We live in a three story house, with our bedroom in the third story loft. On sunny days, it gets too warm up there, even with the windows open (not to mention the neighbor’s 4:00am rooster crowing). The bottom floor (ground level) is heated with a mini split, and also has our heat pump water heater in the laundry room (no door, but at the opposite end of the level from the heat pump). As a result, the laundry area is always very cool. What is the downside of putting in a small mini split with the indoor unit in our bedroom, and the outdoor condenser in the laundry area? I would then be using the warm 3rd story bedroom air to heat the laundry area/ heat pump water heater, and the cool laundry air to cool the bedroom. Thoughts?

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    My first inclination is to say "NO WAY"....however, if the noise from the outdoor unit can be abated, and the units drain can be set up, then I see no reason why not. I don't know of any codes it violates, so I hope the HVAC experts can give us the true answer.
    – RMDman
    Commented Jul 28, 2023 at 20:45
  • The outdoor unit is going to be quieter than the heat pump water heater that is already in there, if the ones I've used are anything to go by.
    – KMJ
    Commented Jul 28, 2023 at 20:50
  • My guess is that this would be a costly solution to a problem which could be dealt with by moving air within the house. how would you route the refrigerant lines? Outside down a wall and then into the ground floor would be unsightly. I believe that it would be better to rig ducting between the ground floor laundry room and the loft bedroom. Maybe one duct from ground floor to 2nd floor and another from 2nd floor to loft. Commented Jul 28, 2023 at 20:54
  • I would never do that, and you would fail a home inspection in most areas. Imagine the winter where everything reverses and now you are freezing the room that once needed heat. Commented Jul 28, 2023 at 21:29
  • A duct adapter for the intake and/or exhaust of the heat pump water heater would be a much smarter and less expensive way to get this done. Or just move the water heater upstairs and plumb a proper drain pan for it.
    – Ecnerwal
    Commented Jul 28, 2023 at 22:32

3 Answers 3


Big commercial multi-zone systems (like Mitsubishi City) actually do this sort of thing internally. They'll move heat around from one indoor unit to another as needed, and only go to the outdoors for heat that they can't get from somewhere else in the system or can't dump to somewhere else in the system.

I believe the Chiltrix units also do this sort of thing, by supporting using the waste heat you pull from rooms as a source of heat for hot water.

What you're proposing is basically a less flexible version of those systems. It won't work if you ever want to heat your third floor, rather than just cool it, since then you'll have all your devices pulling heat from the laundry room. You will also have to worry about condensate management at the outdoor unit if that ever happens. But apart from that, I'm thinking it should work. Something similar is pretty common in warehouses with offices inside of them. The office will have a mini-split installed with the outdoor unit inside the warehouse. It works just fine.

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    I wonder if the warehouse workers think so in the summer time.
    – crip659
    Commented Jul 28, 2023 at 20:53
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    Well 'works fine' and 'doesn't annoy people' aren't the same question.
    – KMJ
    Commented Jul 28, 2023 at 21:33

Before I spent that kind of money, I would get a duct booster fan and a bunch of cheap flexible duct to force the cool air from the utility room up to the bedroom. Temporary and ugly, but it might show that the cooling capacity of the water heater is insufficient.

An overall better idea is to put in a multi zone mini split system. Some are capable of closing a valve in the outside coils, thereby moving heat from one air exchanger coil inside to another.


Bad idea, when compared to the idea of putting the outside unit outside.

Because then, you have an all-season heat/cool system that doesn't affect other rooms, and is able to serve as a backup to the downstairs unit. Think winter and the downstairs unit conks out, you can still heat the house via the upstairs unit and leaving doors open and maybe a fan in the doorway. If you were heating the bedroom by stealing heat from downstairs, that would not work at all!

Maybe add a fan to stir the air in the downstairs area so the cooling effect of the HPWH is distributed throughout the 1st floor.

I agree with longneck, that the proper answer is a multi-zone heat pump equipped to move heat between zones. In fact one of those zones could be an air-water heat exchanger inside a hot water tank, i.e. the HPWH being one zone.

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