Sizing HVAC: 3 vs 2.5 ton AC, 60k vs 40k BTU furnace for 1200sqft

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I'm adding a HVAC system to my single-storey SFH (1200sqft in climate zone 2, San Jose, CA) and the contractor picked out the following system:

  • Furnace: 60k BTU @ 80% Efficiency
  • AC: 3 Ton @ 14 SEER

Got me wondering if it's overkill for 1200sqft? General sqft-to-BTU load calculators indicate I would be enough served by going a bit smaller. Let's say insulation is medium in terms of quality and installation. Would it then be better to go for a size down?

  • Furnace: 40k BTU @ 96% efficiency
  • AC: 2.5 Ton @ 16 SEER

Edit: Did a Manual J calc from loadcalc.net myself. Input R11 insulation in ceiling, and walls, floor tile no insulation, stucco, and double pane windows. Based on the wall dimensions and directions, it came back with only 20k and 17k BTUs for heating and cooling! Not sure if I missed something big here, any advice or is this pretty realistic?

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Best Answer

What was the previous size of the AC? It's typically encoded in the model number, and there are online decoders for your manufacturer.

The size of your AC is based on the heat leakage for your home; but, many AC contractors believe that adding an extra 1/2 ton doesn't hurt. It is generally a safe practice if it is only done once; but, if it's already been done, over cooling a home can have some pretty nasty consequences, Humidity build up (like the water on the outside of a cold soda can) can collect within the home due to air temperature being dropped to rapidly. This typically occurs behind the sheet rock, where it won't be immediately visible. In addition, your unit will run for very short periods as it cycles, likely leading leading to more wear on the installed system.

In any case, the home hasn't moved, the insulation hasn't changed, and unless you had a bad system before, the new one shouldn't require a new tonnage rating unless it was mis-rated previously, or you intend to maintain the home at a lower temperature than before.

Should you believe the home was mis-rated, pay a company to have a "Manual J Load Calculation" done on the home. If it is possible to have this done without installing an AC at the same time, you'll have a better confidence it isn't influenced by the impending sell.

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