Take to the floor for greener heat

Underfloor heating (UFH) is recognised as delivering significant energy and carbon emission savings compared to other heat emitters. If the technology is combined with renewable energy sources those savings are even greater. UFH can therefore be a crucial component in achieving better results under Part L or the Code for Sustainable Homes.

Renewable heat sources work best when they deliver the lowest-possible water flow temperature to a heating system. Every degree increase in the supply temperature degrades the efficiency of the renewable heat source. The measure of efficiency is expressed as the coefficient of performance (COP), which is calculated as the ratio between input energy (to drive the pump) and output energy (heat). For a typical ground source heat pump that transfers the solar energy collected in the ground to the heating system, a typical flow temperature of 35oC would mean the unit provides a COP of 4, or 400% efficiency. Set the flow temperature to 45oC, however, and the COP drops to 2.5.

With an air or ground source heat pump, choosing a UFH system to emit the heat means that the flow temperature can be in the range of 35-40oC, and sometimes (subject to the building fabric and floor covering) as low as 30oC. An alternative is to fit oversized (or ‘low temperature’) radiators. These are typically twice the size of a normal radiator, but require a flow temperature of at least 45oC and often in excess of 50oC, with a corresponding reduction in the efficiency of the whole heating system.

According to a study conducted in 2005 by Denmark’s International Centre for Indoor Environment and Energy and European surface heating organisation EU-Ray, when UFH is compared with low-temperature radiators coupled to a condensing boiler, the energy saving is a modest 5%. However, that figure is still significant in terms of carbon-reduction. When compared with ‘normal’ radiators with a condensing boiler the saving is 15%. The study also showed that the savings are reasonably consistent whether the property is a house, an office building or an industrial building, and the climatic zone does not significantly affect the percentage savings.

Application

UFH can be used for most heating applications, and can offer many advantages over other forms of heating. The reasons include:

• With UFH, the same level of comfort can be achieved by keeping air temperature 1-2oC lower due to the radiant effect of the system. The temperature is also more even across the whole area, so that in buildings such as museums, galleries and libraries, where less heat is desirable, comfort can be achieved at these lower temperatures. It also means that there are no hotspots, reducing damage to artefacts, exhibits and books.
• Where buildings have double-height rooms or atria, UFH is advantageous since it will only heat the lower part of the room. With radiators or convectors, the heat rises to the ceiling and then drifts down to the occupied zone as it cools.
• The safety aspects of UFH systems make them ideal for use in care environments and schools with low surface temperatures and no sharp edges, as found on radiators.
• Many leisure centres and swimming pool operators opt for UFH as the dry floors offer increased safety and user comfort.

However, there are a few places where UFH should not be used. These include buildings where use is intermittent and buildings with high ventilation rates where the intake air has not been pre-heated.

Fitting

UFH can be fitted into most floor constructions, including solid ground floors and solid intermediate floors, floating floors, and timber/intermediate floors. The integration of UFH needs to be considered during the design process. Most UFH companies have similar fitting systems, and generic methods are discussed below.

Solid ground and intermediate floors

The UFH pipework is usually embedded in the screed, but it can also be integrated into the slab if the floor insulation is below the pipework. The pipework can either be stapled to the insulation or, if steel mesh is being incorporated into the pipework, it can be fixed to this.

The requirement of expansion joints within the screed should also be considered. These are dependent on the type of screed used and advice should be sought from both the screed and UFH supplier. As a rule of thumb, for sand/cement screeds an expansion joint should be incorporated every 40m2 or eight linear metres, but for some liquid screeds these can be extended. Typical heat outputs for this type of floor construction are between 75 and 100W/m2 depending on floor finish.

Intermediate or suspended floor construction

Methods of fitting systems into this type of floor construction basically fall into three types which also have to take Part E (resistance to the passage of sound) of the Building Regulations into account:

• Dry sand/cement mix. A board-type insulation is laid between the joists with battens to support it. The pipework is clipped down onto the insulation and a dry 8:1 sand cement mix is tamped down around the pipework and made flush with the top of the joists, to give good contact with the flooring. However, the weight of the mix is around 16kg/m2 which needs to be considered from a structural perspective.
• Foil systems. A lightweight reflective foil is draped between the joists and the pipework is suspended between them. The heat is radiated off the pipework and reflected back to the underside of the floor. This type of fitting is extremely lightweight and it is relatively easy to thread the pipework through access holes within composite joists from one joist bay to the next.
• Plate systems. This is probably the most common method of fitting pipework into suspended floors. Board insulation is laid on battens between the joists and the heat spreader plates can be laid onto the joist.

The pipework is then clipped into the spreader plate grooves. This system can be laid with most joist types, but again, caution should be taken when passing pipework between joist bays.

A typical output for intermediate or suspended floors is around 75W/m2, depending on floor finish.

Floating floor

This type of floor consists of grooved insulation laid over a sub-floor. Aluminium heat spreader plates are then laid into the grooves, pipework is clipped into place and floor grade chipped board or another proprietary product is laid over the top with all joints lapped and glued. The typical heat output from this type of flooring is again about 75W/m2, depending on floor finish.

Floor finishes

Virtually any floor finish can be used, but a few factors must be taken into account:

• Ceramic/stone finishes are probably the best type since their thermal resistance is relatively low and the flow temperature of the UFH system can be kept as low as possible. These need to be fitted with flexible adhesive and grout suitable for use with UFH to allow the tiles to minutely expand and contract as the floor heats and cools.
• Carpet can be used with UFH, but you should select carpets and underlays recommended for use with these systems. The Tog rating of the carpet and underlay should be kept to a maximum of about two.
• Vinyl and linoleum products are ideally suited for use with UFH because they have a low thermal resistance. However, the maximum temperature many of these products like to operate at is about 27oC, which prevents the potential for long-term discolouration. At these temperatures a heat output of 70-75W/m2 is still achievable at normal room temperatures.
• Wooden floors. Many are suitable for use with UFH, if a few precautions are taken. First, the maximum surface temperature of the wood should not be above 27oC. Second, if using solid timber flooring, as a rule of thumb the moisture content of the wood should be below 8% and the profile of the flooring should not really be greater than 4:1. In other words, if the wood is 20mm thick then the width of the plank should not exceed 80mm.
• Polished concrete and terrazzo-type finishes also work very well with UFH.

Underfloor heating in use

UFH can be easily incorporated into building management systems, as required in many commercial buildings, or it can stand alone with its own controls. Optimised start, stop and weather compensation settings can be included with the controls package.

In the future, renewable energy sources will continue to perform best when supplying low-temperature water. A UFH system that has been designed to work with 35oC flow from a heat pump will clearly still be useable if and when the system is served by an alternative low-flow temperature renewable heat source – such as a heat recovery system – at some point in the future. But if the current heat source is connected to radiators, the higher flow temperature they demand will mean an efficiency compromise now and in the future.

UFH system components also offer a long working life. The pipework will typically have a minimum design life of 50 years, outlasting steel radiators.

A UFH system will cost more than a standard radiator system, but often less than a low-temperature radiator system. The fitting cost should be lower and prone to fewer snagging or post-installation costs. For the building owner, the daily running costs are lower because the system is more efficient, and the annual maintenance and repair costs are minimal. Over the lifetime of a building, the cost of UFH is far lower than other heat emitters.

Research by Continental Underfloor Heating has shown that for a newbuild domestic property, the break-even point for UFH versus radiators occurs within six years of the build. In renovations of similar properties the break-even point can be achieved within nine years.