Lower temperatures for higher efficiency

Activities

With the move to using a mix of different heat sources to achieve low carbon heating, system operating temperatures have become increasingly important. Lars Fabricius explains why: In recent years there has been a move to combining a number of different heat sources within a heating system. These include mini Combined Heat & Power (CHP) units that have enabled many smaller applications to make use of this energy and carbon saving technology, as well as heat pumps, solar thermal and the more traditional condensing boilers. Crucially, these technologies operate more efficiently at lower return water temperatures than are traditionally used in UK heating systems – namely 80° flow/60° return. Consequently, there is a need for a re-think in the way such systems are designed.

“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.”

Chartered Institution of Building Services Engineers (CIBSE), updated guidance.

As a result, the difference between the flow and return water temperatures (the ‘Delta T’) is, or should be, now becoming the overriding design consideration if the energy and cost-saving potential of the system is to be realised. In fact, 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.

Furthermore, while the CIBSE guidance refers to CHP systems the same principle applies to many of the other heat sources that are now commonly being used. 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.

Additionally, lower return water temperatures are better suited to the relatively mild UK climate, where heating systems are often ‘over-sized’ for the few very cold days we may experience each year.

Delta T and CHP

One of the fundamental characteristics of CHP is that it generates both heat and electricity. You can’t have one without the other, so the overall energy and carbon benefits are inextricably linked to the run-time of the CHP plant. Consequently, where CHP is included in the system the design should seek to ensure maximum run-times for the CHP plant by achieving a good Delta T and using a thermal storage vessel to store hot water when demand is low. In this way the CHP continues to operate and generate electrical power for use in the building or export the grid.

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

In ensuring that there is effective control of Delta T in the system, the first step is to reduce flow temperatures (e.g. to the 70° described above, rather than the traditional 80°). Then, as long as sufficient heat is removed from the system via the radiators, fan coils or other terminal units, this will reduce the return water temperature. This higher level of heat transfer to the space being heated will be greatly facilitated by reducing the flow rate, so that the water spends more time in contact with the air it is heating. The same principle applies when plate heat exchangers are used for the domestic hot water (DHW) rather than hot water cylinders.

A further benefit of this is that variable speed pumps can be used to control this flow rate, as slower flow rates can be achieved with smaller, variable speed pumps, resulting in lower capital costs and reduced pump energy consumption.

To a non-engineer these measures may seem complex but are in fact relatively simple to incorporate into the design of the system. Teaming up with specialists that understand the benefits of designing for 70/40 flow/return temperatures will help to deliver maximum energy, cost, and environmental benefits.