Ed Morris of Altecnic tackles a common assumption around heat networks incorporating heat pumps, in terms of whether interface units can be installed to deliver on demand and efficiency.
The assumption is as follows: “It is a heat pump led network therefore the flow temperature is low, and I can’t use instantaneous heat interface units (HIUs) as I won’t get enough direct hot water (DHW) output. So, I will have to utilise DHW stores within the apartment to meet my DHW demand.”
DOES A ‘LOW FLOW’ NETWORK RULE OUT INSTANTANEOUS HIUS?
HIUs that have DHW plate heat exchangers that are specifically designed for low primary side temperatures can give very high DHW outputs and primary return temperatures. Current market leading HIUs can output 44 kW of DHW at a 55°C primary return temperature. CIBSE’s CP1 (2020) gives guidance stating that even three bed apartments, with two bathrooms only require 35 kW of DHW, so the HIUs currently available to engineers are efficient enough to be fitted on numerous networks.
Dropping the primary flow temperature further, down to 50°C gives an output of 33.5 kW with DHW at 48°C, 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°C. Using the example earlier in this paragraph, the DHW output increases to a potential 56.5 kW.
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 effect on the network delta T. As a result, network efficiency would be reduced, losses would increase and the potential for the building to overheat would also increase.
DHW STORAGE & TEMPERATURE DIFFERENCES
DHW storage will, inevitably, reduce the heat network temperature differential. As soon as the store gets even halfway up to temperature, the temperature difference on the primary will be significantly reduced. Data from existing projects show that delta Ts on DHW storage networks typically operate on average around 10°C 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 patents 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°C or regularly cycled to this temperature. This dictates further energy input to achieve 60°C, even though the DHW may only be required at 46 -50°C. If the network is running at 50°C or 55°C, the only way for the store to be lifted to 60°C is to install an additional immersion heater. However, this immersion heater will need to run far longer than the instantaneous electrical element that is operated by the HIU only when there is a DHW demand.
A single instantaneous HIU will have a higher instantaneous demand compared to a cylinder. However, the time that the HIU is on that demand will be short. Filling a bath may take around eight minutes, but once filled, there is no demand. If you fill the same bath from a cylinder, the instantaneous cylinder load will be smaller but the time that the cylinder will be reheating will be around 30 minutes or more. This fact dictates that the diversity used for an instantaneous system is far greater than that used for a cylinder system. The result is a longer peak demand for a cylinder system. The shorter peak demand from an instantaneous system allows for that additional short peak to be supplied via a thermal store. The large delta T of the instantaneous system also allows the size of the store to be reduced, saving on plant-room space.
Instantaneous HIUs ensure that the whole system is efficient. However, the lower flow temperatures of heat pump networks require careful selection of the HIU to meet the tenants needs. The focus still needs to remain on the design and sizing of the network first. If this is not the focus of an engineer, the network as a whole and the end users will suffer. Engineers should welcome the ‘new’ heat pump energy source, which will continue to reduce the carbon implication of the electricity that powers them.
Moving forward, engineers need the network to be the most efficient it can be and should continue to reduce the losses by maintaining the widest possible delta T, wherever and whenever they can.
Ed Morris is technical manager at Altecnic