heat pumps and underfloor heating By James Dale, technical manager at Underfloor Warehouse Ltd.
Heat pump technology and underfloor heating (UFH) combined is the perfect partner for space heating
*Calculations based on 16mm OD UFH Pipe, 200mm Pipe Centres, 0.00w/m²K (tile) & 0.15w/m²K (carpet)
Heat pump technology and underfloor heating (UFH) combined is the perfect partner for space heating. With sound design, system integration, careful installation and product selection, a high performance, energy efficient solution with low running costs will be achievable. Due to the low water temperatures required by underfloor heat- ing to generate satisfactory ambient air temper- atures and increased comfort levels, a heat pump is the ideal choice compared to other types of heat source. Air tightness and the level of insulation within
a building influences the efficiency, economy and effectiveness of heat pumps and underfloor heating. Regardless of heat source or heat emit- ter these two factors must be considered so the building has low heat losses and therefore low energy use. This is very important in older reno- vations with high heat requirements. The output of the underfloor heating (UFH) sys-
tem is determined by the supply water tempera- ture necessary to meet the heat requirement of the building, so higher heat losses mean a less efficient heat pump. The performance of a heat pump is measured by its coefficient of perform- ance (COP) and typically air source heat pumps operate at a COP of 3.6 @ 40°C flow, which typically results in an UFH output of 68w/m² @ 20°C internal air temperature (screed floor with tiles).
Another factor that reduces the efficiency of
the system is the floor finishes and it is important to select finishes with low resistances. Ideally a tile or stone would be used to maximise effi- ciency, but this is not always practical especially in bedrooms where carpet is more comfortable. Using the example above but with a deep- pile carpet and screed the system output would reduce to 37.9w/m² @ 20°C internal air temperature with a flow temperature of 40°C. If the project requires a higher output to meet the heat demand then the flow temperature of the heat pump must increase, therefore reducing the COP.
Solid screeded floor Solid floor constructions are one of the simplest, most cost effective ways of installing UFH. The pipes are laid on insulation then covered with screed, which acts as the best heat conductor, making this system the most responsive. There are two types of screed that can be used over the pipes; traditional sand/cement or specialist anhydrite (flow screed). Where the pipe is laid on insulation with sand/cement screed, the minimum screed depth must be 65mm for domestic and 75mm for com- mercial applications. The same situation with anhydrite screed will only call for a nominal screed thickness of 50-55mm coverage. All screed must be allowed to dry and cure,
normally in accordance with the relevant BS Standards*, before initial heating and sys- tem start-up. UFH must not be used to speed up the curing process. The two most common methods of fixing the
UFH pipe to insulation are kombi clip and clip rail & clip. When installing onto floor grade insulation, pipes should be spaced 100mm away from the wall edges. In well insulated domestic buildings the pipe centres are gener- ally laid at 200mm centres across the active floor area (using 15/16mm pipe).
Floor construction methods
Timber suspended: This application is suited for both new and existing buildings. It is essential that insulation is installed between joists as close to the underside of the plates as possible. Typically a minimum 100mm of mineral wool insulation is used. However, where relevant, the insulation used must comply with current Part L of the Building Regulations. All other services such as cables and plumbing pipes should be laid before the underfloor heating. UFH pipework can be laid between timber
joists with the help of heat emission plates. The plates are designed for suspension over joists at 400mm centres and are manufactured with grooves set at 200mm centres. If joist centres are different to the above, a few methods of cross battening are available to give support to
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