Underfloor Heating: Thermal Mass Myth

A common saying in Ireland is that, if you don’t like the weather all you have to do is wait five minutes for it to change.

Underfloor heating in Ireland needs to be designed with this maxim in mind. With such variable weather, your heating system needs to be able to respond quickly, supplying heat when needed, then turning off rapidly as the temperature increases. Greentherm underfloor heating systems are designed first and foremost to be controllable. A dedicated thermal store and a lighteweight floor screed gives a fast response to changing conditions, ensuring optimal comfort and efficiency.

Some installers like to replace the dedicated thermal store with a thick slab of concrete or heavy screed, especially when combined with a heat pump system. They justify this as ‘thermal mass’ doing the same job as a thermal store, saving on the cost of a large cylinder

The effect of ‘thermal mass’ is to make the effective control of the heating system almost impossible.

A system with too much thermal mass will respond far too sluggishly to changes in temperature or demands from heat. The underfloor heating systems will only begin to supply heat to the room once the floor slab surface has exceeded room temperature. In some cases, with heavy concrete floors, this can take hours.

After the thermostatic valves have closed, heat will continue to permeate through the slab, causing the room temperature to exceed the thermostat setting until the air temperature aligns with the heat retained in the slab. Considering the significant heat stored in the slab, it may take several hours for the room to cool down unless ventilation is provided by opening a window.

This graph illustrates the effect. The low thermal mass system in blue has already reached a comfortable temperature, before the high mass system in red begins to respond. The high mass system also overshoots the set ‘comfortable’ temperature, continuing to radiate heat even after it is no longer required.

The analogy can be likened to attempting to push a heavy train without brakes. Initiating its slow movement requires considerable effort, and once it’s in motion, it becomes equally challenging to bring to a halt. As a result, the train overshoots its intended stopping point and needs to be pulled back. Similarly, the system’s response to temperature changes is sluggish, taking a considerable amount of time to stabilise afterwards.

Comparing to this, controlling a Greentherm underfloor heating system is like pushing a lightweight car. It take little effort to get it moving, and once your stop pushing it, it stops all on its own. The thermal store acts as a heat ‘battery’, charging up and then releasing energy exactly when and where it is needed.

Your own comfort is at your fingertips

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