Floor cooling and condensation
Floor cooling runs chilled water through the same pipes that heat the house in winter. The floor absorbs heat from the room, quietly and with no equipment in the rooms. The limit is condensation, and you have seen it: a cold glass of water on a summer day sweats, because the air touching the glass is chilled until it can no longer hold all its water vapour. A floor that is cold enough does the same and gets wet. How cold is “enough” depends on the room air, and that threshold temperature is the dew point.
How floor cooling works
A reversible heat pump, one that can run backwards and move heat out of the house instead of into it, chills the water in the floor circuits. The cool floor surface absorbs heat from the room by radiation and by contact with the air. Because the surface is large, the water does not need to be very cold. That is why floor cooling reaches a better EER than fan coils. EER, the energy efficiency ratio, is the heat removed per unit of electricity, the cooling counterpart of COP. A fan coil is a small room unit with a fan that blows air over a cold coil, and it needs colder water.
The wizard offers floor cooling only with underfloor heating, because the cooling runs through the same floor circuit, and only with a heat pump. An air-source unit runs in reverse. A ground-source system cools passively: the compressor stays off and the floor water gives its heat to the buried loop through a heat exchanger. There are two variants: floor cooling alone, and floor cooling with air conditioning as a backup for hours the floor cannot cover.
Floor cooling is gentle by nature. The difference between the floor surface and the room air is small, so it removes heat slowly compared with an air conditioner. The model uses the same pipe length and maximum water flow for cooling as for heating, and the output is further limited by the water temperature and the dew point.
The supply temperature
In the model, the chilled water is supplied at 18 °C. That is the supply temperature: the temperature of the water the heat pump sends into the floor. It is not a setting in the wizard. EN 14511, the standard for rating heat pumps and air conditioners, and the European ecodesign rules rate machines for cooling floors with water leaving at 18 °C, and the efficiency data the model uses is for that condition. A colder supply would give more cooling power, but it would bring the floor closer to the dew point and cost efficiency.
Why the dew point matters
Any surface colder than the dew point of the air around it gets wet. The dew point rises with humidity, meaning the amount of water vapour in the air. As an illustration with the Magnus approximation, a standard formula for dew point, air at 24 °C and 55 percent relative humidity (the share of the vapour that air at that temperature could hold) has a dew point of about 14 °C. A floor surface kept above that stays dry. On a humid day the dew point climbs by several degrees and the safe margin shrinks.
Condensation on a floor is a practical problem. It makes the surface slippery, can damage wooden floors and encourages mould. Real installations protect against it with a dew-point sensor or calculation that raises the supply temperature or closes the cooling circuit.
How the model guards against condensation
The floor circuits use the condensation control built into the radiant floor model of EnergyPlus, the building simulation program the model runs on. The model advances in steps of 15 minutes, or 5 minutes with a ground-source heat pump. At every step EnergyPlus compares the floor surface temperature with the dew point of the air in that zone (one level of the house) plus a 1 K margin, a safety gap of one degree. If the surface would drop below that value, the floor circuit is shut off for that step. We call this the dew-point guard. The margin is fixed and is not a wizard setting.
The dew point comes from the indoor humidity that EnergyPlus calculates for each zone. That humidity depends on outdoor air entering through ventilation and infiltration (air leaking in through gaps in the building) and on the people in the house.
When the guard shuts the floor off, the room warms until the floor surface or the dew point changes and the circuit can run again. With air conditioning as backup, the air conditioning takes the load in the meantime. With floor cooling alone, indoor temperatures can rise above the cooling setpoint, the highest indoor temperature you asked for. The results raise an overheating alert when the indoor peak passes 28 °C, and the alert suggests shading or an air conditioning backup.
Limits of the model
- The model switches the floor off. Many real controllers instead raise the supply temperature or close a valve. The two behaviours give different comfort and efficiency results on humid days.
- Cooking, showers and drying laundry are not represented as moisture sources, and no dehumidifier is modelled in the living spaces. An indoor pool is a zone of its own with its own humidity control.
- The 1 K margin and the 18 °C supply are fixed.
- The results do not report how many hours the guard was active.
- A thermal simulation does not establish that a floor cooling installation is safe from condensation. The installer needs a dew-point sensor or equivalent control, and that decision belongs with them.