UNDERFLOOR HEATING AND COOLING
Throwing more energy at a building to keep it cool in summer and warm in winter is no longer an option with rising energy prices and the need to tackle global warming. With more innovative methods of regulating temperature required, developments in wet underfloor heating and its companion, underfloor cooling, make them an interesting option.
HEATING These are similar to systems used by the Romans thousands of years ago. Wet underfloorheating systems, however, have been slow to catch on in the UK market, compared with those on the continent. In the majority of applications, underfloor heating uses less energy. This is predominately because of the delivery of warmth through more radiant heat transfer than a conventional radiator system (in which most of the heating effect is via convection).
Generally, air temperatures are lower, with the same level of comfort for the occupants.
Compared with a conventional radiator system, the temperature of the water within an underfloor system is much lower, which makes underfloor heating ideal for use with high-condensing boilers. Often the water can be fed directly into the floor from such boilers without the need to pre-mix down to a lower temperature. Because underfloor heating systems use lower-temperature water they can also be used with nonconventional heat sources, such as geothermal heat-recovery systems. Coils of pipe are either laid 2m deep in a large, open area or placed into piles around the building. The ground will remain constant at about 10ºC throughout the year. With the use of a heat pump, heat energy can be extracted from the ground and used to heat the building.
COOLING Underfloor cooling is simply a conventional, wet underfloor heating system into which chilled water is passed to cool the building in the summer.
Most of the components are identical for heating and cooling systems: the most significant difference is in the control systems. Most underfloor heating systems are controlled by the different circuits or zones running from the manifold, fitted with a zone valve and thermostat. When an individual area reaches the required temperature, the flow to that area is cut. With cooling, the controls are more complex.
Underfloor cooling will need to be linked to a buildingmanagement system. Index rooms are also required and these will be either in a temperature-neutral part of the building or be in pairs:
one north facing and one south facing. The index rooms work with the building-management system to monitor whether to cool or heat the building.
For example, on a hot, sunny day the building-management system switches the main control valve to the chiller side.
Chilled water is then passed through the floor, cooling it below the ambient temperature.
The choice of floor covering has a much greater effect on an underfloor-cooling system than on a similar underfloor-heating system. With the exception of thick shag-pile carpet, underfloor heating can cope with the majority of floor coverings. Underfloor cooling can only work with a limited variety. Coverings will need to be stone or ceramic, or a fully- bonded vinyl or commercial carpet. Underfloor cooling requires the floor to be of a solid concrete construction and it cannot be used in first-floor timber construction.
An issue that must be considered with underfloor cooling is the minimum floor temperature and the dew point.
The floor temperature must never be cooled below the dew point because of the risk of condensation forming on the surface - which could rot a carpet or be a potential slip hazard on solid floors. The ideal floor temperature for comfort reasons should not fall below 20ºC. This is why a building-management system is needed to control it. It can monitor the relative humidity and either restrict the amount of cooling or control the humidity with some form of air-handling system.
In a well-designed system, the switch from cooling mode to heating mode will not take place in the same day, as this would be energy inefficient.
The system will heat in the morning and go off in the afternoon or vice versa.
COOLING POTENTIAL Underfloor cooling will never replace conventional air-conditioning systems.
It offers a level of comfort cooling that can be ideal for our moderate climate. Because of its relatively inexpensive installation costs, it is considered for applications where full air-conditioning systems would not be appropriate.
The majority of new schools under construction are using underfloor heating systems, where it is ideal because of its lack of hot surfaces and radiators. As such projects often also have building management to go with the systems, cooling can be provided for the moderate additional cost of a small chiller.
Large glass atria are another application for underfloor cooling. Having large glazed areas can give huge thermal gain and the size of plant required to control this with air-handling units would be enormous. Underfloor cooling can be installed in such areas with a moderate budget, and the output of such systems is limited to the occupied level - providing optimum temperature at chest height. The rest of the space can remain at a higher temperature without affecting the occupants.
Underfloor heating is now a standard part of many projects and its inclusion is becoming orthodox. Underfloor cooling is unorthodox and its inclusion in projects needs to be considered at an early stage of the design, with the entire design team.
Mike Mosely is major projects manager for Rehau