How low can you go?

Activities

The last few years have seen some significant changes in the ways that heating and domestic hot water are generated in buildings. Most notably, the total reliance on boiler plant has switched so that many systems now include renewable and other low carbon heat sources. For example, whereas combined heat and power (CHP) was once the preserve of large estates and industrial facilities, there are now ‘mini-CHP’ units available for applications such as smaller care homes, residential blocks, fire stations, schools, health centres and larger private residences. Even where CHP isn’t used, there may be other low carbon heat sources such as solar thermal and heat pumps, used in conjunction with biomass and/or fossil fuel boilers.

As many buildings move away from just using boilers for heating to a mix of different heat sources, such as CHP, the system operating temperatures have become crucial to achieving the anticipated energy and carbon savings.

Lars Fabricius

Also, there is now a strong trend towards housing these various heat sources in a central energy centre serving a piped distribution system. This system could be a traditional radiator system, or small or large scale district heating system serving heat interface units in individual spaces.

All of which clearly highlights the variation that can be found in the configuration of heating systems. However, the principles described below are equally applicable to any of these.

Time for a re-think

This sea-change in the approach to heating systems and the need to accommodate different heat sources with varying characteristics presents a challenge, in terms of the way such systems are designed. In fact, the difference between the flow and return water temperatures (the ‘Delta T’) has, or should, now become the overriding design consideration if the energy and cost-saving potential of the system is to be realised.

It is worth noting that where the anticipated energy savings from low carbon heat sources fail to materialise, it is nearly always because the Delta T is wrong, in relation to the requirements of the heat sources in use.

When everybody just used non-condensing boilers things were far more straightforward. Typically the system would be designed for flow/return temperatures of 80°C/60°C because that suited the boilers – and this approach served us very well for many years.

Now, though, the various heat sources in common use have different preferred flow/return temperatures, typically lower than 80°C/60°C. For instance, the optimum Delta T for gas-fired condensing boilers is 55°C/30°C; for heat pumps it is 40°C/35°C. Consequently, we need to re-think our approach to system operating temperatures and, in particular, design more flexible systems using variable temperature heating circuits.

Furthermore, a ‘lower temperature’ mindset is better suited to the climatic conditions in the UK. Our climate is relatively mild but heating systems are generally designed to address the few cold days we experience each year. High flow temperatures are the norm and attempts to reduce them in milder conditions have varying success. With lower flow temperatures as ‘standard’, the heating systems can operate with optimum efficiency for most of the year and then be ‘ramped up’ for a few days as necessary in the depths of winter.

This need for a re-think has recently been recognised by the Chartered Institution of Building Services Engineers (CIBSE). The latest version of the guidance document CIBSE AM12/2013 ‘Combined Heat and Power for Buildings’ states:

“It is recommended that, for new systems, radiator circuit temperatures of 70ºC (flow) and 40°C (return) are used, with a maximum return temperature of 25°C from instantaneous domestic hot water heat exchangers.”

In fact, this guidance applies equally well to all piped distribution systems in any size of building, whether or not CHP is included. So how do we go about achieving these more efficient designs?

Delta T controllers and thermal stores

One of the key features of CHP is that it generates both heat and electricity – you can’t have one without the other. So the cost and environmental benefits of using CHP are linked to the time the CHP plant runs. Therefore, where CHP is included in the mix of heat sources, and a good Delta T is achieved, a thermal storage vessel should also be included in the system. When the demand for hot water is low, any excess heat can then be stored in the storage vessel, so that the CHP can continue to operate and generate electrical power for use in the building

Consequently, achieving a good Delta T and utilising a thermal store are both critical elements in maximising the return on investment in CHP.

The first thing to do in ensuring there is effective Delta T control in the system is to reduce flow temperatures to as low a temperature as possible while ensuring sufficient heat is delivered to the terminal units.

Reducing the flow temperature then has the potential to reduce the return temperature, as long as sufficient heat is removed from the water at the terminal unit (e.g. radiator, fan coil). This can be achieved by reducing the flow rate, so that the flow water spends more time in contact with the heat exchanger and loses more heat to the space. The same principle applies when plate heat exchangers are used for the domestic hot water (DHW) rather than hot water cylinders.

Achieving a good Delta T therefore involves the use of a variable speed pump so that the flow rate of the water can be reduced to maximise heat transfer at the terminal unit. An added benefit of this is that it enables use of a smaller pump that costs less to buy and operate.

In turn, variable speed pumping necessitates the use of two-port valves such as thermostatic radiator valves (TRVs). However, two-port valves are not designed to function with variable pressures, so differential pressure control valves (DPCVs) should also be included to enable the two-port control valves to operate as designed.

When all of these factors are taken into account it is clear that a few relatively simple changes to the way we think about heating system design can help to realise the full potential of low carbon energy sources. Furthermore, adopting the principles of 70/40 flow/return temperatures will deliver those benefits to all sizes of application, from schools and nursing homes to the largest office blocks and factories.