Heat Pumps Today

22 Dispelling myths about using HIUs on Heat Pump led networks The drive to decarbonise the heating and DHW supply in buildings is increasing the number of heat networks that obtain their energy from heat pumps. Some networks have heat pumps working with other energy sources and some networks rely solely on heat pumps for their energy requirement. However, incorporating heat pumps on a heat network requires careful consideration to ensure that the demands of the apartments, both heating and DHW, can be satisfied while also ensuring that the heat pumps run e ciently AND the network operates e ciently. All three of these requirements (demand, networks and heat pumps) can be met by installing a heat interface unit within the apartments. However, some engineers often avoid fitting HIUs, but why is this? Two Common assumptions There are a number of assumptions that are made by engineers when they are dealing with HIUs installed on heat pump led or heat pump only networks. These assumptions are: 1. “It is a heat pump led network therefore the flow temperature is low, and I can’t use instantaneous HIU’s as I won’t get enough DHW output. So, I will have to utilise DHW stores within the apartment to meet my DHW demand”. 2. “I need to design my network around the heat pump to ensure that the heat pump works to the best of its ability/ COP(Coe cient of Performance)”. Both of these assumptions can lead to a heat network as a whole operating sub- optimally. Neil Parry discusses why these August | September 2021 are incorrect and how, with the selection of the correct HIU and hydraulic integration of the heat pump, the performance of a heat network can be improved. Tackling the first assumption… HIUs that have DHW plate heat exchangers that are specifically designed for low primary side temperatures can give exceptionally high DHW outputs and primary return temperatures. Current market leading HIU’s can output 44kW of DHW at a 55-degree primary return temperature. CIBSE’s CP1 2020 gives guidance stating that even 3 bed apartments, with 2 bathrooms only require 35kW of DHW, so the HIUs currently available to engineers are e cient enough to be fitted on numerous networks. Dropping the primary flow temperature further, down to 50 degrees gives an output of 33.5kW with DHW at 48 degrees, covering the requirements of most single bathroom apartments. However, should you require more output, some HIUs available have the option for an inline electrical element to be installed and controlled by the HIU. Under most circumstances, heating and standby, the element is not in use. However, the moment there is a DHW demand, the HIU energises the element and boosts the primary temperature by up to 10 degrees. Using the example earlier in this paragraph, the DHW output increases to a potential 56.5kW. As the element is only energising when there is a DHW demand, the additional electrical load required by the building can be diversified in a similar way to the DHW load using DS439. This makes the total additional electrical load very small. In addition, as the element is on the primary side of the HIU, then the network delta T is not reduced, in fact it is increased. If the electrical element was on the outlet (secondary) of the HIU or in a cylinder, then this would have a negative e ect on the network delta T. As a result, network e ciency would be reduced, losses would increase and the potential for the building to overheat would also increase. DHW storage and temperature di erences DHW storage will, inevitably, reduce heat network temperature di erential. As soon as the store gets even halfway up to temperature, the temperature di erence on the primary will be significantly reduced. Data from existing projects show that delta T’s on DHW storage networks typically operate on average around 10 degrees or less. The smaller the delta T, the more flow rate required for a given energy output and therefore the greater energy use. High return temperatures also increase the network losses and can lead to buildings overheating. There are additional downfalls to DHW stores. One of the most patent of these being that storing the DHW gives an increased likelihood of Legionella growth. Therefore, the temperature of the DHW store either needs to be kept at, or above, 60 degrees or regularly cycled to this temperature. This dictates further energy input to achieve 60 degrees, even though the DHW may only be required at 46 – 50 degrees. If the network is running at 50 or 55 degrees, the only way for the store to be lifted to 60 degrees is to install an additional immersion heater. However, this immersion heater will need to run far longer than the instantaneous electrical element T R A I N I N G Neil Parry, Head of Specification at Altecnic Ltd, has found two common assumptions which need to be discussed in order to move forward with improving efficiency and decarbonising heat networks.