When doing panel load calcs, the section on HVAC loads isn't quite obvious to me regarding multiple zones—hopefully one of you sparkies can clear things up. My house has two-zone HVAC—downstairs is A/C plus gas furnace (7500VA), upstairs is a heat pump (outdoor unit 3000VA, blower 500VA) with aux heat (4500VA). If each was alone, it'd be 7500 or 6425. Together, is it as simple as adding them up and calling the HVAC load 13925VA?

1 Answer 1


You're adding in parallel-series when you should be adding in series-parallel lol.

Put it this way, do you often run the heat upstairs while running the A/C downstairs? I would think not.

So you contemplate the house in heating mode/season. Figure a Load Calc with all the air conditiony stuff taken out (A/C no, heat pump yes, strip heat per the formula).

Then contemplate the house in cooling mode/season, this time with all the heaty stuff taken out. No auxiliary heat, however 98% of a furnace's electrical load does apply with A/C because you are using the furnace's air handler to push the A/C around the house.

  • I mean, I knew a person would use the higher load of cooling vs. heating, but the question was more along the lines of "do I just use the higher of System A or System B (whether both are cooling or heating) or do I add them together?"
    – Huesmann
    Commented Sep 26, 2023 at 12:23

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