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Getting more from underfloor heating

Getting more from underfloor heating

While most people accept that underfloor heating is an energy-efficient form of heating for many spaces, people are often frustrated by the slow response and uneven temperature distribution of some systems. This blog seeks to explain how underfloor heating can be made more responsive and provide more uniform heating across the floor.

This will be achieved by exploring the differences between low heat dissipation and high heat dissipation systems – the former being the cause of so much disappointment.

Low heat dissipation

In low heat dissipation systems the heat from the water in the pipe dissipates at a relatively slow rate. This results in an extended ‘thermal lag’ between the system heating the floor and the heated floor warming the space above it.

This long delay is often frustrating to the building operator, who may then try to heat the space faster by increasing the set point temperature. However, although the room sensor closes the heating circuits when the set point has been reached, the heat stored in the floor continues to heat the space for some time.

As a result, the system ‘overshoots’ the set point and the space is too hot for some time after the heating has been switched off.

Another issue with low heat dissipation systems is that they operate with relatively high temperature water that circulates slowly through the embedded pipework. This low flow rate, combined with a high temperature differential between the water and the surrounding floor, results in a high proportion of the heat energy being transferred to the areas of floor closest to the beginning of the underfloor heating circuit.

As a consequence, by the time the water reaches those areas furthest from the manifold it is considerably cooler and transfers less heat energy to those areas of the floor.

In fact, with a low heat dissipation system less of the floor mass surrounding the underfloor heating pipes – around the entire circuit – is thermally activated. The resulting uneven heat distribution with regions of hot and cold is far from ideal from a comfort or control perspective.

Pipe layout design can help to mitigate this problem, with the bifilar or counter-flow spiral configuration being the most effective, but does not completely resolve the issue.

High heat dissipation

Further mitigation can be achieved by designing the system to produce high heat dissipation, the benefits of which are being increasingly recognised.

This approach uses lower water temperatures with higher flow rates so that the temperature differential between the start and end of the circuit is minimised, resulting in a floor surface that radiates thermal energy evenly.

As well as comfort considerations, this more even temperature distribution is better for the flooring. The warranty for some hardwood and vinyl floors require no more than a 1°C temperature difference across the whole floor.

A system that minimises such fluctuations will help to extend the service life of the flooring.

Maximising energy efficiency

Another major benefit of high heat dissipation is that it results in the water returning to the heating plant at lower temperatures than would be the case with a low dissipation system. It has been shown that a high heat dissipation system will deliver the same heating capacity as a conventional system but with a 6°C cooler flow temperature.

This increases the efficiency of plant room heat sources that deliver their highest efficiencies with low return water temperatures. These include combined heat and power (CHP), condensing boilers and heat pumps. Consequently high flow rates can only be justified in an energy efficient system if the heat dissipation is sufficient to achieve low return temperatures.

In terms of the energy consumed, higher flow rates in a high heat dissipation system clearly produce a slight increase in pump energy consumption. However this is more than compensated for by the increases in efficiency elsewhere.

Control systems can learn the thermal characteristics of each room and vary the start of the warm up process to compensate for slow reacting systems. However, it is clearly more efficient to have a faster responding system.

Design considerations

Achieving high heat dissipation involves the use of an aluminium multilayer pipe fixed directly to aluminium panels. Experience shows that using a multilayer pipe with a 17mm diameter is more effective than the standard 15mm diameter piping as it produces lower pressure losses and enables faster flow rates through longer pipe runs.

Crucially, the use of an aluminium layer in the pipe ensures a high heat transfer coefficient for efficient heating of the surrounding floor screed. The ductile nature of the aluminium also facilitates installation as once the piping is formed it will not revert to its original shape.

The floor screed used also influences the performance of the system. There are two distinct types of screed typically used, each with different properties and potential applications.

The most common type of screed is made of dry sand and cement typically applied in thicknesses between 70mm and 100mm. This is relatively cheap and easy to install but has a low thermal conductivity and high thermal mass, thereby increasing the thermal lag of the system.

This type of screed is best where a constant temperature is to be maintained and response times are not a major consideration, such as care homes and hospitals.

An alternative is liquid self-levelling (anhydrite or cementitious) screed that can be applied in thicknesses as low as 20mm above the pipe and has a much higher thermal conductivity than dry sand and cement. This supports high heat dissipation, allowing faster response times – even approaching those of a conventional radiator system.

Improved accuracy of control also reduces the risk of temperature overshooting or undershooting, making it ideal for residential applications.

It is often the case that the perceived disadvantages of underfloor heating lead to its use being rejected in favour of less energy-efficient systems. Therefore, there are clear financial and environmental benefits to making underfloor heating more responsive and consistent. High heat dissipation is clearly the way to achieve this.

Further information

http://www.sav-system.com