to keep the main blower on.

Take a volt meter to see if the 24 volts is showing up on the pressure switch, if yes, then check the heat exchanger for cracks.

Do you have a CO detector in the house?
 
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Old 12-30-04, 08:28 AM
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Watch the flame as the blower kicks on and see if they get pulled up off the burners or move around alot.
 
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Old 12-30-04, 06:58 PM
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Talking Furnace shuts off

Jay11j

Thanks for quick reply.

Thanks for your response, you go along what I hasard: something to do with the air pressure, but didnt expect a crack heat exchanger...

There are two switches side by side, one marked "L" and the other "F".

I disconnected the AC supply to pry the connectors out then put the AC back:
On the F switch there is 125 Volts (using an old VOM not the Fluke) across the two wires. One wire goes to a Normallly closed contact (NC) located in an enclosed box with the other relays and transformer. The other side of the contact goes to one of the several windings of the furnace blower. The second wire is brown and goes to the primary winding of the transformer marked 115 V.

On the L switch an orange wire is connected to a Fasco Vent Motor type U21B, then goes through a pair of contacts called C.S. and eventualy to the Heat Relay contact (N.O.) and to the secondary (24V) side of the transformer. The second orange wire (smaller) comes from the SW to the log connector "TH" of the Fenwall Automatic Ignitor System. There is no voltage when the connectors are disconnected, due to the N.O. contacts I guess.

Hope this helps. If not cracked, the limit switch comes on for some reason.
By the way thanks to Matteson: yes we do have a C.O. detector, I smell a faint smell of natural gaz when the gaz is shut off (believe 5 to 6 seconds). BTW there are no burners, I mean round rods with holes, but rather a funnel type with a deflector at the end. (HSI)

Thanks again for your input

P.A.T. (Senior)
 
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Old 12-30-04, 08:41 PM
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I am lost here......... Are you able to take a pic or scan the system's wiring drawing??
 
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Old 12-31-04, 08:34 AM
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What sense flame in this furnace ? It should have a flame sensor on the oposite side of the pilot. It's a metal rod about the size of a q-tip, have you tried cleaning it off?
 
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Old 12-31-04, 11:59 AM
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Furnace shuts off

Mattison
My system is a HSI with single funnel type burner with a round deflector at the end. Underneath, in the flame there is what seems like a metal thing which touches the flame all the time,is that it? If so, how do I get at it?
Tks for your patience
 
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