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I've never tried one, but they seem uniquely suited to clothes dryers.

Use the cold side to dehumidify the air, then pass it over the warm side to heat it up.

Net-net, no extra heat in the room it's operating in (beyond the motor and such), and a nice self-contained unit that doesn't need external venting.

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You can also get heat pump water heaters, although they take heat from the room.

I've wondered about these - why don't they do the same split that HVAC pumps use? It seems really bizarre as a design. Components would also mean replacing the tank would be more cost effective and circuit routing would be easier.

Most modern clothes dryers work exactly like that.

A number of newer ones are ventless (although still very much the minority in North America), but only some use an actual heat pump to recycle the heat. IME most of the cheaper ventless dryers just reject the heat (but not the moisture) into the room they're operating in.

This is still a win because it doesn't depressurize the house (any air you vent out of the house is eventually replaced by outside air, sometimes via dirty areas like crawlspaces and attics). But the heat pump ones can run off of a low-power (~1500W) circuit whereas the condensing ones typically need a higher power (~5kW) circuit.

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