Underfloor Heating Installation

Maximum hourly losses or thermal load of wet underfloor heating - these should be calculated, just as you would for any other heating system, using preferably a proprietary software package or data taken from SAP rating our other reliable source. For an underfloor heating system the criteria is slightly different and this can result I lower heat losses than would be calculated for a radiator system.

Floor Heat Emissions

As more than 50% of the floor heat emitted is radiant transfer this heats bodies, objects and the building fabric without heating the air. This feature of underfloor heating increases the rooms mean radiant temperature (MRT) which means a lower room temperature can be used without loss of comfort. The same comfort will be experienced with a design temperature of 20°C when compared to a radiator system with a design temperature of 21°C because of the heat radiated from the walls, floors and the very fabric of the room. Consequently a 1°C reduction in normal room air temperature is used. For bathrooms, wet areas, and walk in wardrobes we recommend a design temperature of 23°C. This is to compensate for the reduced floor area, added ventilation and the fact you will often use these rooms with little or no clothing.

Air Changes

When compared to radiator and forced convection heating systems wet underfloor heating creates weaker convective currents. This in turn creates a more even temperature and pressure within the heated space. This fact enables a lower air rate change to be used.

Floor Losses

The floor is the equivalent of the radiator. When turned on, the floor will always be warmer than the room temperature. Consequently downward losses from the floor should not be included when sizing underfloor heating coils. Floor losses will only need to be included when sizing the boiler, heat source and distribution pipework. Downward losses should be limited by insulating the floor slab and perimeter edge.

High Ceilings

Because underfloor heating does not produce the same convective gradients as radiator heating no additions are required to compensate for high ceilings.

Addition for Intermittent Heating

Underfloor heating should be designed to operate at a constant temperature during the day with a night or unoccupied setback of approximately 3°C. This ensures the room warm up periods is reasonably short and no adjustments to the heat loss calculation are required.

Heat Output For Wet Underfloor Heating

The formula for working out the floor capacity to heat the room is as follows:

Total Room Heat Loss (W)/ Active Floor Area (m²)

So for a room with heat losses of 1257W and a heated area* of 14.5m² the heat output required is:

1257/14.5 = 86.68 W/m²

* Heated area is that available to under floor heating. Sterile areas should not be heated e.g. under fridges, freezers, cupboards, baths and sanitary ware, and anywhere else you do not want under floor heating or where you may required to drill and fix into the floor.

Most rooms in new build properties will require a design heat flow of between 50-70 W/m². Possible exceptions where floor heat emission required can exceed 100W/m² are:

Conservatories or rooms with large glazed areas
Bath and shower rooms where the available space for under floor heating is reduced by fixtures
Rooms with open flued appliances the air change rate is greater affecting heat losses
Rooms with very high ceilings due to the increased wall heat loss

Floor Surface Temperature

BS EN 1264 floor heating and floor surface temperature. Floor surface temperature is critical to comfort, as well as to heat output. BS EN 1264 gives guidelines on the maximum values for floor temperatures.

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Underfloor Heating Design