CPD PROGRAMME | HEATING AND HOT WATER capacity/cost/availability constraints whole-life carbon considerations a heat pump that is sized for a load that is only likely to be exceeded for a certain proportion of hours or when the ambient temperature exceeds or falls below a certain temperature the level of uncertainty in loads since an undersized heat pump will lead to overreliance on secondary systems. ASHPs provide a particular challenge in a bivalent system when paired with condensing boilers owing to the different requirements for flow and return temperatures. Typically, heat pumps work best at lower flow temperatures (30C to 50C) and a flow/ return temperature differential, , of 5K to 10K. While condensing boilers also operate more efficiently at lower temperatures, the range for a typical commercial boiler is 10K to 40K. At its simplest, it may be convenient to design to a 10K differential which, coincidentally, is similar to the 11K , such as 82C flow, 71C return, as employed in older commercial heating systems. However, this loses the advantages derived from employing higher differential temperatures, which could include smaller pipe sizes, reduced volume flow and lower pump duties. Employing a thermal store allows the accumulation of heated water, so flattening the peak demand, at the same time also providing opportunity to hydronically decouple the two generators while maintaining thermal connection. A thermal store allows lower capacity heat generators than would otherwise be needed, and will also reduce their on-off cycling frequency, since at every on-off cycle there is a decrease in overall efficiency as a result of start-up losses. AM17 notes that an excessive number of on-off cycles can damage the compressor (in a heat pump) as well as impact the efficiency, and manufacturers typically limit the number of on-off cycles to a maximum (dependent on the system), which could mean that a heat pump may not restart within 15 minutes (or longer) of shutting off. The thermal store will also provide a resource for any defrost cycle for the heat pump. Using a load assist method, the ASHP would run as the lead provider of heat to meet the base load, with boiler(s) used to assist as heat demand increases. However, the primary flow temperature and consequent temperature differentials must be suitable for both the ASHP and boiler technology. Running a full system with a of 10K would not be an issue for most boiler technologies, but it can reduce the performance of many ASHPs that typically perform better when operating with a of 5K to 7K. To ensure that the condensing gas boiler operates in a condensing mode, the return water temperature must remain under 54C (and preferably lower). An example of a load assist arrangement is shown in Figure 1. In this system, the ASHP is used to heat the thermal store, and then the heated water from the thermal store provides the lead heat source. The thermal store discharge pump is modulated to match building load. It should never be allowed to fully deplete the resource of heated water from the thermal store, as this would disrupt the thermal stratification and potentially deliver a flow temperature that is below the maximum potential. To avoid this, the boilers should be cascaded on, to assist with the demand prior to the store being depleted. The load ratio of boiler to ASHP in this arrangement would be project-specific. If both the ASHP and boiler(s) are required to satisfy peak demand, any risks arising from the lack of redundancy must be considered. The advantages of the load assist arrangement are flexibility and scalability, since both generators can run together or independently as demands fluctuate. An alternative is the thermal store method that utilises a boiler and ASHP to feed into a common thermal store, and the heating power required to satisfy system loads is shared. The ASHP maintains a stratified warm layer at the bottom of the tank to heat the cool return. The boiler draws warm water from the top of this layer and raises its temperature to the target store outlet temperature. Under non-steady state load conditions, care should be taken to avoid the boiler being required to top up the temperature by less than 10K, otherwise a temperature overshoot could occur in the tank, since most condensing boilers are unable to operate with less than 10K . The introduction of a mixing valve to blend flow temperatures on the demand side of the thermal store can help resolve this. The controls should ensure that the boiler contribution is held back until absolutely required. This is achieved through close monitoring of tank temperatures, at multiple points, together with the feedback of boiler and ASHP flow temperatures. The injection method is very similar to the thermal store method. This approach uses the boiler(s) to boost the flow temperature to the required set point at times when the ASHP is unable to satisfy the demand. The system, illustrated in Figure 3, delivers heat from the ASHP to a return header prior to the low-loss header (LLH). During periods of low or zero demand, the thermal store is charged by the ASHP, with a discharge pump injecting into the return header. This preheats the return to the LLH, which is then topped up by the boilers. When the thermal store is charged to a usable flow temperature, the system can hold off boilers until required. As the thermal store starts to deplete, the control system will actuate the boilers, providing a top-up to the preheated water. Typically, there is approximately 1%-2.5% drop in efficiency for a gas condensing boiler when preheating a 30C return by 5K-10K, as a result of reduced flue gas condensation. Figure 1: Example of a load assist arrangement 94 November 2022 www.cibsejournal.com CIBSE Nov 22 pp93-96 CPD 205.indd 94 21/10/2022 16:55