I replaced the original forced air oil furnace (circa 1985) with the new gas furnace about a year ago and it simply isn't moving enough air to heat my 2-story 2400 sq ft house. Additionally, a part (forgive me, I don't know exactly which one) keeps failing and the company keeps replacing it (they say it's over heating). The company that installed the furnace can't figure out why it's not blowing very much air. They called in another HVAC company who suggested that the return ductwork was not large enough and they installed an additional air return. I'm sorry to say that didn't solve the problem. The air output is still minimal on the ground floor and barely exists on the second floor. Does anyone have any idea why this is? Is it a wrong size furnace? Is it a function of faulty ductwork? Both HVAC companies say that the ductwork is hooked up properly; although I can see that some of the round runs are held together with duct tape and not silver metal tape. Should this matter? I did have some foundation work done (helical piers installed) and they had to move one duct just prior to the installation of the new furnace. Would this have an effect? Would it matter if I have a circa 1985 leaky house and ductwork? I desparately need help because the local guys can't figure this out.

Figure out what part keeps failing? Motor/blower?

Was a manual D calculation done on the duct system?

Was a manual J calculation done to size the unit?

Was the static pressure measured after installation?

Was the CFM set properly?