There are numerous advantages of installing a heat pump system. To briefly summarize some of the most important points: the purchase cost of a heat pump is undoubtedly higher than that of traditional condensing boiler, even a high-quality model, however due to the higher efficiency and the subsidies available in many countries, the upfront cost can be paid back relatively quickly. In addition to the economic benefits, we also should not forget that vapour compression heat pump systems – the most common type on the market – don’t use fossil fuels but rather electricity, which is increasingly produced from renewable sources. This significantly limits the emissions of harmful substances into the atmosphere, as well as CO2 emissions, helping achieve climate-neutrality objectives. If the building also has a photovoltaic system for producing its own energy, combined with a hot water storage system, this becomes the ideal solution. Finally, the massive investments made by manufacturers have made it possible to build units that can be installed even in particularly harsh climates, or can be retrofitted in older systems while still ensuring high performance and minimal maintenance.
In this article, we will now examine some of the aspects that need to be taken into consideration when choosing a heat pump, given that there are several different models, configurations and capacities available on the market.
Obviously, this is merely general information, and indeed the choice should be made by a specialist technician. A do-it-yourself approach can help make a more informed choice and understand whether the proposed solution is in line with the actual needs of the home, yet does not necessarily guarantee the best results. Indeed, these units may seem simple, as they are now almost comparable to an ordinary household appliance, yet this is not the case, as they feature different technologies and therefore can achieve different levels of performance.
Let’s start with the sizing of the heat pump
Just like with all electrical and non-electrical equipment, a system should be adequately sized so that the work it carries out is sufficient to meet demand, while at the same time without any unnecessary waste (oversizing). In this specific case, the heating system needs to be able to compensate for all the heat losses in the building due to transmission and ventilation, while ensuring an adequate and sufficient level of performance throughout the year. Heat losses in fact affect the performance of the system significantly, in both heating and cooling modes. Furthermore, each building has different dispersive elements and layouts: walls, ceilings, floors, thermal bridges, doors, windows; they may be detached or semi-detached homes (in the latter case, the homes in the middle will dissipate less heat) or apartments heated by conduction from those adjacent to them.
The calculation can be done quite accurately by knowing all the dimensions and the composition of the building; obviously, this is not perfectly straightforward, and so specific calculation tools are used.
Heat pump performance at various temperatures is shown in blue: at low temperatures, efficiency will be lower, vice-versa at high temperatures it will be higher. For heat pumps with on/off compressor technology, performance would be represented by a straight line that continues at the same gradient as the initial section. Nowadays however inverter solutions are increasingly widely used; these limit and therefore reduce the capacity delivered, which otherwise it would be excessive.
The curve of capacity required to heat the home is in orange. The trend will be the opposite to the previous one, as heat losses will be greater at low outside temperatures, and vice versa.
Manufacturers can decide the best configuration for achieving the capacity needed to meet the total heating demand, indicated as HP max. For example, in the graph above it can be seen that the heat pump has been sized for a lower capacity, called HP nom, and that the total capacity required needs to be reached by supplementing with another heat source (for example, an electric heater). This configuration can also provide a backup in the event of an alarm that shuts down the heat pump, putting part of the circuit out of service, i.e. can either be used as a supplementary heat source, as described, or as a replacement source. In this case, heating demand would be met by the heat pump (sized based on the design temperature) and, where necessary, by the heater.
The balancing point, represented by the intersection of the building’s heating demand with the maximum compressor speed, indicates the extent to which the heat pump is able to deliver heat without the electric heater; this will then be activated to make up for the deficit (hatched area).
Therefore, if the heat pump is undersized, its operation will be very limited at low temperatures, and the supplementary source (heater) will need to be activated quite often, with high costs and inefficiencies.
In the same way, oversizing causes inefficiencies and greater power consumption, as the unit does not operate in optimal conditions, at times reaching the set point too quickly and then significantly reducing the compressor speed, with swings in operation. Furthermore, it makes no sense to oversize, as the heat pump is chosen based on the design temperature (for example 10kW @ -7°C), but rarely will a home have a heat loss of 10 kW; as homes are normally kept at a certain temperature, the heat loss to be offset is much lower.
To reduce heating requirements, the building can be insulated. This will move the orange line downwards, as heat losses are lower, bringing a simultaneous increase in efficiency. The graph below illustrates what has been described here for an on/off heat pump.
DHW production has not been mentioned, if a storage system is also provided. Approximately 55 litres of water per person can be heated and made readily available, avoiding the unpleasant inconvenience of cold water when opening the tap or the need to adopt alternative systems that use fuel or have low efficiency. From the sizing seen above, no further heating energy is needed, as the unit is usually equipped with a three-way valve and therefore DHW production is alternated with the production of hot water for heating. Furthermore, the house as a whole and the water used in the systems tend to have a certain thermal inertia, so even if hot water production is interrupted, there will not be a significant decrease in room temperature.
What type of heat pump should I choose?
There are multiple types of heat pumps on the market, classified based on the outdoor source that the refrigerant in the circuit absorbs or releases heat from or to (air / water / ground) and based on the type of heat exchange inside the building (air / water). The type of heat pump is therefore simply a combination of the two. Which type should I install?
For some time now, most manufacturers have been shifting their attention and investments towards packaged air/water heat pumps, i.e. with the entire refrigerant circuit housed in one single unit. These provide excellent performance in most operating conditions, and cost much less than those that use water as the outside source. Generally, even down to -20°C, air-source pumps still work with reasonable performance; indeed is no coincidence that they are often also used in Nordic countries. In these cases, air/water heat pumps may suffice, even though every single error is amplified and can cost dearly. In the event where a higher upfront investment can be afforded and high efficiency is desired, irrespective of the outside temperature, the best choice will be a water- or ground-source heat pump. However, care must be taken with ground-source units as, unlike air/water appliances, these are much more complex in terms of assembly. Both technologies, however, require specific skills for optimum management.
As regards the brand, there are obviously those that are better known, better recognised and more expensive, and apply refrigerant circuit technologies and variants with higher performance. It should not be forgotten, however, that it is the installation as a whole that makes the difference: units, piping, distribution system and integration between the refrigerant circuit and electrical parts must all be optimised by the system integrator.
To get an immediate idea, the unit’s COP (coefficient of performance) or better its SCOP (seasonal coefficient of performance) can help. The latter, unlike the former in which the data is instant, takes into account the average temperature trend throughout the year for a specific climate. Better units will have higher SCOPs. If the unit is reverse-cycle, the cooling efficiency in terms of EER and SEER can also be evaluated.
The refrigerant is another element that is specified for heat pumps, even if the residential market is now undoubtedly oriented towards natural solutions, above all propane. This allows excellent performance to be achieved and at the same time sufficiently high operating temperatures, even up to 70°C, ideal for replacing gas boilers without needing to change the distribution systems in the rooms (e.g. radiators). Obviously, the ideal solution involves medium/low temperature distribution systems so that the unit can operate in less stressful conditions (less work for the compressor to do, given the lower pressure difference between suction and discharge).
This article has examined just some of the elements to consider when choosing a heat pump. It’s good to be informed, but always rely on expert technicians who can advise and support customers from sales through to installation and after-sales service. The choice of technology depends mainly on the location and the desired investment, even if the market is moving in the direction of air/water units. However, the units always need to be adequately sized, in order to optimize energy consumption and save on electricity bills, while at the same time ensuring the best indoor comfort throughout the year.
Climate Systems provides fresh commercial HVAC innovations to the Sioux Falls area. Call 605.334.2164, email info@climatesystemsinc.com or visit climatesystemsinc.com to learn more.
Source: “How do you choose a heat pump?,” carel.com
