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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  • Have you had your contractor run a Manual J calculation for your house? Aug 21 '20 at 2:32
  • I have not, are the various online calculators not accurate enough?
    – Shubham
    Aug 21 '20 at 2:49
  • There are a few things online that claim to do Manual J calculations, but I can't personally vouch for them. (Note that anything that claims to be "plug square feet in, get BTUs out" is rule-of-thumb, not Manual J, and thus should be avoided) Aug 21 '20 at 2:52

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.

  • I found a manual J calculator, based on the specs on my wall insulation, flooring, windows, N/E/S/W directions, it tells me I only need 20k in cooling, and 17k in heating. Which would mean I'm going way overboard. Would you advise these are more realistic numbers or something is very amiss here? (added a screenshot in the question above)
    – Shubham
    Aug 21 '20 at 3:36
  • @Shubham I haven't done a Manual J Calculation, my experience comes from replacing dozens of air conditioners as the husband of a home flipper, the brother-in-law of a builder, personally maintaining air conditioners in rental properties, and having a brother with 15 years in HVAC. I'd love to tell you that your numbers are right or wrong; but, you'd be much better off getting a licensed technician or a reputable shop involved.
    – Edwin Buck
    Aug 21 '20 at 3:52
  • I really wish this "short cycle wear and tear" nonsense would die. Wear happens when things move, so the less you move the pistons/vanes in the heat pump, the less it will wear, period. There's a fixed amount of "startup wear" each time a pump starts moving, because usually the lubrication is sub-par when the pump elements start moving from standstill. So, if the number of cycles is the same, then the shorter length of the cycles is in fact beneficial to the longevity as long as the cycles are long enough to properly distribute the lubricant inside the pump. Aug 21 '20 at 18:25
  • That's an interesting thought. What I read very often is that although an oversized AC will cool to your desired temp more quickly, it will cycle on and off more frequently. Here's what I have to wonder....isn't that second part only true of the cool is escaping quickly from poor insulation? if that's the case, then a proper sized AC will then anyway have a hard time reaching your desired temp. Which makes "short cycle wear and tear" kind of a fallacy because it's a symptom a of an issue that has nothing to do with ac size, right?
    – Shubham
    Aug 21 '20 at 20:30
  • @ReinstateMonica Maybe the short-cycle wear is suspect, but it's still not a good idea to have a mis-sized AC, for the other reasons stated above.
    – Edwin Buck
    Aug 22 '20 at 18:00

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