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portable ac heat

[email protected] | Posted in Mechanicals on

I live in a two-story home with a oil furnace in the old half and a pair of mitsubishi hyperheat minisplits in a newer addition. The two sides can operate largely independently of one another. I recently bought a Midea inverter AC with heat in the hopes of using this on the old side to reduce reliance on the furnace (and to provide cooling in that season). Have just recently begun using the unit and am finding, I think, that makeup air through the dual hose setup at temperature lower than 40 F keeps the unit from running at all, or at least causes it to take a considerable time to go to the heating mode. In zone 5, this significantly reduces its usefulness. 

This makes me wonder whether arranging to have the makeup air taken from the already 55 F interior air, with the exhaust still blown out through the outside, would be a better arrangement. I realize that the negative pressure created¬† would draw some outside air through the myriad ways that the old farmhouse portion offers for entry, but would it mean that I’d net no meaningful amount of warmth (or so little that we’d all be better off if I burned oil)? I don’t have any appliances or systems with pilots, so don’t think I’d be creating a safety concern. Help me understand why this wouldn’t work.

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  1. Expert Member
    BILL WICHERS | | #1

    I think what you are thinking is that using the indoor air to help heat the "outdoor" part of the heat pump unit will help. I put "outdoor" in quotes because it sounds like you have a packaged unit that uses ductwork to bring outside air into the unit, and then to take exhaust back out again.

    This won't really work, even though it might appear to. The reason why this is a problem is that heat pumps do not MAKE heat, they MOVE heat from one place to another (hence the name heat "pump"). They basically scavenge heat from outdoors, then concentrate it inside. Think about this like a large solar panel on a cloudy day being able to make enough electricity to make a small light bulb still run very brightly -- you are concentrating energy into a smaller space.

    Anyway, since the heat in your home was already "pumped" there by the heat pump, you can't then use it as input heat for the heat pump to pump into the house -- that heat is ALREADY in the house. It's just like using that bright light bulb to try to light up the solar panel to make more light in the light bulb. That's a form of "perpetual motion", which physics, unfortunately, says cannot happen.

    If your heat pump can't work below 40F, then you need one of the cold weather heat pumps. Some of these are "hyperheat" models. There are several manufacturers of such units, although I'm not sure if any are available as portable machines.


  2. maine_tyler | | #2

    Don't think of it as 'make-up' air because that's not what it is. The outdoor air is literally the heat source. One way or another, outdoor air* must pass over the coil.

    For you to be gaining net heat, the exhaust air must be colder than the outdoor air temperature. And if the exhaust air IS colder than outdoor air temp, then you are scavenging heat from the outdoor air-- just as you are in normal ducted operating mode-- but via random air infiltration instead of via the duct work. So it may technically be possible, but whether it is efficient, effective, or comfortable, I couldn't say.

    Do you have any specs on the heating capability of that unit? A quick look at it, and it seems to talk mostly about cooling... it may not heat well save for mild fall/spring days.

    *meaning the energy from the outdoor must be extracted and deposited inside the building envelope

    1. maine_tyler | | #3

      To clarify on "whether it is efficient, effective, or comfortable"

      You can only 'break even' (at best) in terms of heating efficiency with this setup vs normal ducted mode given the constraints of the outdoor air temperature being constant in both scenarios. Even if breaking even were possible, I suspect it may be less comfortable given the infiltration.

      Any EXTRA heat dumped out of the unit to the indoors in your exhaust only scenario VS the ducted scenario would be heat from your oil system. Think in terms of exhaust temp: unless the exhaust air was COLDER in your scenario than in the ducted, it is not more efficient. And how could it be colder? The interior handler may expel air that feels warmer, but the unit still has to pass the same mark after that extra energy is expelled that the ducted set up has to in regards to outdoor air temp (energy source).

  3. [email protected] | | #4

    I get it. Thanks for your clear and thoughtful replies.

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