Turning Down The Heat

As lower grade heat sources for space heating become the norm, and fabric insulation levels improve, it's important to take a fresh look at the choice of heat emitters in classrooms. David Shuttleworth of Dunham-Bush explains how to get the best response from the system. There have been some significant changes in the ways that heat for space heating is generated in schools over the last few years, but in some cases the approach to selecting heat emitters in the classroom hasn't kept pace. Consequently, many teaching spaces have heating systems that fail to maintain the required comfort levels. One of the major changes has been the move to condensing boilers which now offer a good turn down, where flow and return temperatures of 55°C/35°C will maximise condensing and deliver maximum energy savings. Similarly, many more local authorities are encouraging the use of heat pumps, requiring flow and return temperatures of around 50°C/45°C to improve the COP in cold weather. All of which means that the heating in the classrooms needs to be suitable for lower temperature hot water than would have been the case just a few years ago. Very often, the first option that springs to mind is underfloor heating but it's important to consider whether this will provide the rapid warm up or the responsiveness to changing conditions, necessary to conserve energy. As school buildings have become better insulated, the steady state heat loss has reduced but the thermal mass has remained almost constant, leading to protracted warm-up periods at full load. Once up to temperature, it's important for the emitter to match the heat loss and accommodate any internal heat gains with as little overshoot, or undershoot, as possible. Clearly, underfloor heating simply isn't designed to offer this level of rapid response, so a teaching or meeting space with variable occupancy is likely to experience dramatic temperature fluctuation during the day, exacerbated by the changing orientation of the sun. In contrast, fan convectors offer a fast response on warm-up and to changing internal temperatures, taking advantage of internal and solar heat gains to reduce energy consumption. In fact, recent tests carried out by an education authority on two similar schools with similar condensing boiler plant suggested that low grade heat fan convectors used 30% less energy than underfloor heating to maintain the same design conditions. Furthermore, fan convectors have been designed to work with a wide range of water temperatures, including the 55/35°C and 50/45°C flow/return temperatures that optimise condensing boiler and heat pump performance. They can also be supplied in low surface temperature versions (not exceeding 43°C surface temperatures) where this is required by local guidelines. However, it is a fact that some local authorities have chosen to move away from fan assisted heating because of perceived energy disadvantages. It could be argued, though, that the motor power consumed is insignificant to the savings that can be achieved with a highly responsive system. Where there is a requirement to use heating systems based on natural convection; perimeter, trench or individual natural convectors with low water volume heating elements and local controls will still provide rapid response, improved comfort and energy savings. The use of steel panel or aluminium radiators in classrooms will present problems such as excessive surface temperature and the use of greater wall space when compared with either fan assisted or natural convectors.