The correct heat pump selection
When selecting a heat pump, mistakes can be made that do not depend on the manufacturer, but on evaluations concerning the system itself. The manufacturer, based on own experience and knowledge, can suggest some alternative or additional considerations that could help
select the most suitable unit for the specific requirements of the project.
- Selecting the unit based on the peak winter requirement: this choice is very common in the field of heat pumps. Such a choice has the benefit of protecting the plant by oversizing the unit in order to ensure that for a few hours or a few days a year the plant can still supply the required capacity at peak conditions. However, since the unit is oversized, there are some negative consequences for the both the plant and the proper functioning of the unit:
- by ensuring a performance at a peak ambient air temperature means that the unit is oversized throughout most of the year; it will therefore operate with a higher number of ON-OFF commands to the compressors, thereby reducing its life cycle;
- oversizing the unit means that the whole system is designed to follow this unit sizing requirement: pumps, water pipes, insulation, storage tank, valves, by-pass valves, electrical cables, electrical protections, electrical consumption are all designed to follow this peak capacity selected;
- selecting the unit and therefore the system to be oversized generally translates to increased consumption costs for the end user, increased footprint of the unit, and naturally higher investment for the unit itself.
- Failing to consider that in a reversible heat pump the heat exchanger is the same both for cold and hot water supply: since the unit is with reversible cycle, the heat exchanger is designed to work with the same constant flow both for the chilled water and the hot water supply. When this is paired with the common choice of selection at peak load, it leads to questionable selection results. When selecting the size of the heat pump, even if it were at the maximum winter peak conditions (water temperature will also be mentioned shortly), one must be aware of actual site conditions. Normally, the flow rate in summer and winter will always the same (in 2-pipe systems), therefore it is necessary to select the unit fixing flow rate (usually the chilled water supply at 35 °C ambient is taken as standard) and leaving water temperature. In this way, regardless of the delta T between the incoming and outgoing water (which will be a direct result of the heat exchange), the selection of the pump is effective for its operation in the field.
- Taking into account inertia volumes: as manufacturers, we see the trend to minimize inertial accumulation. It should be noted that during the operation of the unit there are buffer periods such as defrosting, which, due to the reversible nature of the unit, would cause the unit to produce water at a temperature opposite to the setpoint. It is therefore important to make correct use of inertial volumes that will allow minimizing this effect at plant level. It will also guarantee a lower number of power on/off commands of the unit, enhancing its reliability over time.
A word of advice from Hecoclima
- Operating range and work in hot water mode: according to the type of application, the model and the refrigerant gas used, we will offer heat pumps with different operating envelopes and therefore applicability. It should always be kept in mind that, up to the minimum temperature guaranteed, the heat pump would be able to supply hot water. This means, for example, taking into account the local climate conditions and curves to assess how critical is it to guarantee the LWT of the heat pump in the most extreme conditions. This also means evaluating the plant when the selection is done at optimal site conditions (not at peak winter conditions), accepting, only when outside ambient T is very low, that the LWT of the heat pump might be lower than the set point. Such conditions will anyhow be compatible with the compressors’ envelopes and the characteristics of the gas used. Such reduction of the LWT of the heat pump, normally for very short periods if not only a few hours a year, can be compensated by: adequate inertial reserve and adequate insulation, electric heaters to the tank, high temperature heat pumps of small capacity to act only as booster if needed.
- Always compare two units and the modality of their selection: as mentioned, the manufacturer’s suggestion remains to avoid selection of the reversible heat pump at its peak load, since that would mean unbalanced operation of the unit for its entire life cycle. This imbalance becomes evident if we compare, given equal flow rate, the effective cooling and heating capacities. Such comparison allows a better understanding of the effective over-sizing of the unit and the correlated accessories within the system (a higher cost for system materials and power consumption management of the plant for the end user). Given the above consideration, a better strategy would be to start offering, as an alternative, the selection of the unit at standard conditions, as suggested by the manufacturer. From this selection the consultant or project manager can evaluate the final cost of the system, associated management costs and evaluate effectively how the unit will operate at the required conditions (simulations at the minimum external air temperature, at constant flow rate, maximum outlet water temperature guaranteed under critical conditions).
- Check and evaluate the actual working point of the heat pump: when discussing choices of a particular refrigerant gas or the suitability of a particular compressor type for a certain type of application, there are particular advantages of specific product series compared to others. Pointing out these benefits might aid when approaching
the marketplace, in order to be able to offer valid alternatives even to the competitors’ most standardized products. Upon evaluation of the actual working conditions of the heat pump one can notice how the water temperature varies with variations of the ambient temperature (as previously mentioned). This analysis can then be used to include the end user or the designer in the unit selection process according to the specific needs of the application by asking the following:- For how long will the heat pump actually have to work at the set point “X°C” given outside ambient at “Y°C”?
VERY LITTLE » the suggestion is to select the unit at standard conditions and check operation of the unit at extreme ambient condition.
A LOT » the suggestion is to verify with the manufacturer the best heat pump, which will guarantee long-term operation at extreme conditions (in order to still try to avoid oversizing the unit).
ATTENTION: in Hecoclima’s experience, the second case is very rare. - Does the system have the required inertial reserve (buffer tanks) to be able to manage, in case of unit selection at standard conditions, the eventuality of the unit producing water at a lower-than-set point temperature?
YES » the unit can be selected at standard conditions and it can integrate the buffer tank during operation at extreme conditions.
NO » the suggestions is always to include a buffer tank in the system, in Hecoclima’s TDS there are always recommendations in order to estimate the proper sizing. - Does oversizing the unit make the system inefficient for the end user for the rest of the year?
YES » an in-depth evaluation of what this means for the system is needed. The presence of two oversized units, for example, can bring one of the units to always be OFF, which would mean an inefficient investment evaluation at design stage.
NO » the offer will be made with an oversized unit as required.
- For how long will the heat pump actually have to work at the set point “X°C” given outside ambient at “Y°C”?
- Delta T (dT) in reversible HP units: as previously mentioned, the manufacturer suggests to always select the same dT for both the cooling and the heating water temperatures. In the case of the unit’s selection is based on a very low external ambient temperature compared to the standard selection, the suggestion is to focus on the heating capacity, selecting the correct water flow and the leaving water temperature, in order to work with data as close as possible to the effective operation on site. The units are anyhow able to manage a minimum and maximum water flow and one should always verify these values with the manufacturer.
