Underfloor heating – better comfort by design

Brian Sensecall
Helping design better underfloor heating systems – Brian Sensecall on new standards.
A new standard for calculating the performance of underfloor heating systems is not only simple but very accurate. BRIAN SENSECALL explains how it works.New standards developed in Europe are changing how we think about comfort — and helping us design better systems. They also make it easier to prove why underfloor heating is so much more comfortable than other systems. Adopting these new standards can only improve matters by reinforcing the message that every underfloor heating (UHF) system is a unique solution to a unique situation which depends for ultimate success on careful design, specification, procurement and installation. Necessity To begin at the beginning, accurate heat loss calculations are a necessity, despite heating loads having fallen in recent years. For the last 20 years or so, UK engineers have used a simple calculation method based on the average air temperature 1.5 m above floor level, plus an average of the surface temperatures of floor, walls and ceiling. European Directives BS.EN 7730 (1996) and BS.EN 12831(2003) require the more stringent operative temperature calculation based on a blend of 50% of the air temperature surrounding the occupant (not that which exists overhead)and 50% of all radiant heat, from the surrounding walls, floors, ceilings, windows, furniture, lights, appliances etc. Traditionally, focused radiant energy, needs ‘solid-angle’ mathematics with fourth-power Absolute temperatures to calculate the effect from hundreds or thousands of facets in one room, which is for practical purposes impossible. However, underfloor heating designers have a simple alternative, requiring no more than basic mathematics, to provide an accuracy very close to the real thing, and that is by using distance-weighted mean radiant temperature (DWMRT) where any surface area is divided by its straight-line distance to the occupant A chessboard analogy enables a designer to place an occupant in any square metre within a ‘room’ and use a spreadsheet to calculate the DWMRT for that position. Underfloor designers typically use 1 m as the floor-to-body distance, a logical value for seated persons. Comfort Warm floors and ceilings emphasise the feeling of comfort, due to the short distance between these heat sources and recipient. Changes in activity-linked occupancy, metabolism rate or different clothing can be quickly determined from BS.EN 12831(2003) to complete the comfort picture. Convection from underfloor heating systems is less than from panel radiators or warm-air systems, but, since all the convection currents produced by the floor have to pass around the occupant in their 5 to 10 minute journey to the ceiling, maximum benefit is derived. Such dedicated local heat input makes a subtle contribution to an individual’s comfort without being obtrusive, or leaving dusty trails in its wake. Interpret Completion of heat-loss calculations is by no means the end of work for a UFH designer. On the contrary, this is when the success of every UFH system depends on the skills of the careful professional to interpret the placement of distribution manifolds for most effective results and ensure that energy is correctly directed towards cold spots rather than spread thinly and evenly over the whole area. BS.EN 1264 Parts 2 and 3 serve the needs of the UFH designer by providing the parameters and equations for accurately calculated, efficient results. UHMA regard pipe standards to be so important that no member will accept new pipes which are of lesser performance. Over the past 100 years, changing fashion has seen brief periods of popularity for wrought iron, black mild steel, soft copper tube, nylon tube, rubber tube etc., but all have at last been totally overwhelmed by long-life cross-linked polyethylene, polybutylene, or polypropylene plastics. Mindful that the European construction-materials directives require the UK to specify only those components which have a minimum life of 50 years, quality and performance has to be spotless. UHMA members use pipes that comply with BS 7291, DIN 4726, and BS.EN.ISO 15-874(PPC), 15-875(PEX), and 15-876(PB). Other components in UFH systems do not escape the rigours of standards. Indeed BS.EN 13163 and BS.EN 826 is used for thermal insulation, while for the last 30 years BS.8204 Part 1 has reigned supreme for the solid floor screeds used with floor heating systems. BS 8204 Part 7 covers the latest pumped calcium sulphate flowing screeds. Installation One last consideration must be installation, which may not seem to be a design issue, but the design engineer has the most intimate knowledge of his system and should be encouraged to influence correct installation procedures. To this end, compliance by installers with the designer’s intentions is covered by BS.8201 (Timber floor finishes) BS 8203 (Finishes for solid floors) and BS.EN 1264 Part 4 on how to best integrate the installation of an underfloor heating system. Brian Sensecall is with the Underfloor Heating Manufacturers’ Association, Belhaven House, 67 Walton Road, East Molesey, Surrey KT8 0DP.
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