Running hot, cold and green

How about a floor slab which slashes build times, saves on construction costs, running costs and is good to the environment? Warmafloor, which supplies underfloor heating pipes, normally cast in-situ, has teamed up with Tarmac Precast Concrete to come up with exactly that.

Thermocast is a system of precast floor units, with cast-in polybutylene pipes through which you can run cooling or heating water. You can't control temperature as closely as with conventional air-con and heating systems, but you can open windows. And it uses far less energy.

Research by Oxford Brookes University and the Steel Construction Institute found that water-cooled slabs in a steel framed building used 20% of the energy of conventional fan-coil air conditioning system, and reduced Carbon Dioxide emissions by 8kg CO2/m2 per year. To put that in context, for a 10,000 sq m building that's about a quarter of what an average car produces in a year.

After two-and-a-half years of development and testing, Tarmac's talking to a number of contractors about using it on their projects. The lower running costs and easier maintenance make it an attractive proposal for PFI jobs.

Cool idea, warm reception
Underfloor heating is an ancient technology which the Romans first employed by passing hot air through voids in their floors. In the UK, buried pipe technology was first used in the 1930s, becoming popular in the '60s.

Swiss engineer Robert Meierhans is considered the first person to make use of the cooling effect, around 15 years ago, and since then it has become popular in the Netherlands, Germany and Switzerland. It's only in the last four to five years that the use of pipes in concrete for cooling has taken off in the UK according to Mike Lamb, managing director of Warmafloor and subsidiary Structural Conditioning, set up for the new venture, has installed its pipes for heating and cooling in hundreds of buildings, including 300 schools.

All these UK projects see the pipes installed in-situ, whereas in Germany they come attached to prefabricated reinforcement cages. Precast units were once used on a building called the Basilica in Basildon which was completed in 1999 for client the Barclay Group which wanted an energy-efficient building. Engineers at Troup Bywaters + Anders came up with the idea of creating a passive cooling system by running water from a lake through concrete units with exposed undersides.

The project was a success, but Troup Bywaters + Anders have not been able to use a similar set-up on other jobs. Engineer Rex Bunker explained that economies of scale mean that only big projects are viable. The Basilica is 10,405 sq m.

Warmafloor approached Tarmac Precast Concrete to develop a standard precast system. It has been two and a half years in development and testing.

The advantages of off-site are less risk of injury to workers and increased quality. Getting a good finish is key because of the bare concrete soffit. The polybutylene pipes, first cast in at the Tarmac works, will be tested before the units go to site. Spanning up to 13m, the exposed soffits can arrive with the light-fittings in place. Once the units have been craned into position they are connected together and to the cooling and/or heating circuits.

Thermocast is a form of 'structural conditioning' which means it uses part of the structure of a building as a heat exchange. Heat is transferred through the large surface area of exposed concrete ceiling by a combination of radiation and convection.

The cathedral effect
Because of the large surface area and the ability of concrete to absorb large quantities of heat, cooling can be achieved with relatively high temperatures of water running through and heating with relatively low temperature. Concrete's ability to absorb, store and radiate heat or coolth is sometimes referred to as 'thermal mass' or 'the cathedral effect'.

Christian Struck, an engineer with Buro Happold, explains that chilled beam systems require water at 6 degrees C, whereas water at 12 to 14 degrees C can lower the surface temperature of the concrete units to 20 degrees C, which can cool the room. "You can forget about mechanical ventilation generally if you have the possibility to use windows," says Struck.

This means that water from a lake, river, or canal can be used to cool the water which passes through the units, by means of a heat exchanger. While lakes or rivers are not always around, boreholes are often an option in the UK, says Lamb, as groundwater is plentiful. The advantage of groundwater is that it is renewable, but chillers can be used if that is not an alternative.

Sometimes cooling is required throughout the year, but if heating is required, raising the temperature of the circulating water to between 24 and 30 degrees will achieve the same effect as heating to 55 degrees C for a boiler driven system, says Struck.

David Arnold of Troup Bywater + Anders who designed the Basilica calculated that a Thermocast-type system which uses groundwater costs £50 per sq m less in capital costs, £3 per sq m less to run and £3.35 per sq m less to maintain.

For the occupiers of a building, there are advantages in that the system produces no noise and there is no spreading of germs or 'sick building syndrome'. But since concrete takes a lot of energy to change temperature, it isn't possible to change on request. Opening windows is a benefit in some locations, but an impossibility in others.

For developers, there is the advantage that less floor space is taken up by hefty plant rooms. But concerns over tenant comfort and therefore the risk of not letting the space outweigh that. And unless the promise of lower energy bills translates into higher rents, which it doesn't currently, property owners won't be interested.

But reducing CO2 emissions is moving up the political agenda, as is sustainability. And since 90% of a building's environmental impact comes during its usage, Thermocast may be able to make the argument in the public sector and with sustainability conscious owner-occupiers.