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Different Finishes Work Differently With UFH

RECENT changes to SAP and SBEM have had the effect of giving a building a better energy rating if it has UFH in a timber floor than if it has UFH in screed/concrete, or if it has radiator heating. The improvement in SAP is sufficient that, if a house with timber UFH is powered using water heated by a ground-source heat pump, a house which would otherwise be rated Level C will instead be rated Level B.

The most important reason for this is because timber floor constructions have lower thermal mass than screed/concrete. As a consequence, timber UFH has the potential to change the floor surface temperature much more quickly and with a much smaller injection of thermal energy, and this means heating power can be changed quickly in response to changes in heating demand.

For floorlayers, a very important advantage of timber floor UFH is that the construction is all-dry and that means that timber floor finishes can be laid immediately and safely – the same day if necessary – and there is none of the time-delay associated with waiting for screed/concrete to dry. UFH has become the preferred form of heating throughout the residential sector and is also now commonly specified for schools and medical facilities, for offices and factories, and even for boats and zoos. It can be equally easily installed in new-build or refurbishment projects. UFH mostly warms by using radiation and this is the most natural and comfortable form of heating because it is the way the sun warms us.

It is safe because it creates no hot surfaces, it is unobtrusive, more hygienic and more energy-efficient. With UFH, the temperature throughout the room is very even and the floor is the warmest part. With UFH, the floor surface only needs to become about as warm as the palm of your hand (25-27degC) to provide effective heating, even on a very cold day. The right form of UFH can achieve this surface temperature while using water which is much cooler than even over-sized radiators require and this ability can not only make the heating system more energy-efficient but it also opens up the prospect of being able to obtain this lower water temperature from renewable energy sources.

UFH has the potential to wean a building off a dependency on fossil-fuels. Most UFH suppliers are able to supply an effective form of UFH when it is set within a screeded or concrete floor. However, when it comes to UFH in timber floors, not all forms of timber UFH are equally good. While some are very good, others are surprisingly poor. Timoleon commissioned the test laboratories at BSRIA in Bracknell to measure the effectiveness of 14 different forms of timber floor UFH.

Specifically, BSRIA measured the water temperatures needed to achieve the same heating power from the different floor structures, and it is worth looking in more detail at how some of these different forms performed in these tests because the results are particularly relevant. In each of four example floor constructions described below, the water temperature shown is the Flow water temperature needed to produce the same heating power of 50W/cu m, when the timber floors are covered with a carpet and underlay having a combined thermal resistance of 1.5Tog.

Even if a floor might start out without carpet and underlay, it is likely that it will be covered at some stage during the floor’s life. Comments have been added alongside each floor construction about the ability of each to create squeaking timber floors because there is nothing more frustrating than creating a timber floor which squeaks when you walk over it. n A heating pipe is fixed to the top of insulation and the timber floor deck is then laid over the joists/battens.

This creates an air-space beneath the floor deck, which is heated by the pipe. n Comment: This water temperature is so high it completely rules out the building ever being heated by solar thermal panels or a heat pump. It is sufficiently high that it can gradually dry out the floor deck and the tops of the joists/battens, with the potential to create squeaks. n Foil-faced bubble-wrap is draped over the tops of the joist/battens and between them, and held in place by clips which also hold the pipe.

It also relies on heated air and some reflected energy from the pipe to heat the underside of the floor deck. n Comment: The water temperature is almost as high as in example one. By having foil-faced bubble wrap across the tops of the joists/battens, it is impossible to use a glued joint between joist/batten and floor deck (now recommended practice) making squeak-free floors almost impossible. n Rigid aluminium diffuser plates are nailed to the tops of the joists/battens. Each has channels which hold the pipe. The timber floor deck is then laid over the joists/battens.

The aim is to have some contact between the top of the diffuser plates and the underside of the timber floor deck but, in practice, as a plate heats up, it expands and loses contact with the underside of the floor deck. n Comment: The water temperature is lower but still too high for efficient use of a heat pump, and beyond solar-heated water. The floor deck cannot be glued to the joist/batten, making creation of squeak-free floors almost impossible. n T+G flooring panels – typically 22mm P5 chipboard or plywood – factory-machined with a pattern of channels into which the heating pipe is pressed. These are over-laid with the same 6mm ply used with regular T+G panels but fully glued into place.

The resulting composite acts like a glulam and tests at TRADA show it to be stronger than regular 22mm P5 chipboard or plywood. n Comment: This water temperature is a full 25degC cooler than the first example and is suitable for efficient heating using either a heat pump or solar-heated water. By comparison, this form is more than 30% more energy-efficient. The floor deck can be glued down and the chances of creating a squeak-free floor are maximised. Also, the nature of this construction is that the heating performance will not deteriorate during the installed life of the floor. The BSRIA tests showed it is preferable to avoid all forms of timber floor UFH which rely in any way on air to transfer thermal energy between the pipe and the top surface of the floor. Separate floor-strength tests conducted at TRADA specifically investigated the effect on floor deck construction of having to comply with the new EN12871 gamma q=1.5 factor, as compared with previous, long-established gamma q=1.0. They showed that successful compliance with gamma q=1.5 requires substantial strengthening of the floor deck.

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