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11 years sounds premature for a heat exchanger, so that makes me wonder about airlfow, which would lead me to believe the air distibution system needs to be checked.
 
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It is not always the case but many times when a HE goes out early it is from lack of air flow. The lack of air flow can come form many places, dirty filters, dirty evap, improper setup when the unit was installed, etc. Since you mentioned increasing some part of the ductwork I have to assume there is some issue there. However you must understand that you could double the size of duct but if there are not enough branch runs, registers, if they are to small, not enough return, etc, you will still have a low airflow issue which could take out the new HE prematurely. The new furnace may have a stronger blower than the old furnace which could overcome the resistance of the duct, but not n=knowing your furnace, duct, or house I have no way ao knowing.

If the furnace had been on LP before that could account for the failure but if it was good when it was put in it should not have deteriorated very fast after that assuming everything else was ok ie properly fired with adiquate air flow.

I hope that clarifies this for you.
 
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Yes he should be able to measure static pressure at the unit and get the total air flow {CFM} from the blower data sheet. If that is not correct, or if the temp rise through the furnace is to high then things need to be adjusted. If the static is to high then all the duct details I talked about earllier come in to play and need to be tracked down to see where the restriction(s) are at.
 
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