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Doesn't this suffer the problem that you're then putting heat into the device, which is in your house, during the warm season when you want your house to stay cool?

Think of the water tank as being in but insulated from the house.

In the hot summer the hot air within the house is cooled by heat pumping the heat into the water tank which is slowly over weeks bought up from cold to mean summer tempreture.

In the winter the heat is extracted from the water and transfered to the house.

The house and the water mass are out of phase by six months.

It's a large enough volume of water, that adding insulation is feasible.

But then why put it in the house and not e.g. in the ground?

1. Not possible everywhere 2. Probably more expensive

You don't even need water, you can just transfer the heat to the soil under your house

This is super cool