Low carbon ready under floor heating (UFH)
As an installer you will be coming across a lot more consumer demand for the alternatives to radiators as the UK transitions to a low temperature, low carbon ready heating system, such as UFH. Therefore, it is important to not only understand the design principles of floor heating, but to understand whether the products you are buying are actually fit for purpose and low carbon ready. This article is aimed at simply breaking down the UFH system into some basic key technical areas to aid you when purchasing and installing a UFH system.
I have not long finished some extensive research into underfloor heating with a leading university that has highlighted some key things for installers to consider before attempting to specify a UFH system. My first choice for a low carbon ready underfloor heating system type is a low profile screed system with its high heat outputs and quick response times. However, if I am forced into offering a dry solution such as an aluminium foiled panel, then care and attention is needed.
Basic UFH system types
Screed systems: The most widely used warm-water underfloor heating system is one where the pipework is embedded within a screed. This system incorporates many variants, such as the pipework stapled to rigid insulation or pipework laid into preformed castellated plastic panels. Traditional sand and cement screed thickness is usually around 75mm, resulting in approximately 50mm of screed over the pipework. However, following recent innovations in both fixing systems and screeds, there are now low-profile screed systems available as thin as 15mm or 20mm. Figure 1 shows a low-profile screed system where the screed thickness is determined by the type of supporting sub-floor, the type of screed used, and the final floor covering chosen. In combination with a quick drying levelling compound, this system provides a perfect, low carbon ready, high heat output UFH solution ideally suited to tackling retrofit challenges. Historically, traditional 75mm sand and cement screeds were not very responsive. However, the new low-profile screed systems offer the best of both worlds; a quick response combined with high heat outputs at low flow temperatures, and the added benefit of easier system balancing due to flexible pipe centres compared to the fixed pipe centres of grooved panels.’
Aluminium foiled insulation panels: Because aluminium has good heat conduction properties it is commonly used in dry-fit UFH systems in the form of either grooved and foiled insulation panels or heat spreader plates, typically ranging from 45micron thick foil to 0.5mm thick spreader plates (Figure 2). To achieve similar heat outputs, these system types require much higher flow temperatures than those required for in-screed systems and are supplied with specified pipe centres compared to the flexible pipe centres of an in-screed system. In addition, there are some critical elements to consider when purchasing grooved and foiled insulation panels to ensure you are compliant with BS EN 1264 and to meet any structural performance requirements. These are discussed in more detail later in this article.
UFH System mean water temperature An underfloor heating system is not too dissimilar to a radiator in the fact that the heat output increases with a greater difference between the mean water temperature (MWT) and the air temperature, known as the ΔT. The mean water temperature is the average of the flow and return temperatures.
mean water temperature – desired room air temperature = ΔT
Therefore, it can be seen that increasing the mean water temperature increases the ΔT and hence the UFH heat output.
Figure 3 demonstrates how the heat output from an in-screed UFH system increases from 54 W/m² to 70 W/m² (30% increase) for an increase in mean water temperature of 5°C (40°C MWT rising to 45°C MWT).
Figure 4 shows how the heat output from a grooved and foiled insulation panel UFH system increases from 33 W/m² to 41 W/m² (24% increase) for a similar 5°C increase in mean water temperature.
ΔT and flow rate
As discussed in the previous paragraph, the ΔT plays a big part in determining the heat output of an underfloor heating system. However, whilst it is possible to increase the heat output of a UFH system by increasing the flow temperature, for a fixed flow temperature an increase in mean water temperature is only achievable by increasing the return temperature. This will require an increase in flow rate, which will add greater resistance to the whole system if the correct primary pipework is not selected.
Figure 5 illustrates that, for a fixed flow temperature, increasing MWT by increasing return temperature in order to increase ΔT by 5°C, doubles the flow rate required. This would require the primary pipe size to be increased from 28mm to 35mm for a 20 kW heat load.
The floor is not reaching temperature, is it due to pressure drop in the primary system?
Sometimes when we receive a UFH design from the supplier, it is easy to overlook the pressure drop in each component from the heat source to the mixer valves on the UFH manifold. As we transition to higher flow rate heat pump systems, the pressure drop across all components starts to play a significant role in potentially limiting the heat output of the floor heating system. There is a common misconception that the thermostatic mixing valves on UFH manifolds act as a form of hydraulic separation, however, applying this theory to all UFH systems is risky business. Always check the pressure drop across each component in the system when selecting zone valves, magnetic filters, manifolds etc as an incorrectly sized valve can be the difference between a warm or cold house during peak cold conditions.
The term pipe centres is the terminology used to specify the distance between the centres of each of the UFH pipes within the floor. The closer the pipe centres, the greater the heat output of the UFH system for any given mean water temperature.
The effects on heat output with different floor covering types
One of the main construction factors that affects the heat output of an underfloor heating system is the thermal resistance of the final floor covering laid on top. A 1.5 TOG carpet (R0.15) can significantly reduce the heat output compared to a thin floor covering such as Amtico which typically has a thermal resistance as low as R0.02.
Figure 6 shows a reduction in heat output of 32 W/m² when comparing a 1.5 TOG carpet (R0.15) with Amtico (R0.02) laid over a low-profile in-screed UFH system operating at a MWT of 40°C.
Therefore, it is critical that you know what floor coverings will be fitted in each room before the UFH system is designed because the choice of floor covering will affect the heat output of each floor and could lead to challenges when commissioning and balancing the system.
What to avoid and why
BS EN 1264 stipulates that to optimize heat output the heat diffusion device (aluminium foil) should be in full contact with the underfloor heating pipework.
Figure 7 illustrates a non-compliant grooved and foiled insulation panel where the panel has been manufactured with the aluminium foil over the main pipe grooves. To enable the UFH pipes to be fitted into the grooves, the aluminium foil first needs to be cut through on site. Not only does this add a time-consuming step to the installation process, but the accuracy with which it is done can vary considerably, resulting in a significant reduction in performance (up to 30% reduction in heat output). To compensate for this inefficiency, a much higher flow temperature would be required, leading to higher fuel costs and, in some cases, a cold house.
Figure 8 shows a compliant grooved and foiled insulation panel where the panel has been manufactured with the aluminium foil fully formed into the main pipe grooves, ensuring maximum contact between the UFH pipe and the aluminium foil in order to optimize heat output. With these panels there is no need to first cut through the aluminium foil, not only avoiding the additional time involved in doing so but removing any risk of compromising performance.
Figure 9 illustrates the results from the research comparing the performance of the panels in Figure 7 (foil over the grooves) with those panels in Figure 8 (foil into the grooves) for aluminium foil thicknesses ranging from 45microns to 200microns. The results shown are for a 16mm O.D. pipe @ 150mm centres with a 15mm engineered wood floor covering floated directly over the panels. The results clearly demonstrate the significant reduction in heat output achieved as a result of the foil not being fully formed into the pipe grooves.
The CIPHE as a professional body with the purpose of protecting the safety health and welfare of the public, expect to see fit for purpose low temperature heating systems being installed in consumers homes. As the UK transitions to low carbon solutions, it is important that installers prioritise safety, comfort and efficiency.