HEAT PUMPS | HIGH TEMPERATURE Figure 3: Cascading solution, using upper and lower portions of thermal store overall efficiency of the building sufficiently, without the need to replace air handling unit (AHU) coils. Figure 1 shows a two-boiler reverse return header setup, with CT and VT circuits, that is typical of many 82/71C legacy designs. Using data from the current VT setup (if available), the VT minimum temperature can be reduced below current settings to assess whether the target space temperatures may still be maintained. Even a modest reduction in flow temperature will ensure higher efficiencies. Most HT ASHPs would deliver a coefficient of performance (COP) of approximately 2.2 at 65C flow and -2C ambient conditions. If we were able to drop the weather compensation to below 60C, output from medium-temperature (MT) and HT ASHPs may be blended, potentially reducing any siting or budget complexities of a full HT. In the blended MT/HT example solution shown in Figure 2, the heat pumps are cascaded, with a three-port diverting valve being used to deliver heat to the calorifier. Typically, the MT ASHP(s) would act as lead for the directly weather-compensated circuit, supported by the HT ASHP(s) during peak demands. When higher temperatures WHY HEAT PUMPS SUIT FABRIC FIRST The main reason heat pumps are tied so intrinsically with fabric rst is related to ow temperatures. ASHPs are more efcient when the ambient temperature is higher and/or the sink temperature is lower. Ambient temperature is highly variable in the UK and not within our control. The sink temperature, or ow temperature, is within our power to reduce, when and if we can. Lower ow temperatures reduce emitter outputs if they were originally designed for higher mean water temperatures, but if less heat is required because of fabric upgrades, the equation should balance itself out. are required for more challenging design conditions, the HT ASHP can increase the thermal store temperature up to 80C. If Figure 1 (the reverse return arrangement) had been designed on a T of 20K, one solution is the alternate cascading method shown in Figure 3. This uses the thermal store lower and upper stratified sections to provide a cascaded temperature rather than the load. This is an identical philosophy to that used with most hybrid solutions. Temperatures are still key, and with good weather compensation a blend of MT and HT ASHPs can still work. However, this solution is more suited to HT ASHPs as, at a higher design temperature of 80/60C, MT ASHPs are unable to delivery any useful heat. Managing expectations As clients start to review boiler-replacement projects and the achievable options to decarbonise asset heat, we must still ensure fabric options are considered first. Emitter replacement to suit lowtemperature heating will tend to yield a higher efficiency overall than a hightemperature heat pump system. Heat pumps should not be over-specified in terms of capacity, for economic and spatial reasons. Ultimately, perfect is the enemy of the good when it comes to decarbonising buildings unable to offer a significant improvement from fabric upgrades. However, a well-considered assessment of the building thermal profile delivers essential intelligence for a clearer understanding of the impact of the potential system solutions, to ensure true life-cycle benefit. CJ RYAN KIRKWOOD is engineering solutions manager at Baxi OMICRON Zero Eurovent certified performance First air-source multifunctional heat pump and chiller unit using propane as a near zero GWP, natural refrigerant solution. 36 March 2024 www.cibsejournal.com CIBSE March 24 pp33-34, 36 Baxi Heat Pump.indd 36 23/02/2024 13:48