I have a 2 story house and I want to replace our old window AC. I was initially thinking of replacing it by a single heat pump unit, but in order to get government funding, I would need to get one heat pump per floor. I calculated that it would cost me around $1000 more to have the 2 units system and I was wondering if it would be worth it in the long run.

So, my question is : will cooling the house in summer and heating it in winter be more energy efficient with a 2 units system?

(currently, my house is heated by electric baseboard heaters and cooled by a 12 000 BTU window AC)

  • The largest factors are external to the systems you speak of. Energy lost (winter) or heat ingress (summer) is largely a function of home construction practices, materials used, and insulation. As such it’s impossible to quantify how much more efficient independent zones might be. As far as comfort, yes the two independent zones are beneficial. If you mean that total out of pocket by you increases $1000 but allows you to get govt money, then no I’m not certain you could justify that with “future energy savings”
    – Tyson
    Commented Jun 27, 2018 at 14:47
  • @Tyson I understand that the biggest factors are external, but they should stay constant between the 2 scenarios, so I was hoping that some comparison could be made. the 1000$ more would be the increase in cost after applying the govt funding.
    – Traceur
    Commented Jun 27, 2018 at 14:56
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    I don’t think you can save energy by having 2 systems. Your house still leaks the same amount, and still requires the same amount of heat to be added/removed. Can you increase the comfort level? Yes.
    – Tyson
    Commented Jun 27, 2018 at 15:10
  • @Tyson fair enough, I thought maybe the fact that each machine only does half the work instead of one doing all of it could have an impact
    – Traceur
    Commented Jun 27, 2018 at 15:18
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    The question is are the SEER numbers the same? In other words if the two units each have a higher SEER than the single unit, then yes that’s more energy efficient.
    – Tyson
    Commented Jun 27, 2018 at 15:22

1 Answer 1


Without the performance curves, it's impossible to say, but in general, equipment is more efficient when it's not operating near either the maximum or minimum load, so yes, it's possible that two units, each operating at 40-70% of their rated capacity, could be more efficient than a single unit operating at >90% of its capacity.

This link is actually about computer power supplies, but it does illustrate how many performance curves look. Realistically, this should serve to illustrate proper sizing so normal operation is near peak efficiency with peak performance matching maximum anticipated load.

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