If you are looking for a unit to do only slab heating, that makes the system less complicated than one that both heats and cools. If you go with a brazed heat exchanger and a floor slab, you should not need a big water tank. Especially if you are interested in a variable speed compressor unit. That's the big value in using one: no big tank. With a single speed outdoor unit that can dig into the cold temperatures without losing too much capacity, a heat store tank makes a lot more sense.
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