The heat of the moment
It’s now more important than ever to ensure that space heating systems are tailored to reflect the characteristics of individual buildings and the activities within them. Ant Wilson of AECOM explains
It’s probably true to say that the building-services industry has undergone more change in the last 10 years than at any time in its history — all driven by a panoply of regulations. Not just those relating to services design, but also others that impact on building structure and how workplaces are used. The result is that it is more important than ever to try to ensure that each heating system is optimised for the building in which it is installed.
Space-heating systems are a case in point because their performance is inextricably linked to the building fabric and the activities in the building. Consequently, an appropriate system needs to address the type of heat generator, the distribution of the heat around the building, the heat emitters, the controls and how the system is commissioned and maintained through its life.
For instance, the increased use of condensing boilers and heat pumps dictates designs that use lower-temperature hot water which, in turn, influences the choice of heat emitter. There is also an increasing need to design for integration of different heat sources — making the control aspects important.
However, the starting point has to be the building itself, and it is obvious that thermal insulation levels and solar heat gains in the spaces will impact on the type of heat source, its capacity and the nature of heat emitters used. For instance, with a modern façade there should not be any cold surfaces on exterior walls, so the direct impact of the heating system on the space becomes more important than its ability to prevent heat losses at the perimeter.
Similarly, while large spaces such as factories have traditionally been fitted with radiant heating, as the buildings themselves become better insulated and more airtight there is less concern about losing warm air through the fabric, thus giving the designer more flexibility with the types of system that are appropriate.
This, in turn, raises the question of whether we are heating for comfort or for the thermostat?
Heating for comfort is very subjective and depends on circumstances the specifier should be aware of. A rather extreme example could be a man working close to a furnace all day; his face and chest will certainly feel very warm, but his back will feel cold in comparison. Warming the air to 21°C will not make much difference to his perception of comfort, but a radiant heat source warming his back will.
So understanding the use of the spaces is another important criterion. In a space that has virtually constant and predictable occupancy, with predictable internal heat gains, a system that maintains a steady condition and uses minimum energy through the day (or night) — such as underfloor heating — may be ideal. Underfloor heating also makes effective use of lower-grade hot water.
However, such systems are less suited to providing a fast response in spaces with variable occupancy. A more responsive system is likely to offer the best option, and, again, the type of building will influence the decision. For instance, a church may only be used for limited periods, so there is little point in heating the whole area all the time. Also, most traditional churches are quite ‘leaky’, so heating the air could be wasteful. This situation, therefore, might lend itself to a zoned radiant heating system that heats the people, when there are people there to heat.
In a well insulated space with variable occupancy, however, a fan-convector system will offer a more responsive solution that can also take advantage of lower-grade hot water. Clearly, there is a balance to be reached between the energy savings from a more responsive system and the extra power consumed by the fans, and that needs to be part of the design process.
In parallel with these considerations, it is important to be clear whether the key criterion is energy consumption or carbon emissions. A radiator system will have significant embedded carbon because of its constituent materials. A fan convector has a lower mass of materials but some will need to be replaced more frequently, thus we need to take account of the whole life costs and sustainable procurement arguments of the installation.
Underlying all these considerations is the sustainability imperative to ‘do more with less’. However, there is a danger that we are starting to ‘do more with more’ when putting together hybrid systems that integrate a number of different heat sources simply to tick sustainability boxes. To avoid this, we need to stick to tried-and-tested engineering principles and apply them within a design that takes account of all the factors that have been discussed here. Then we can be sure we are giving end clients the best solution.
Ant Wilson is a director of AECOM (formerly Faber Maunsell)
