Is your underfloor heating design floored?

Published:  18 April, 2017

Paul Harmer, technical director at the Chartered Institute of Plumbing and Heating Engineering (CIPHE), calls for a review of design standards within the industry.

The design of an underfloor heating (UFH) system is not just a case of drawing lines on paper, it also heavily involves the use of engineering science. Over the last decade, the UK has seen an increase in the use of underfloor heating, with the actual performance of the heat emitter often overlooked or unknown at the design stage.

UFH systems were traditionally installed in newbuild properties within a 75mm thick sand and cement screed, or more commonly a 50mm thick anhydrite flowing screed. As UFH increased in popularity, the market started to develop new systems to be used within retrofit applications and older properties. Typically, these new, retrofit systems can be categorised as either a "floating floor" or a "suspended floor", with an increasing emphasis on low profile systems.

Underfloor heating provides the most comfortable, even warmth of any heating system. It is economical to run and virtually maintenance free. UFH systems are designed to operate at lower temperatures than radiator systems, making them especially suitable for condensing boilers and heat pumps, resulting in reduced energy consumption and lower heating costs for the property.

The technology itself is fairly straightforward. Heated floors act as low-level radiators, distributing heat evenly into each room, steadily warming the living space through a combination of radiant energy and heat conduction. In a modern, well-insulated home where heat loss factors have been taken into account, UFH can comfortably perform as a primary medium for distributing heat, removing the need for radiators and allowing more open space and fewer restrictions within a room.

'Theory alone is useless. Practical alone is dangerous'

The market as a whole has concentrated heavily on designing systems that only deal with the practical application of the product, such as floor to ceiling height restrictions within existing properties, rather than the actual performance and heat output of the emitting surface.

This has led to widespread confusion for both the installer and the consumer. Depending on what type of floor heating system is used, an intermediate layer needs to be added to prevent point load issues. A common mistake made by designers is planning a system without this intermediate layer, for example, a 10mm plywood layer between a grooved dry screed board and a carpet. Adding this thicker, structural layer, reduces the risk of point load issues, but it also consequently reduces the heat output of the floor. It is very much a balancing exercise.

Industry relies heavily upon the BS EN 1264 Part 2 standard for calculating the thermal output of a heated floor, which sets out both a manual calculation and a procedure for carrying out live tests. Due to the amount of variables present within a UFH system, the manual calculation for type B systems becomes inconsistent, and this has led to other, more accurate methods being adopted, such as detailed CFD and conjugative heat transfer simulations using the finite element approach.

Accurate calculations are crucial

The overall heat output of a floor heating system is affected by many parameters, most commonly: pipe spacing, pipe diameter, water flow rate, Delta T or ΔT, water temperature, floor covering resistance and the thermal properties of the heat conducting layers.

In addition to this, the fluid mechanics of both the air above the finished floor covering and the flow within the pipe can significantly affect and vary the overall result. It is worth pointing out that due to the complexity of these calculations, these are beyond the scope of this article.

Once a system has been tested or simulated, then the following formula stated in BS EN 1264 Part 2 is used to create heat output tables at varying water temperatures:

q = KH ⋅ (∆ϑH) n

q = Specific thermal output

KH = Equivalent heat transmission coefficient

∆ϑH = Temperature difference between the mean water temperature and the air above

n = exponent

The greater the kH value and the temperature difference, the greater the heat output of the floor surface in W/m2. The kH value is different, however, for each type of UFH floor construction, and is the result of detailed testing.

The CIPHE believes that there needs to be a review of the BS EN 1264 standard and the way the industry is currently designing systems. It has become common practice for UFH systems to be installed without any correct heat output data to support the designs - with some marketing material often being misleading, too.

That said, a correct set of UFH heat output data is useless without knowing the true heat loss of the building for which the system is to be installed. Therefore, if the information provided at the very start of a design is incorrect, for example the room by room heat loss data, then the whole design will not be fit for purpose. It is therefore critical that a valid and accurate set of heat output data is known before designing or installing the complete UFH system.

Educating the Industry

To support the future education of the industry, the CIPHE is carrying a research project in conjunction with its Industrial Associates into the thermal output of systems.

Please contact Paul Harmer at†paulh@ciphe.org.uk†for further information.

Pictured: Not including a plywood layer between a dry screedboard and a carpet is a common error when calculating heat outputs at the design stage.

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