A heat pump in summer: why a cool floor does not always replace air conditioning

If a heat pump can produce chilled water, using the existing underfloor heating for summer cooling is a natural idea.

This can be part of a sound system. But the Cooling button does not turn every floor into a fully fledged air conditioning system. Cooling capacity, humidity and condensation protection must be checked.

Producing and delivering cooling are different tasks

A reversible heat pump supplies water at a specified temperature. Heat from the room must then be transferred to that water through the floor, walls, ceiling or a fan coil unit.

There are fan coil units specifically designed for use with heat pumps in both heating and cooling modes. These are separately selected terminal units, not a function of the outdoor unit. (Daikin — Altherma HPC FWXV-ATV3)

For surface cooling, the active area, finish, construction and temperatures all matter. If these limit heat transfer, a higher-capacity outdoor unit will not remove that limit.

The dew point matters more than the desired coolness

Condensation occurs when a surface cools to the dew point or below. Uponor specifically highlights the link between surface cooling, humidity control and ventilation air treatment. (Uponor — surface cooling in humid climates)

For air at +26 °C and 60% relative humidity, the calculated dew point is approximately +17,6 °C. At the same +26 °C but 70% humidity, it rises to around +20,1 °C. These values are rounded using the approximate Magnus formula for dew point.

A floor surface at +20 °C in the second scenario is therefore already close to the condensation threshold. A controlled safety margin is needed, determined during design with allowance for sensor accuracy and system response. A single fixed water temperature setpoint for the whole summer is not enough.

Water temperature and surface temperature are not the same either. The coldest unprotected components must be monitored, and chilled-water pipework must be suitably insulated.

Cooling without dehumidification may not provide the comfort required

A surface that must remain dry cannot be used like a conventional condensing dehumidifier. Excess moisture must be removed by another system designed for that purpose. Uponor considers surface cooling, ventilation and dehumidification as parts of an integrated solution. (Uponor — surface cooling in humid climates)

We would therefore not suggest removing all air conditioning units simply because the house already has hydronic underfloor heating. First, we assess the sensible and latent cooling loads separately.

Floor area can limit performance

For an illustrative room, assume an active cooling area of 70 m² and a design heat transfer rate of 30 W/m². The second value is for illustration only; in a design, it is determined by the construction and permissible operating conditions.

Q=70×30=2100 W=2,1 kW

If the sensible cooling load is 3,5 kW, the shortfall is 1,4 kW. Any dehumidification requirement is a separate task on top of this.

Combining floor cooling with fan coil units may make more sense than trying to make one circuit perform every function. The floor covers a justified share of the load, while the air-side heat exchangers handle the remainder and the necessary moisture removal.

Controls complete the design; they do not replace it

We would provide zone temperature and humidity monitoring, cooling limits and a clearly defined response to sensor failure. The level of analytics can be developed further, but basic condensation protection must not depend on a cloud-based forecast.

A heat pump can provide cooling in summer. Comfort comes from a correctly selected cooling distribution and humidity control system.