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tomgoe, HVAC Technician
Category: HVAC
Satisfied Customers: 1578
Experience:  32 years servicing HVAC equipment
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I have a 40 yr old International gas furnace in a mobile home.

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I have a 40 yr old International gas furnace in a mobile home. When it comes on the air is only warm. After running a while the air gets cool. What should I look for?

Please do the following


Turn your unit off.

Wait one minute.

Turn the unit back on.

Watch the operation.

Normally you would see the following:

  1. The induced fan comes on (if you have one).
  2. Then you see the igniter spark, or get hot depending on which you have.
  3. After that, you will see the flame ignite.
  4. Once ignition has been accomplished, the unit will heat up, and the indoor fan will come on.


Please watch your unit and tell me where your unit deviates from the description I have given.

If you would like to approach this another way I can do that too.
Customer: replied 7 years ago.
It appears to go all the way through the cycle you mentioned and then comes back on with cool air output
Can you see the flame ignite and stay on?
Customer: replied 7 years ago.
Do you feel a good amount of airflow coming out of the discharge grills?
Customer: replied 7 years ago.
<p>the flame stays on but the air gets cooler each time it comes back on.</p><p> </p><p>yes on the amount of air coming out of the grills. Normal</p>
Tell me this. Does your unit stay on untill it meets setpoint, or does it shut off before it meets your setpoint.
Customer: replied 7 years ago.
it stays on
The unit ignites. The fan comes on. The unit stays on untill it meets the setpoint.... I think I am going to opt out and let someone step in to help you. I am out of ideas.
Customer: replied 7 years ago.
Thanks very much for the help anyway
Your very welcome. I sent an email to someone to get them to take over.


What i have seen on INTERTHERM furnaces that are of the age you have is at the top of the heat exchanger ( your heat exchanger is made like a barrell ) it will get cracks - sometimes quite large and when the blower runs air is getting down into the burner compartment and the burner is on but not heating the exchanger. Luckily on mobile home furnaces this is a sealed combustion furnace and so you are not getting carbon monoxide in to your home but 40 years is way longer than these heat exchanger were meant to last. To check this you need to take the outer pipe off the flue and the top of the furnace off and look down on to the top of the heat exchanger and feel around for cracks . I've seen some that I could stick my fingers in. IF that is the case - no fix- replace the furnace.

tomgoe, HVAC Technician
Category: HVAC
Satisfied Customers: 1578
Experience: 32 years servicing HVAC equipment
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Customer: replied 7 years ago.
This is something to look for. I have been looking for cracks ,but so far have not found any. I even put my camera in the furnace ant took pictures without any luck. Thanks for the info and I will check it out.Tell Kenton thanks too.