Underfloor heating with water pipes.
When an air-water heat-pump (or ground-source heatpump for that matter) is used to heat a house, the ideal way to distribute the heat around the house is with wet underfloor-heating.
PEX hoses are used. This is a special oxygene impermeable polyethylene pipe. We use this in both the concrete foundation and between the floor joists under the floor for the upstairs. Scandinavian Homes has used this extensively in hundreds of houses in Ireland since 1991. We are very experienced in designing this. The layout is remarkably simple. Depending on the size of house we usually install 2-4 circuits in the concrete slab and 2-4 circuits between the joists to serve the upstairs.