RENEWABLES
Retrofitting with high temperature heat pumps
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High temperature heat pumps provide increased options to decarbonise hard- to-heat buildings, but investigating the building and system first is essential to ensure appropriate design and specification, says Baxi’s engineering solutions manager, Ryan Kirkwood
s businesses and organisations consider the achievable options to reduce emissions from heat, one option that is nearly always fast, efficient
and affordable is a hybrid solution combining heat pumps and boilers. But what if the client brief is to move straight to full electrification of heat? Some may argue that this is the very purpose of high temperature air source heat pumps (HT ASHPs). Unfortunately, the solution is not as simple as it is sometimes made out to be. This article considers possible solutions for
retrofitting HT ASHPs and key areas to investigate before undertaking such as project.
Understanding design and operational temperatures
The main challenge with decarbonising heat in older buildings is that older systems typically operate at flow and return temperatures of 82/71°C or potentially 80/60°C. But although modern R290 (propane) ASHPs, like our own Baxi Auriga+, can deliver up to 80°C flow temperature, this at the very top end of the performance envelope. Even then, it falls slightly short of the 82°C flow required by 82/71°C circuits. A further consideration is that most HPs prefer to operate in the 5-10°C ∆T range, making a straight swap on 80/60°C circuits not impossible, but challenging. Adding to the challenges involved in the detailed design of swapping out boilers for heat pumps is the lack of design information on dated buildings. For this reason, a degree of investigative engineering is recommended before embarking on a project of this sort. The aim should be to build an understanding of the original design temperature and loads, any hydronic inefficiencies or changes to the building and/or heating system and any bounding constraints. Knowledge of electrical capacities, whether the budget allows for standalone ASHPs, and any factors that could offset the potential higher running costs, such as PV, will also influence the design.
Installing ultrasonic heat meters, undertaking a full heat loss calculation where possible and utilising known data such as gas meter readings will provide better insight into the building profile. With real measured data, solutions providers like Baxi can add support by engineering different design options, backed with predicted energy and carbon savings and capital
Fig 1. Typical two-boiler reverse return header system
Fig 3. Cascading solution Fig 2. Blended MT/HT HP solution
expenditure modelling. Optioneering is a valuable process that can help clients and designers make the best choice for a specific building within the project parameters.
Bracketing
One such option might be to bracket the heating system to avoid running all circuits at 80°C or 82°C all year, improving running costs. For example, if the survey data shows a sizable constant temperature (CT) circuit serving an air handling plant exclusively, one option might be to “bracket” this out of the overall heating system. Serving it directly from its own heat pump plant would allow a change of the tempering or reheat coils to suit a 55°C flow temperature (or lower). This decision alone could increase the heat pump efficiency up to 150% from the current design temperature of around 80°C. The same principle can be applied to variable temperature (VT) circuits when the CT circuit is unable to deviate from its current design flow temperature. VT circuit bracketing can deliver considerable efficiency rewards as the weather compensation can now be done directly at the plant without the use of mixing valves. With direct weather compensation on HT ASHPs, the flow temperature could range from 35°C-80°C. If heat losses have been carried out, changing the emitters might be a possibility, which would allow a more aggressive curve. The weighted aspect of bracketing involves understanding the split in capacity required for each circuit. If VT equates to 80% of the overall load requirement, then addressing that purely in isolation, with CT remaining on 80°C flow, may impact the overall efficiency of the building sufficiently without the need to replace air handling unit (AHU) coils. Figure 1 shows the two-boiler reverse return header setup, with CT and VT circuits, typical of most 82/71°C legacy designs. Using data (if available) from the current VT setup, the VT minimum temperature can be reduced below current settings to assess whether the target
18 BUILDING SERVICES & ENVIRONMENTAL ENGINEER JULY 2024
space temperatures may still be maintained. Even a modest reduction in flow temperature will ensure higher efficiencies.
Most HT ASHPs would achieve a coefficient of performance (COP) of approximately of 2.2 at 65°C flow and -2°C ambient conditions. Dropping the weather compensation to below 60°C will allow for a blend of Medium Temperature (MT) and HT ASHPs, potentially reducing any siting or budget complexities of a full HT solution.
In the blended MT/HT example solution shown in figure 2, the HPs are cascaded with a three port divert valve 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 are required for more challenging design conditions, the HT ASHP can ramp the thermal store up to 80°C. If Figure 1 had been designed on a ∆T of 20°C, one solution is the alternate cascading method shown in Figure 3 which utilises the thermal store lower and upper stratified sections to cascade temperature rather than load. 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 80/60°C, MT ASHPs are unable to delivery any useful heat.
Precision engineering
Precision engineering is essential to ensure the HPs are not over-specified in terms of kW capacity for economic and spatial reasons. Considering fabric options at the outset of every heat decarbonisation project is therefore vital to reduce heat losses and heat demand. Where the fabric cannot be improved, designing a perfect solution is almost impossible without compromise. However, a thorough understanding of the building thermal profile will enable a clearer understanding of the impact from each of the potential solutions, to ensure optimal performance and value.
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