I had a new Lennox G61MPV-60C-110 installed a few months ago ( also a G61MPV-36C-090 at another location) The 090 works just fine. Both furnaces were spec'd by the heating contractor afetr doing all the measurments. However the 110 emits a high frequency noise that is transfered through the ductwork and drives my daughter nuts (extremely acute hearing). I can barely hear the noise, and it is not being produced with the 090 unit. The motor seems to be moving or trying to move a lot of air through the ducting and when it shuts down we get a siren like noise as it winds down. Again, no noise with the 090. I believe the furnace is at fault, Lennox and the dealer insist its the ducting....not enough air supply. If I open the cabinetry the noise is still present, so personally I am not convinced it is the ducting. I think they made a mistake on the recommended size of furnace since both houses are similar is size (100sq ft difference). Anyone have any suggestions?
Update: I just had another heating company come and estimate the job..their recommendations 80,000 BTU.
Opening the cabinet may not indicate that it is an equipment problem, for all that does is change where the blower is getting its inlet air from. If the duct work is too small, it is still to small. It is also possible that the fan was not set up properly. Did they do a duct static pressure check, air volume and velocity checks at the registers? It could be one or more registers causing the noise. As to the size of the unit, no one on this site can say without knowing all of the specs. I wont pretend to know how much homework you did before the purchase, but this is why I advise that the dealer/installer is at least as important as the type of equipment chosen. Keep pursuing the answer from the dealer and/or Lennox until they can prove with certainty that the problem is the duct system. Good luck...........Gasman