Bigger motor needed for A/C ?
Results 1 to 7 of 7
  1. #1
    Join Date
    Jul 2005
    I was looking for a little advice. I have a Lennox furnace and I added central air a few years ago. The house is older (ductwork etc). The furnace is probably 10-15 years old, but still works great. I got several bids on the AC and all rated the house needed 2 tons, but 2.5 to be safe. That is what they installed. It blows very good cold air, but just not enough of it. I did the tempature check, and it is over 30 degrees cooler coming out than going in. I am wondering if the cold air is heavier and the motor just doesn't push the air as well?? I set my thermostat at 78, and it runs pretty much all day and almost all night in 95 degree weather. It will bring it down to 80ish, but almost never below that. I guess I may have some duct leaks, although I don't think they are significant (is there a way to check for them?).

  2. #2
    Join Date
    Apr 2004
    A 30 degree temp. diff. is awful high. It should be around 18 to 20 degrees. When the air is being cooled down that much, it is usually due to lack of air flow. Could be a dirty filter, blower wheel, evaporator coil, or it could be that the duct work is not sized properly for the amount of cooling you have. Does the evaporator freeze over with ice? What is the humidity level in the house? Remember, when you had the furnace installed there was no A/C, so it may have been sized to the furnace without regards to adding air conditioning. The furnace requires less air flow than the air conditioner.
    A Veteran is a person, who at some point in their life, wrote a blank check payable to the United States of America for payment up to and including their life.
    Gene Castagnetti-Director of the National Memorial Cemetery of the Pacific in Hawaii

  3. #3
    Join Date
    Jul 2005
    Difference was actually only 26. Filter is new. Tried to clean up the wheel with the last filter. Coil is very clean. I've never seen it freeze over, and have checked it numerous times. Humidity is quite low (I'm in Utah). I appreciate the thoughts. I could be a duct work problem. I did ask about duct work when both bidders came in, and they said it would be fine. One bid was the local Lennox dealer/rep. He spent an hour on windows, insulation, heat loss, mothers name, and everything else. Any way to increase the air flow or could it possibly be a motor/size problem. The unit is in a somewhat restricted space but not significantly. Thanks again for any ideas.

  4. #4
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Your duct system may be too small to have added air conditioning.

    The fan delivers a certain amount of air ,based on the speed selected and "static"(resistance) of the duct system and coil you have.

    Without the A/C coil the resistance was a lot less ,so it could move more air.

    30 difference is too great a temp. drop,indecates low air flow,which lowers the capacity of the system.

    So, call around ,find a company that says the can check the "static"of the coil and duct system,and hopefully has the "fan specs' on the old furnace to compare.Be prepared to have the duct system redesigned and redone to allow proper air flow.

  5. #5
    Join Date
    Jul 2004
    Kent, WA
    Couldn't he just crank up the blower speed, either by picking a different speed jumper setting or increasing the motor pully diameter? It is possible that the fan is going as fast as it can, in which case you either need a bigger fan or less duct restriction.

    If you suck in the filter and blow it throughout the house, then you know you've gone too far with the speed setting.

  6. #6
    usually when systems are upsized the return air is left and not increased. higher splits means low air flow. also is your furnace a 3 ton drive if not its to small. if evap coil was not replaced its to small and if you have any of these problems youll be replacing compressor frequently.

  7. #7
    Join Date
    Mar 2005
    burlington county n.j.
    model # of furnace will give us blower capacity. no belts or pulleys on lennox in a loooooong time.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event