Technical Information Sheet
Whatever the method used, the purpose of all space heating is to create an acceptable level of human comfort within a defined area. “Comfort” however, is a subjective concept. It will vary from person to person according to their age and activity level. There is therefore no universal ideal design temperature for all occasions – a sheltered housing project may require air temperatures of 21ºC, while just 15ºC may be adequate in a gymnasium or indoor sports hall.
The principle of UFH is very simple. Rather than mounted metal panels on walls, pipes are laid in the floor and warm water circulated so that the floor effectively becomes a large radiator. Because the floor is so large compared to a normal wall-mounted radiator, it needs to run only a few degrees above the air temperature to provide enough warmth to gently heat the whole room. The primary aim of the floor heating design is to create an even, uniform surface temperature across the entire floor area within the building in order to ensure a consistent comfort level throughout the structure. When the floor temperature is higher than the air temperature, the floor will emit mainly radiant heat.
The heat output from the floor is directly related to the temperature of the floor and that of the surrounding air. Loops of pipes are normally installed beneath the whole floor area. These loops are connected to a central manifold, which is supplied with hot water from a suitable heat source – such as a boiler or heat pump – heat pumps are becoming ever more popular due to the potential energy savings. Usually, with boilers as the heat source, the central heating water is mixed before it reaches the manifold to reduce the water temperature to that suitable for the UFH system. Controls reduce the water temperature to maintain the correct design temperature and pump the warm water through the UFH pipes.
Heating with UFH
UFH is a true radiant system and heats from floor to ceiling. UFH avoids wasted heat at high level and since the whole floor is heated evenly, optimum comfort is achieved everywhere in the room. In fact, the room thermostat can be set 1 – 2ºC lower than a radiator system and the room will still feel more comfortable! Running the system at a lower temperature and reducing the heat wasted at levels above head height makes for significant savings on fuel costs. The exact savings that can be expected are difficult to determine, as there are operational factors that also need to be considered.
It is the client responsibility to check that heat losses of the building, carried out by a heating consultant or engineer, are compatible with the outputs given. Generally, the maximum output from an UFH system is often stated at between 70 and 100 W/m2. The actual output achieved is a direct relationship between the difference in floor surface and room air temperatures. The floor construction, floor covering material, pipe size, pipe spacing, and the temperature of water circulating through the UFH pipes are major factors that determine the floor surface temperature. When designing conventional heating systems it is necessary to know the required heat output to be able to size the heat emitter. However, for UFH the size of the emitter is fixed – it is the floor area. Hence, the heat output is a function of the operating temperature of the floor, the floor area, and room air temperature.
Heat Requirements & Supplementary Heating
Given the low U-values stipulated in current Building Regulations, it is unusual to require outputs greater than 70W/ m2, based on a 21ºC internal design temperature. It is important to note that poorly insulated buildings, conservatories, areas with high ceilings and rooms with high internal temperature requirements, may require supplementary heating during midwinter conditions. The heating consultant or engineer should provide heat loss calculations. Heat losses are calculated in the conventional way and the boiler size will be similar whether UFH or other heating system is used.
Establishing the correct operating temperature for the floor surface is a balance between not having the temperature so high that it causes discomfort, but high enough so that sufficient heat output is provided to meet the calculated heat losses. BS EN 1264-2:1997 states that the ‘physiologically agreed’ maximum floor surface temperature is 9ºC above the room temperature. This results in a maximum floor surface temperature of 29ºC in typically occupied areas with a room temperature of 20ºC. A 9ºC temperature difference will equate to a floor heat output of 100W/m2.
Floor Construction Type
Floor construction is another key factor in the design. Screed floors, suspended wooden floors and floating floors all require individual consideration to ensure optimum performance and an even distribution of heat across the surface of the floor.
The screed or solid floor system relies on the conductivity of the screed or concrete to conduct the heat from the pipe surface to the underside of the floor finish. Because the screed is itself heated to conduct the heat it tends to store considerable amounts of heat and thus provides a slow response when both heating up and cooling down.
Timber floor systems rely on the conductivity of components fitted within the floor to conduct the heat from the pipe to the underside of the floor finish. In order to achieve good results the pipes must transfer their heat evenly to the floor surface. Inadequate heat dissipation and hot spots can cause unsightly shrinkage, particularly with natural wood boards. Because the mass of a timber floor structure is less than the mass of a screed floor, the system response of a timber floor system is usually much faster.
The floating floor system is predominantly suitable for sheet flooring or some stronger laminates. The grooved insulation is structural and lay on top of a prepared base. Additional insulation may be required to ensure compliance with Building Regulations and to minimize downward losses.