To me the furnace is a bit undersized to begin with.
First thing first,
If blower motor has a speed tap, it should run slower than when in
cooling mode.(warm air is light vs. cold air) Does cooling mode work OK
in summer? Had a look at evaporator coil? Is it clean?
What exactly are you telling the techs? If you are telling
them the air coming out of your vents is too cool, they might be
thinking that it is doing the job and tell you nothing is wrong.
Your problem, as I understand it is that you can't keep your
home as warm as you would like. Tell that to the techs. Don't over
think it, that is the job for the techs.
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