Heat pumps with underfloor heating
Underfloor heating has the largest emitter surface in the building, so it can heat a room with water only a few degrees warmer than the room itself. That low water temperature is exactly where a heat pump is most efficient.
Why low water temperature matters
A heat pump's COP depends on the temperature lift between the source (outdoor air) and the water it delivers. Every degree less on the flow temperature reduces the lift and raises the COP. The RV DC inverter series is rated at A7/W35, meaning outdoor air at 7 °C and water leaving at 35 °C, which is a typical underfloor design flow temperature. At higher water temperatures the same unit delivers less heat per kilowatt-hour of electricity.
Design flow and return temperature
The floor surface temperature, the room heat loss and the floor covering together set the flow temperature. A well-insulated building with tiled floors may need 30–35 °C at the design outdoor temperature; thick carpet or timber raises it. These figures come from the heat-loss calculation and the manufacturer's floor output tables, not from a rule of thumb.
ΔT and water flow
The difference between flow and return (ΔT) and the heat output fix the water flow:
Heat pumps usually work with a ΔT of around 5 K, smaller than a boiler system. The pipework, manifold and circulation pump must carry that flow, and the unit's minimum flow must be met at all times. Each RV model lists its nominal and minimum flow on its product page.
Manifold, circulation and hydraulic balancing
Each loop is fed from a manifold. Loops of different length have different resistance; without balancing, short loops take most of the water and long loops stay cold. Set the flow per loop at the manifold to the design value, then confirm the total flow at the heat pump.
Buffer tank and minimum system volume
Room thermostats that close individual loops reduce the open water volume. If the open volume falls below the heat pump's minimum system water volume, the compressor short-cycles and defrost has too little stored energy to draw on. A buffer tank or a permanently open circuit solves both. Size it from the minimum volume on the model's product page and the volume already in the open circuit.
Room thermostats and control
Underfloor systems respond slowly. Stable flow temperature, set by the controller from outdoor conditions where that function is used, often works better than frequent on/off switching of loops. Room thermostats then trim individual rooms.
Cooling through the floor
A reversible heat pump can circulate cool water through the same loops. Floor cooling capacity is modest, and the water temperature must stay above the dew point to avoid condensation on the floor, which needs dew-point or humidity control.
Related: Fan coils · Radiators · Weather compensation · Compare RV models
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