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Under the surface: The increasingly multifunctional nature of concrete floors

Surface heating and cooling using pipes embedded close to the soffit is proving an increasingly popular technique. Tom De Saulles of the Concrete Centre looks at the merits of this approach.

There is an old adage that goes something like, ‘Since time began and things were made, nothing has changed in the building trade’.

It’s a great line that still resonates, though perhaps a little less true than in previous years.

One area where change is quite apparent is in the increasingly multifunctional nature of concrete floors, expanding to include aesthetics and building services alongside their primary structural role.

Increasing use

The use of embedded water pipes to provide surface heating and cooling is becoming a common element, though even this is not entirely new – it was first tried back in the 1930s by Frank Lloyd Wright in America and Oscar Faber in the UK.

This proved effective, though the steel pipes used in these early systems eventually failed due to corrosion.

“Surface heating and cooling using the soffit is simple, discreet and more energy-efficient than conventional air conditioning”

With the arrival of plastic pipes made from materials such as cross-linked polyethylene (PEX), this problem has been overcome.

PEX has been used in underfloor heating for some years and is now being incorporated into the exposed concrete soffits of low-energy buildings too, for the purpose of both heating and cooling.

Only a few years ago, this would have been considered unconventional, but it’s now becoming a mainstream technique used in a variety of building types.

Surface heating and cooling using the soffit is simple, discreet and more energy-efficient than conventional air conditioning, which raises the question of why it has only recently started to take off.

The answer may partly lie in the British Council for Offices’ Guide to Specification.

In 2009, this raised the recommended internal summertime temperature to a more easily achieved 24 deg C, making the comparatively modest cooling output of surface-based systems a far more viable option.

Built-in best of both worlds

Essentially, the system comprises plastic pipes located about 60 mm above the soffit and arranged in a serpentine pattern across the floor slab, with spacing of around 150 mm.

Typically, the pipes are fixed to the steel reinforcement prior to the concrete pour.

During the summer months (or whenever cooling is needed), chilled water is pumped through the pipes, at a temperature close to that of the occupied space.

This is made possible by the large surface area for heat transfer provided by the soffit, and allows the use of efficient sources of heating and cooling, such as ground-water and ground-coupled heat pumps.

Thermal mass provided by the floor slab also plays its part by attenuating peak heat gains and smoothing out the cooling demand across the day.

“In many ways, surface heating and cooling offers the best of both worlds”

This in turn allows the building’s cooling needs to be met by a relatively small building services installation and also by natural ventilation if required.

The combined effect is lower capital and operating costs, plus less space needed for plant.

When considered from the perspective of the developer and operator, there is a more subtle benefit over many passive cooling techniques: risk.

The ability to continually regulate the soffit temperature regardless of what the weather is doing outside ensures greater control of internal conditions and the ability to avoid overheating – something that can be more difficult with a passive approach reliant on natural ventilation.

In many ways, surface heating and cooling offers the best of both worlds.

Active water cooling provides better control and cooling output, while still being able to take advantage of free cooling from passive ventilation whenever conditions permit.

What to do with services

While surface-based systems avoid the need for elements such as radiators and fan coil units and their associated pipework, they also introduce the problem of what to do with overhead services normally hidden by a false ceiling.

Happily, there are a number of standard solutions that can be used to address this.

These include:

  • Grouping systems such as lighting, fire alarms and sensors into a services raft suspended from the soffit.
  • Using floor voids, perimeter bulkheads and ceiling voids in corridors for ventilation ductwork.
  • Using the cores in hollowcore slabs for pipes and cabling.
  • Integrating services into the design of in-situ or precast floors – that is, casting in rebates and conduits for wiring, pipes, ventilation grilles, smoke alarms, lighting, etc.

Floor and soffit options

Usefully, surface heating/cooling does not limit the range of design options for concrete floors, with both precast and in-situ solutions proving equally suitable.

For many, a straightforward off-the-shelf solution using precast may be the preferred option.

This can be supplied in the form of hollowcore or lattice girder units, both of which provide similar thermal performance but a different visual finish.

The lower-cost hollowcore option is produced using an extrusion process, giving a slightly more utilitarian appearance to the soffit, which is usually painted.

Lattice girder units are cast on a steel bed, giving a smoother surface, more suited to a fair-faced finish, if required.

Whichever precast system is used, both benefit from the pipework being installed under factory conditions, where it can be pressure-tested before site delivery.

“The growing uptake of surface heating/cooling in concrete floors is testament to the synergy that exists between the two”

Alongside these proprietary precast systems, there is also the option of using in-situ concrete floors. Alternatively, bespoke precast floor units can be specified.

These two approaches offer design flexibility, including the option to specify a profiled slab, which can increase both the span and cooling output, and is not currently available with the standard precast systems.

Post-tensioning can also be used without interfering with the embedded pipes.

The drawback, as always with a bespoke approach, is the need for more upfront design and, in the case of in-situ floors, an increased potential for damage to the pipework during construction.

However, consultants and suppliers can help with the design and ensure the build programme runs smoothly.

The growing uptake of surface heating/cooling in concrete floors is testament to the synergy that exists between the two, which is helping to meet requirements for comfort, energy efficiency and whole-life performance both in terms of cost and carbon emissions.

Tom De Saulles is senior manager – building sustainability at the Concrete Centre

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