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Planning An Underfloor Installation
Installing a commercial underfloor heating system is the efficient solution for businesses big and small.
Heat losses or the thermal load of wet underfloor heating should be calculated as you would for any other heating system, preferably using a proprietary software package or another reliable source. For an underfloor heating system the criteria is slightly different and this can result in lower heat losses than would be calculated for a radiator system.
Floor Heat Emissions
As more than 50% of the heat emitted from underfloor heating is radiant, this heats 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 from an underfloor heating system set at say 20°C compared to a radiator system at 21°C because of the heat radiated from the walls, floors and the very fabric of the room. 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 and added ventilation.
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 stable pressure within the heated space.
Floor Losses
With underfloor heating, the floor is the equivalent of the radiator. When turned on, the floor will always be warmer than the air temperature of the room. 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.
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 period 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. Some 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 be required to drill and fix into the floor.
Most rooms in new build properties will require a design heat outputs of between 40-60 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 where the air change rate is greater, affecting heat losses
- Rooms with exceptionally 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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