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Condenser and coil sizing for HVAC unit

Installing a HVAC system (Unico) , Calcs say I need 24,000BTUh for the house and 5500BTUh for latent cooling. For a total of 29,500BTUh.  I live in Mpls where humidity removal is a priority.

Mfgr has two air handler sizes (2.5T & 3.5T)  and allows the condenser to be sized +-0.5 ton for each (Ex: you can run the 2.5T unit with a 2T, 2.5T, or 3T condenser).

So I can go 3T Condenser and 2.5 T coil for 29,000BTUh or

3T Condenser and 3T coil for 31,000Btuh

The airflow for either unit can be adjusted to match the BTU and the required outlet CFM. But the larger unit has about 2X the fan size and hence 2X the amps.

I favor the smaller size becuase I've heard its better to undersize these systems. Plus I found a deal on a 2.5T airhandler (saves $600).

Looking for advise on performance and operating costs for the options.


  • meplumber
    meplumber Member Posts: 678
    I prefer to undersize too.

    I am in coastal Maine and we dehumidify more than cool, as well. I like to run a smaller condenser on a larger coil for that purpose. I adjust the air flow for the smaller condenser, but having a larger area for the coil will increase contact for the air.

    I like the 3ton air handler and the 2.5 ton condenser, but just my opinion.

    Good Luck.
  • Which Calculations?

    How was the heat gain on the structure calculated? Did you use an average load calculation or a peak load calculation? If the structure has AED (adequate exposure diversity) meaning windows on all (at least three) sides, you should be able to reduce the cooling requirements since the entire structure will not experience a peak load at all times.

    Having thrown that out there, I still always recommend that a "matched" set be used, although many manufacturers say that mismatches are okay, within reason. I would go with the 2.5/2.5.

  • John Mills_5
    John Mills_5 Member Posts: 935
    Watch real capacity

    The high velocity systems have ratings lower than you'd expect. They say with their higher latent capacity you can do with less total.

    For example, a Rheem 14AJM30 basic 2.5 ton which gets 14.50 to 16.00 SEER with a matching Rheem air handler ends up at 24,600 BTU and 11.15 SEER with a Unico 2430. The 14AJM36 with 3642 air handler is needed to get 30,000 BTU.

    see for matches with the outdoor unit & air handlers you are considering.

    We always use conventional duct systems even when adding cooling to an old house with baseboard electric or hot water systems. Sure high velocity tiny ducts is nice, but pricey and as you can see, the unit capable of up to 16 SEER only gets 11.15 with high velocity. JMHO
This discussion has been closed.