Heat pumps are increasingly being used to supply heating and hot water in commercial premises. Andrew Robinson, Managing Director of Exi-tite, discusses the challenges still encountered during design and installation.
Taking advantage of the economic and environmental benefits of heat pumps is the primary driver when consultants consider their use. However, experience with poor designs and installations still leaves them shrouded in doubt and confusion. Typically, a commercial system is not subjected to the same level of hydronic design issues that we often see in the domestic market, with designers generally aware that lower flow temperatures and appropriate flow and return temperature differences will achieve higher system efficiencies. However, we are still asked to investigate problem systems that have replaced boilers without attention to hydronic design or control. We find heat pumps short cycling due to oversizing or a lack of water content, operating at inefficient high water flow temperatures when trying to maximise the effect of existing emitters, and a lack of performance in winter due to excessive defrost cycles.
In addition to the typically manufacturer-recommended low flow temperature and low delta T principle referred to above, in many instances, the ‘turn down’ of the system is not considered. The lower the return water temperature of a heat pump system, the lower the condensing temperature required and, therefore, relative efficiency is increased. As zones close down, however, the calculated delta T of the system is reduced, leading to increased return water temperatures and loss of efficiency. Inverter compressors reduce the mass flow rate of refrigerant as load decreases to maintain this design delta T. Still, their minimum speed, usually defined by oil circulation requirements, dictates the system’s minimum heating output. Once this is exceeded, the system will cycle on and off due to thermostat control. Multiple inverter compressors or multiple modules allow for better regulation of capacity control and, in most cases, manufacturer control logic allows for capacity staging. However, understanding the principles of operation assists in further system control during design.
Potable hot water
When it comes to heat pumps for potable hot water, we have to remember that the systems will typically use much higher flow temperatures than for space heating. This means that system efficiencies by default will be lower than we are used to. If the system is a relatively small commercial system with low water use, a combined heating and hot water system can be used. Because a heat pump system is not designed for instantaneous hot water like a gas boiler might be, it is essential to understand the pattern of water use in a building in addition to the peak load. There are a lot of systems installed that are undersized when considering recovery rates and demand. This is usually addressed by increasing the storage temperature of any calorifiers and, therefore, providing more usable energy or operating any electric immersion heaters, which reduces system efficiency. Understanding heat pump operating principles such as delayed starts, oil returns, defrost logic and calculating energy in and out of a system rather than litres provides a solid foundation to design heat pumps for potable use. Often, calorifier reheat times are required to meet a building’s pattern of use, but these times are based on the heat pump delivering 100% load, which, from a standing start, is not available. When a heat pump receives a thermostat ‘on’ signal, it enters a start-up process; this is a period where it judges its surroundings, in terms of ambient temperature, its requirements, in terms of target temperatures, and also what the current water temperature is returning to the heat pump. The inverter will start at a low operating speed and slowly bring the system to its desired operating cycle. This process can take 5-10 minutes, so during this time the system will not be producing full capacity and, in turn, extending the calculated reheat times.
Defrost is one of the most misunderstood concepts of heating with a heat pump and, again, commonly miscalculated in system design. An air-to-water heat pump operating in heating mode will remove energy from the air that passes over its evaporator and deposit condensation on the evaporator fins. This condensation turns to frost and ice, eventually restricting air flow and heat transfer. There are two main types of defrost used; a reverse cycle, where the role of the evaporator and condenser are reversed, absorbing energy from the water circuit to be used to defrost the heat exchanger, or, more commonly in commercial heat pumps, a hot gas defrost. Failure to understand the differences between the two causes disagreements when identifying systems designed incorrectly or ‘not fit for purpose’. Hot gas defrost does not remove any energy from the water circuit; it instead diverts hot vapour refrigerant away from the condenser that produces hot water and injects it into the evaporator, defrosting any frost or ice build-up. Whilst heating production ceases, there is no adverse effect on the heating circuit.
The effects on the heating capacity of defrost are generally shown in the product data and should be used when selecting equipment to meet design requirements. A lot of the time this is overlooked. However, one of the main misconceptions is that defrost lasts for hours when, in fact, with a well-manufactured system, defrost typically only lasts 5-10 minutes. The problem is that accounting for defrost is more complex than allowing one defrost per hour because the process is not based on operating time alone. Control variables include accumulated compressor operating time, ambient temperature, evaporator heat exchanger temperature difference to ambient temperature, and performance. The length of the defrost is also not easily calculated because a similar type of control logic is applied. Whether a system can defrost one module at a time or a section of its heat exchanger at a time, defrost will always impact the reheating times of stored water.
Reducing installation costs
Onsite installation costs of any equipment are getting out of control, and we live in an age where we are still building and constructing in an uncontrolled environment rather than in favourable conditions where quality can be observed. Installed incorrectly, heat pumps can be an expensive problem. We assume that the mechanical installation would cause most problems and, while this is true in part, a commercial system, if designed and procured correctly, will generally be OK. The main issues are usually found in the control side of the system and integration into building management systems, along with the logistics of delivery to the site.
With multiple parties involved during the installation, the commission can appear to be successful. However, intermittent faults or faults caused by work post-commissioning are challenging to find and without effective command chains blame can quickly be apportioned. Considerable resources are spent protecting brand identity and systems like the Exi-tite heat pump packaged for potable hot water alleviate many of these issues. The system is designed to meet requirements, constructed off-site in a controlled environment and presented as a plug-and-play heat pump solution. All controls are prewired with a simple Modbus or BACnet connection for integration with a building’s BMS.
Understanding heat pumps enough to maximise their potential and engineer projects away from the typical specification is more than just a training-based exercise that can be addressed in a day or two, unless people are only specifying one type of heat pump all of the time. Working with heat pump specialists like Exi-tite and its packaged products gives access to a wealth of knowledge and support to ensure projects are concluded without issue.