How big do you need to design a buffer tank for the heat pump?
The dimensioning of heat pumps is often talked about. Depending on the desired building area, temperature level, insulation standard and consumption of heating energy, the need for heating power varies greatly. Also the bathing/showering behavior can influence the heating power once again. All this makes sense, because the heat pump must eventually be able to heat even at the coldest temperatures and maximum consumption as much as the household needs.
So these are the big parameters that dictate how big the heat pump must be. Practically speaking, the size of a heat pump is not about the mass and the space it occupies, but about power - measured in kW. If the heat pump is too big, energy is wasted and often money as well. Often the machine runs sub-optimally at very low load. If it is too small, the building cannot be kept warm on the coldest days.
What is less often discussed is that the buffer tank must also be dimensioned in the optimal size. This is strongly related to the size of the heat pump and the parameters mentioned above. In contrast to the heat pump, the size of the buffer tank does not mean the output, but the volume. It is therefore important how much heating water the buffer tank can hold. Arguments for why a buffer tank should be installed with a heat pump can be found here (link WP buffer tank benefit).
Use of large buffer storage tanks
The advantage of a large buffer tank for the heat pump is that you can store more heating water and thus achieve a greater buffer effect. You can bridge a longer period in which the heat pump does not have to be switched on. For example, because electricity becomes more expensive at this time, the heat pump is working inefficiently due to very low outdoor temperatures, or simply to reduce noise. In the case of a PV system, this also means you can temporarily store a larger PV surplus as heat.
Use of small buffer storage tanks
In favor of a smaller buffer tank to your heat pump is that they are cheaper to buy, take up less space in the boiler room (and transport into it), and also have less heat loss when used less, because they are filled and emptied in a short time and the water does not stand for days.
Thankfully, there are some common rules of thumb for the decision. In addition, when you buy, you are typically very well advised on the size.
Selection of the optimal size
The easiest way to make a decision is to make it dependent on the size of the heat pump and the heating demand. If the heat pump is fully modulating (it has a wide power spectrum, which it covers efficiently), it is not possible to go by nominal power alone. In this case, the heating demand is used without the hot water portion (because it is stored in the boiler). Here, the approximate rule of thumb is that each kW of heating capacity should be covered by 50-70 liters of buffer volume. If there are more, inefficiencies arise. If there are less, the bridging time becomes rather low.
Necessary criteria for choosing the size of the buffer tank
This remains only a rule of thumb, but it is a very good starting point for deciding the right size of buffer tank relative to the heat pump. You can make adjustments to this rule by including the following factors.
- Type of heating: So far, we have talked exclusively about heat pumps and buffer storage. However, if you have a fossil heating system that can easily provide higher flow temperatures at any outdoor temperature, the necessary buffer volume decreases. After all, you can start the heating directly without the need for a buffer to bridge inefficiencies. Similarly, if the heat pump draws energy from geothermal or groundwater for its buffer storage, the buffer volume can be smaller. In this case, the buffer volume can be smaller than with an air-source heat pump, because the buffer effect on efficiency becomes much smaller.
- Heating requirements of the occupants: this already indirectly flows into the heating output of the heat pump. But it is still important to realize that different occupants may have different heating behaviors. For example, it is usually the case that the rooms of children and older people should be somewhat warmer. There are also often differences in the desired temperature between the sexes.
- Insulation standard and heating delivery system: Also important is the temperature at which the heating water circulates through the delivery system. For radiators, this is relatively high. In large volume heating delivery systems, such as underfloor heating, the temperature is significantly lower. Accordingly, the difficulty of heating water spontaneously differs. This influences the necessary bridging time and thus the ideal buffer volume.
- Heating demand and heating capacity: Although already stated above, this is certainly the most important point. Heating capacity and size of buffer tank are closely related. If the storage tank is too large, there is a risk of heat loss over several days. If it is much too small, the heating power of the machine may even overheat the storage tank and damage it in the long run. In the case of heat pumps, however, this risk is much lower than with fossil systems
- Connected systems and energy generators: If a PV or solar thermal system is on the roof or otherwise connected to your house, it is worth storing this energy. Because battery storage is expensive and also has limited capacity, a buffer tank can help. If this is generously dimensioned, then you can store more energy as heat and do not have to allow unnecessary backfeeding.
- Size of the cellar and cellar passage: Even an optimal storage tank size and tuning to all factors brings nothing if you can not place it. The space required in the cellar for a 1000-liter buffer tank is much larger than for a 300-liter tank. In addition, the transport to the cellar should not be forgotten. If doors are small or access via the stairs is difficult, this can be the limiting factor. Therefore, be sure to check the tipping dimension of the buffer storage tank.
Conclusion
As we can see, the interaction between heat pump, buffer tank and size is not that simple. Nevertheless, with competent advice, it succeeds in most cases. It is important that they are clear about the individual circumstances of your building and the occupants and know what they want approximately. The more details you can provide on this, the better the advice will be. If you purchase the buffer tank together with your heat pump, then many of these parameters will have been clarified in the course of their consultations anyway.
