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  1. #1
    Join Date
    Feb 2005
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    I have a question on sizing multistage heatpumps.

    My house has been figured for 3.5 Tons. It seems that the multistage heatpumps (Carrier, Trane, Lennox at least) only come in whole sizes now. Given this is a 4 Ton unit be acceptable or would you advise on only using a 3.5 Ton single stage?

    The variable speed air handlers do come on the half sizes so would you mix a 3.5T AH with a 4T compressor (I suspect not) or would 4T AH and 4T 2 stage work for 3.5T with extra capacity for those really cold days?


  2. #2
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Four ton would be the choice,IF,your home really needs a 3.5 ton system.I'm sure you know the 3.5 is not too small,but likely wouldn't know if it's too large.

    Have a Manual J load calculation done,or do it yourself at the "bullseye" above.Consider adding attic insulation or tinting east/west glass to reduce the load to 3 tons,which will reduce the purchase price and operating costs.

    One concern when going from a 3.5 to a 4 ton ,is adequate size duct system,for the extra airflow.If it will be set at 350 cfm a ton ,this helps,but best to check the duct system before installing as it may be on the small side for 3.5 tons ,already.

  3. #3
    Join Date
    Aug 2002
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    Dash is the man!! Listen to him...
    Get a Load Calc.. Upgrade insulation..
    One other thing as mentioned get the ductwork properly checked!!
    I am so tired of customers calling me that had other companies install variable speed units on poor ductwork systems and now they complain of noise from the unit and ductwork.. A variable speed unit will try to ramp up as high as possible to meet the required cfm which can become noisy if the ductwork is not adequate.. All I get is these customers telling me that their old unit was the same size and it was very quiet compared to the new system their contractor put in... The problem being the old system never moved the proper cfm but the new variable speed unit is trying to move the required cfm..
    Have your ductwork checked!!
    Just look on this site,, almost every complaint for a variable speed unit leads to a ductwork problem..
    Good luck

    I love the extra work from these jobs but to deal with angry customers is not my idea of fun.. I would much rather see other contractors doing things properly to establish a better homeowner view of hvac contractors..

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