ENERGY SAVING
Lessons in heat pump installation
There are many powerful reasons to specify and fit heat pumps in a range of applications. Tim Mitchell, sales director of Klima-Therm, outlines the lessons we can learn from his own company’s experience of this versatile technology.
T
The industry is still
developing its understanding of definitions associated with heat pumps. So, for example, a chiller with heat recovery will have four pipes so could rightly be called a four- pipe system.
he arguments in favour of electrifying heat are well- rehearsed but compelling. They essentially boil down to the pressing need to reduce carbon emissions to help tackle the climate emergency. To this end, there have been major advances in renewables, with 2024 set to be the first full year where UK low-carbon renewables generate more electricity than fossil fuels. These have helped ‘clean up’ the National Grid, making electric heat generation far more environmentally friendly than fossil fuel- based heating.
So, what lessons can we learn from my own company’s involvement in supplying what we believe to be the largest range of heat pumps available in the UK? The first is that system water volume is key, especially for ASHPs that inevitably need to run defrost cycles. Put simply, to take heat out of the air using the vapour-
compression cycle, that air must boil the refrigerant, and for space heating, the evaporating point of the refrigerant is likely to be sub-zero, which will cause ice to form on the coils. In most heat pump installations, the defrost cycle involves the machine reversing. In reverse, the machine now pushes hot gas through the coils, which melts the ice (the defrost cycle). However, this means you are now removing temperature
from the system you are trying to heat. With an infinite volume of water, it wouldn’t matter if you cooled some of it. Conversely, too little water volume and as soon as you start running the heat pump in reverse as a chiller, you could take all the useful heat from the system. So, if follows that there is a balance point: we would
recommend not less than 12 litres of water per kW of heating.
This can also help with night-time load requirements. More is always better; less is a compromise.
Also, if the system volume is to be made up with a buffer vessel, the location of that vessel matters. because it will do a slightly different job depending on where it is placed in the system. Don’t therefore assume that you can place the buffer anywhere and it will do what you want it to do. You need to understand what you want the buffer vessel for and to find the optimal position for it based on this. So, ask these questions – Is it for storage of energy or for bulk? Is any level of delivery temperature dilution acceptable? Furthermore, water quality is key: this is a particular
problem in retrofit systems. With hot water systems, you will often have a scaling issue in the pipes. In a heat pump system, any level of scaling or water quality problems are exacerbated by the fact that the temperature difference between the delivered hot water and the control temperature are typically not as great as with a boiler.
Inside the heat pump the plate heat exchanger (PHE) is usually quite small so any compromise on water quality will be compromising the heat exchange quite dramatically. Therefore, sometimes a demountable intermediate plate heat exchanger may be worth the parasitic losses. Part load performance is also key – it is tempting to select units at their limits, and indeed this has often been necessary for some early adopters, especially with retrofit systems, but this can seriously backfire when the system operates at part loads and parts of the compressor operating envelopes are breached.
Because of the cost of per kW of heat pumps, it is tempting to push them to their limits, but that can cause problems at less than full load. So, try to sit comfortably inside the operating limits of the heat pump to account for all load conditions. And remember to design for system start-up. Most heat pumps will have a window of operation that is not always conducive to starting in an ambient loop. Typically, on the discharge side, where the load is, you are unlikely to get many heat pumps that are happy to start below 20-25°C entering water temperature. You therefore have two choices – keep the
22 January 2025 •
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