Underfloor Heating Design Principles

Basic information about how the underfloor heating system is designed and highlights points to consider before the design work commences.

Space Heating

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 21°C, while just 15°C may be adequate in a gymnasium or indoor sports hall.

Principles

The principle of UFH is very simple. Rather than mount 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 factor throughout the structure. When the floor temperature is higher than the air temperature, the floor will emit mainly radiant heat. The output from the floor is directly related to the temperature of the floor and that of the surrounding air.

Loops of PEX pipes are normally installed beneath the whole floor area. These loops are connected to a central manifold which is supplied with hot water from any suitable heat source. The hot water from the boiler is mixed before 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 UFH pipes. Geothermal heat pumps do not require the mixer arrangement as they provide low temperature warm water.

Heating with UFH

Underfloor heating 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 1-2°C lower than a radiator system and the room will still be 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.

Scope of Design

Earth NRG designs the system up to and including the manifold. Any type of heat source may be connected, providing it can supply water to the manifold at the specified temperature and flow rate.

Earth NRG normally design controls for operating the system and maintaining both water and air temperature levels.

Heat Outputs

Note: Earth NRG do not calculate the heat loss of the building. It is the client's responsibility to check that the heat losses of the building are compatible with the outputs given in the design.

Generally the output of any underfloor heating system is 70W/m² for wooden suspended floors and 100W/m² for concrete floors. 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.

The heat output is a function of the operating temperature of the floor and the floor area. Given the low U-values stipulated in current UK building regulations, it is unusual to require outputs greater than 70W/m². This equates to a floor surface temperature of 27°C.

Boiler Sizing

The heating consultant or engineer should provide heat loss calculations. Heat losses are calculated in the conventional way and the boiler size will be the same whether UFH or other heating systems are used.

Earth NRG will specify maximum heat outputs for the floor and air temperatures specified. Providing the project complies with current building regulations, particularly with regard to thermal insulation levels, these outputs should be more than adequate to exceed heat losses and provide full comfort conditions.

Design Limits

BS EN 1264-2:1997 states that floors may be raised to a maximum of 29°C and up to 35°C in peripheral areas.

For screeded floors with maximum design surface temperature of 29°C, the floor will emit 100W/m² with an indoor temperature of 20°C.

Floor Construction Type

Note: Earth NRG must be notified of the floor construction type before design commences.

Floor construction is another key factor in the design. Concrete screed, wooden suspended floors and floating floors all require individual consideration to ensure an even distribution of heat across the top surface of the floor.

The concrete screeded floor is an excellent medium for heat transfer, diffusing heat evenly from the warm pipes to the floor surface. With wooden suspended or floating floors however, it is necessary to use aluminium heat diffusion plates in order to achieve an even distribution of heat.

Calculating Size of UFH Pump

Note: Earth NRG will provide calculations to define the pump duties necessary.

The smooth inner surface of PEX pipe minimises pressure loss optimising the pipe length that can be used. The temperature drop across the pipe loop and the maximum required heat emission determines the water flow rate required through the pump.

When sizing pumps, it is necessary to take into consideration all regulating valves in the system. However, with the minimum pipe bending curvature recommended by the pipe manufacturer of 100mm radius for the PEX pipe, the pressure drop through the bends is negligible and can be ignored in pressure drop calculations.

Pipe Spacing

Based on many years of design experience, Earth NRG have established that pipes should generally be spaced at 300mm centres to achieve optimum working efficiency. In areas of high heat loss, the pipe spacing may be decreased to 200mm.

Screed Thickness

For solid floor construction, a normal floor screed can be used. No special additives in the screed are required. Where the pipe is laid on insulation, the minimum screed depth must be 65mm for domestic applications and 75mm for commercial applications as specified in UK Building Regulations. Where heavier floor loadings are required, the construction engineer should advise on the screed thickness. Where special screeds are specified, the manufacturer must be consulted as to the screed thickness.

Floor Coverings

Note: Earth NRG must be notified of the floor covering type before design commences.

While floor coverings laid above the UFH system do not normally present problems, they must be taken into consideration at the design stage to ensure that a good design is established. After installation, the UFH system can be "tuned" to match the variations in floor coverings from room to room by adjusting the Lockshield valves for the individual loops.

Timber Floor Surfaces

Careful consideration of the type of floor to be laid must be established before the system is designed and installed. Particular attention should be paid to moisture content of the wooden floor. Consider if the timber floor will be floated on a screed or fixed to battens. This will determine which type of UFH system will be used. Not all timber floors are suitable for UFH and advice should be sought from the flooring supplier from the trade association TRADA. Details of the British Standard BS8201 should be observed.

Glazed Areas

In most areas of a building the heat losses are relatively similar and unless very large areas of glass are used, the pipe centres do not need to vary. When designing the UFH system, the warmer flow pipe should be positioned to run immediately next to outside walls or other potential cold spots. This ensures a slightly higher heat output in these areas. Extensive areas of glazing in modern developments may require special design consideration. A closer pipe spacing of 200mm may be chosen.

In double height rooms, e.g. barn conversions or conservatories, the pipes may be spaced at 200mm centres. During extreme winter conditions, supplementary heating may still be required in glazed areas such as conservatories where the heat losses are excessively high.

Positioning the Manifolds

It is important to fix the positions of the manifolds at the beginning of the design process. The position of the manifolds should be as central as possible in order to minimise the lengths of pipe tails and unequal loop lengths. One manifold set typically serves a floor area up to 150–200m² in domestic systems and up to 250–400m² in commercial systems.

Optimising Loop Lengths

To allow for different loop lengths, heat outputs and different floor coverings it is necessary to balance the loops on each manifold. In summary, the flow in short loops serving small rooms needs throttling relative to longer loops serving larger rooms. If the loops are not balanced, the shortest loop would "steal" all the water flow from the longest loops, resulting in reduced heating to the largest rooms.

To help achieve flow balance in the system, loop lengths are normally designed to be of similar length. To avoid many short loops serving small rooms, some areas may be linked into one operating zone, e.g. hallways, toilets and common areas. Two or more loops may be required to serve larger areas.

Insulation

A layer of insulation should be laid immediately below the heating pipes to prevent downward heat losses (not normally supplied by Earth NRG).

For screeded floors, Earth NRG normally supplies a plastic Clip Rail for fixing the pipework. Clip Rail is laid onto the insulation to which the pipes are attached. This means that the client can use the insulation material of his choice to lay underneath the fixing system.

Minimum insulation thickness and type should be specified by the architect and/or builder to comply with the current Building Regulations.

Thermal Response Time

The better the U-value of the building and the higher the degree of insulation, the faster the response time of the system.

In wooden suspended or floating floors, system response time is comparable to a radiator system. However, there is no thermal lag with these types of floor construction and good levels of insulation are required under these systems. Where it is necessary to allow a small gap between the insulation and the UFH system, there must be no air movement in this air gap.

Where the UFH system is to be installed in wooden suspended ground floors with an open void below, all air movement must be prevented between the insulation and the UFH system to ensure maximum efficiency from the heating system.

For systems installed in screed operating on a daily basis, the response time is fast. This is because the temperature drop during "off" periods of up to 8 hours is of the order of only 3°C due to the thermal lag in the floor created by the energy stored in the screed. By carefully setting the system time clock, comfort can be maximised whilst minimising the system "on" time.

Sound Insulation

Depending on the type of insulation installed, the insulation will provide a degree of sound insulation. If a specific level of sound insulation is specified, details must be advised to the design team before the design process begins.

Design Check List

Before the design begins, Earth NRG require the following details:

• Heat losses
• Floor construction types
• Floor coverings
• Large window or glazed areas
• Preferred manifold positions
• Any specified insulation
• Any requirement for sound insulation