Furnace Blowing Cold Air

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Old 01-06-10, 07:01 AM
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Unhappy Furnace Blowing Cold Air

First I would like to say a few things I know about my furnace.

1. Coleman Evcon Furnace/AC unit (10 years old)
2. It has Solid State ignition (no pilot)
3. The thermocouple is not burned/corroded
4. It starts up fine - I can see the igniter heat up and a flame start and it burns solid while the heater is on.

From what I know about heaters everything seems to be working fine.

except: The air is cold coming out of the vents.

I thought well if everything is working fine the problem MUST be in the duct work (some sort of exposure underneath the mobile home). So I crawled under the mobile home and found what I thought was the problem. A Huge hole torn in the insulation from raccoons. I re insulated that hole and patched it up nice and tight hoping my house would soon be warm again. I was wrong. This only made the house about 2 degrees warmer.


Is there anything I'm missing? I do not know how big the flame in the furnace should be, so I'm not sure if maybe its not big enough to heat the air adequately or what.

Any help would be greatly appreciated.
 
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Old 01-06-10, 08:32 AM
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I take it you have a hot surface ignition system that has an ignitor that glows white hot to light the gas.

It sounds like the burners light. How long do the burners stay lit? If that shut off does the fan that circulates the air around the house continue to run until the burners light again?

Is the fan motor starting and operating?


This sounds like the furnace is shutting off on the high temperature limit switch, which shuts off the burners when the furnace overheats but continues to run the fan to strip heat away from the furnace and send it through the warm air vents into the house.


The most common reason for the limit switch to open like this is plugged furnace filters, return air grill(s) covered over, an air conditioning evaporator coil on the furnace plugged with dust and dirt.
 
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Old 01-06-10, 08:44 AM
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I also thought that perhaps the burners were shutting off, but that is not the case.

I can see the burners lit constantly, they do not shut off while the heater is on. I have sat and stared at them for 10+ minutes to make sure of this.

---------------

unless it reduces the flame without actually turning it off, however i do not think they do that.

----------------
Also, How can i clean the "conditioning evaporator coil"?

Thanks for your response!
 
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Old 01-06-10, 08:59 AM
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Ten minutes might not be enough. Twenty ought to be enough.


If the burners remain on continuously either the limit switch is bad or the heat is going somewhere.

Is the warm air plenum that collects the warm air from the furnace notably hot or just warm? I'd drill or punch a hole in that sheet metal and insert a thermometer to measure the air temperature the furnace is producing --- it can be covered over with a piece of duct tape.

If the warm air plenum temperature was notably hot, I'd test the high limit switch to see if it is functioning properly, but that involves methods I wouldn't suggest to a DIYer.
 
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Old 01-06-10, 10:27 AM
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Actually on the bottom (since furnaces are basically upside down in trailers - as the vents go through the floor not the ceilings) that tin plate (plenum (had to look this up)) is almost cold to the touch. There was already some metal tape covering a pipe going in this hole from something else. it was enough space to slide a meat (10 inch) thermometer into. this was only reading about 75 degrees.

now im no scientist but im sure the heat coming out of the vents in a house should be warmer than that. and when the thermostat reaches a certain tempurature it shuts off? am i correct?
 
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Old 01-06-10, 11:07 AM
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Were you measuring that with the fan on? If so, it's certainly much too low.

Now I'm suspicious that the burner input is too low. Is this natural gas or propane fired? Do you have other gas appliances? If so what are they and have they been operating normally?

Do you associate this problem with some notably cold weather for your area? Where are you located?
 
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Old 01-06-10, 11:59 AM
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Chilly Iowa (10 degrees here right now)

I actually called a local HVAC guy and you 2 I think are on the right track. He said the input for the gas was probably not enough and I should check the brass ring nipple hole on the gas output. When i get home i will take off the tip to see if any cobwebs have formed in it. If this is the case I will love you both forever for saving me 1-200 dollars!

You will surely hear from me later.
 
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Old 01-06-10, 04:49 PM
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A problem I have found on Coleman Mobile home furnaces is an insect will get in behind the burner orfice and build a cocoon/nest.
Remove the gas valve and you will see a brass orfice on the end of the nipple that inserts into the burner. Remove this orfice and check the nipple for a cocoon. If it is clear you will see a screen if not dig that sucker out.
 
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Old 01-06-10, 05:55 PM
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So I take it you can't hear the corn grow today?
 
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Old 01-07-10, 09:12 AM
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OK!

so I opened up that nipple and sure enough there was a spider nest in there which was restricting gas flow, making for a very small flame. After I removed hit you bet your butt there was an enormous (normal) flame.

Now I have another problem. - the flame wont stay on.

The igniter gets hot - the fire lights - big flame.
I can hear the fan slowly turning (centrifugal switch im guessing)
Then the fan really kicks in and the flame goes out
THEN
the ignition cycle starts again, and sometimes it stays on, and sometimes it goes out.
it stayed on for about 4 hours last night, and we had a nice warm house, but this morning it wouldnt stay on.
 
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