What is a buffer tank and how is it different from a boiler?
The hot water tank or often called "boiler" is a term most people are familiar with. Have you ever gotten up so early that the shower just didn't get hot? Or did your roommates sing whole arias in the bathroom, so that only cold water flowed out of the shower for you? Then there was probably no more hot water in the boiler.
In contrast to the hot water tank, buffer tanks are much less common, but they perform a very similar task. However, not for your domestic hot water, but for the heating water. We can therefore also speak of heating storage tanks - even "heat pump storage tanks" can be read on many an offer. But the name buffer storage tank actually says it all: If you connect a heat pump to the buffer storage tank, it efficiently bridges the period between heat generation and use. This storage of thermal energy thus effectively serves as a buffer that compensates for fluctuations and the time lag between the best time for generation and consumption.
Why is the buffer tank only an issue with heat pumps?
Theoretically, the buffer tank could also be used with gas or oil heating. However, the added value of connecting buffer storage to the heat pump is significantly higher. This is due to a fundamental challenge in heat generation with these machines. Heat pumps obtain thermal energy free of charge from the environment and not by burning fossils at several hundred degrees Celsius. Regardless of whether a heat pump uses air, the ground, or groundwater as its source, the source temperature is at its lowest precisely when heating is needed most. If it is cold outside, the heating system has to work - but at the same time it also produces the least.
So, on the one hand, the temperature fluctuates throughout the year. In summer it is hot outside (high source temperature), but the heating is only used in winter. On the other hand, the temperature also fluctuates strongly during the day. Between day and night, the temperature difference can be as much as 10°Kelvin. Heat pumps and buffer storage tanks can take advantage of this together. In good conditions, the heat pump heats the heating water in the buffer tank, where it is temporarily stored. During the cold night, the machine makes no noise and the stored heat circulates through the heating delivery systems. So a buffer tank can also bring some advantages.
How do you benefit from connecting a buffer tank to the heat pump?
There are a handful of mechanisms through which the buffer tank makes an entire heat pump system more efficient. This leads to lower heating costs and high comfort during operation. To choose the right buffer tank, it is helpful to be aware of these mechanisms. This then directly results in the criteria that the right storage tank must fulfill.
Storage mechanisms
Fewer switching cycles
Heat pumps do not run continuously, but adjust to demand. They switch on, generate heat for a while and then switch off again. This is called a switching cycle. Basically, don't have too many switching cycles in a short duration, as they shorten the life of the compressor. A buffer tank helps to reduce the number of on-off switching cycles, thus extending the service life of the installed components and the entire heat pump. This also has a positive effect on maintenance costs.
Optimization of heat pump operation in the most efficient range
Modern heat pumps operate at variable speed. This means they clock their output flexibly according to demand and do not always provide full power. These are known as inverter or modulating heat pumps. Nevertheless, there is an optimal power range in which the machines work most efficiently. You can compare this with a car. The fuel consumption is lower in higher gears with lower speed, but at a relatively low speed for the respective gear and of course only if you can keep this speed constant. If you have to cover the same distance in a very short time or if you have to brake and accelerate again and again, the consumption is significantly higher. Thanks to buffer storage, fully modulating heat pumps can also operate as constantly as possible in the optimal range.
Availability at peak loads
In the event of sudden high heat demand in the household, for example if you have accidentally left the windows open during a severe cooling or want to set your radiators warmer on a cold day, the buffer tank can provide support for the additional heat without overloading the heat pump. This relieves the heat pump and contributes to energy efficiency.
Energy storage
Especially for owners of solar or PV systems, the combination of heat pump and buffer storage is an excellent option to save money:
If there is a PV surplus without buffer storage, you have to feed the electricity back into the grid, but in return the system owners receive only a fraction of the market price of the fed-back electricity. If you need exactly this electricity an hour later, you pay the significantly higher market price. Therefore, in the case of PV systems, it is important not to feed electricity back into the grid, but to use it yourself whenever possible.
With a buffer storage, you are in the case of surplus electricity from the PV system, the heating tank already in stock with heating water to fill. In return, you can operate the heat pump for free, so to speak.
Blocking times & power outages
Your electricity company has the possibility to determine "blocking times" during peak load. This means that when the general power consumption is very high, heat pumps do not run in order not to overload the power grid. So during this time your heat pumps cannot use electricity, but thanks to stored heat in the buffer this is not a problem. A buffer also pays off during power outages, because if it is very cold outside and the heat pump is not running, then the water supply pipes are in danger of freezing. Because these are sunk approx. 50-100cm in the ground, this goes anyway some time. But if there is still warm water in the buffer, both power failure and minus temperatures would have to last for several days before the water could even begin to freeze.
What are the criteria for choosing the right buffer tank?
Those who can connect the buffer storage tank to the heat pump therefore benefit from a variety of advantages. Depending on the personal and building-specific circumstances, however, some points outweigh others.
This should then also determine which type of buffer storage tank you connect. Thus, on the one hand, there are different types of storage tank (combi storage tank or heat pump buffer storage tank), and on the other hand, different designs of the same storage tank (primarily the storage volume). If you buy the heating storage tank together with the heat pump, you should be able to count on the competent advice of your salesman. However, there is no harm in knowing the basics.
Choosing the right type of
On this point, there is usually clarity relatively soon. Manufacturers, sellers and installers of heat pumps have usually negotiated very good terms with storage manufacturers and can quickly assess what kind of storage they need based on your situation.
Classic buffer tank, serve only for the storage of heating water. The boiler for hot water is a separate storage tank. Nevertheless, there is the possibility of installing a so-called combi-storage tank, which can do both. Because the architecture is a bit more sophisticated in this case, you also pay a corresponding surcharge for a combi-storage tank. Another option is a so-called hygiene storage tank. In the hygiene storage tank, the hot water is not stored directly in the tank, but flows continuously thanks to continuous heating. This makes it very difficult for legionella to form, which is also the reason why the term "hygiene" storage tank is used.
The more systems that are connected to a buffer storage tank, the more connections it must typically have. A key difference between a normal heating buffer tank and a buffer tank with a solar system is an additional heat exchanger. This helps to efficiently supply the additional energy from the system on the roof to its storage tank.
But again, do not worry too much about choosing the right type of storage tank. This question should quickly become clear in the course of discussions about the package around the heat pump.
Choosing the right model
The same type of storage tank is usually available in different versions. Literally the biggest difference is the decision on the storage volume. Buffer tanks are available from 100 liters up to several thousand liters. But how to decide on the right size for the tank?
Fortunately, there are some rules of thumb and decision-making aids for this. More volume basically means more heating energy can be stored. So most of the benefits mentioned above become greater the more storage volume they have. Nevertheless, the largest storage tank is not the same as the best storage tank. This is because even though modern buffer tanks are very well insulated, there is some heat loss over several days. So if you choose a buffer tank that is oversized, the water that has been heated with energy will slowly cool down before it is used and will have to be heated again - an unnecessary loss of energy. In addition, heating a large volume of water to the desired temperature also takes noticeably longer.
A common rule of thumb is that for every kW of heating capacity, 50-70 liters of buffer volume makes sense. Important here: even if the hot water production runs on the same heating system, then do not include it - after all, for the hot water you have a special hot water tank with different specifications. The exception is the combined storage tank.
"50-70 liters of buffer volume per kW of heating capacity is a common rule of thumb".
Other factors that influence the best buffer volume are:
- The type of heating: heat pumps require more volume than fossil heating.
- Heating requirements of the occupants: Children or older people usually want somewhat higher temperatures and the heating requirements differ accordingly. Interestingly, there are also differences in the ideal temperature between the sexes.
- The source and source temperature: geothermal and groundwater heat pumps are slightly more efficient at delivering higher temperatures and therefore require a slightly smaller storage volume than air source heat pumps
- Heating power provided: There are direct dependencies between heating power and storage tank volume. If the storage tank is very small, there is a risk that the same output will overheat the heating water, which can damage the storage tank. With heat pumps, however, this risk is significantly lower than with fossil heating systems.
- Insulation & heating delivery system: Depending on the energy status of the house and the necessary flow temperature, the ideal storage volume increases.
- Connected systems: If a PV or solar system exists, it makes sense to choose a slightly larger storage tank so that more energy can be stored.
By the way, using the laws of physics, you can also calculate how long a buffer tank could completely bridge a failure in the worst case (at maximum heat pump output).
Relevant for this are:
- (A) The heat capacity of water (that is, how much heat energy can be stored in water), this is constant at 1'163 kWh per kg of water.
- (B) The temperature difference between supply and return (measured in Kelvin). By how many degrees Kelvin must the water in the storage tank be heated so that it runs through radiators/floor heating again at the desired flow temperature.
- (C) The volume of your buffer tank in cubic meters (divide the number of liters by 1000).
- (D) And of course the power of the heat pump so you know how fast the necessary work can be done.
The maximum bridging time in h is then equal to = [(A) * (B) * (C)] / (D)
Attention: before you reach for the largest storage tank now, you should remember that you also have to install it in your basement. Firstly, the necessary space must be available in the basement and secondly, you must be able to transport the storage tank into the basement at all. Especially the last factor is often underestimated. If cellar entrances have normal door sizes or are only accessible via winding stairs, then you should still measure them before purchasing. To do this, pay particular attention to the so-called "tipping dimension" of the storage tank, which indicates the size when transported overturned.
So, as you can see, storage volume is a big issue in choosing the right model. The other options you can still decide on deal with the materials used. Since a storage tank is naturally exposed to a lot of water, it may be worthwhile to connect the heat pump to a buffer tank that is made of stainless materials. Here you have a choice between stainless steel or an enameled storage tank. Both come at a premium but a slightly longer life with less maintenance.
Conclusion
In general, the core function of a storage tank is to increase the overall efficiency of the system. This is done by preventing inefficient operation of the heat generator and balancing the time between the generation and consumption of heat, which are naturally separated.
If you connect the buffer tank to the heat pump, you can protect the pipes from freezing/stiffening, especially on cold days and during potentially prolonged power outages. The heating storage tank thus also acts as an insurance policy, so to speak.
Disadvantages of buffer storage tanks are actually only the additional costs in the purchase, which, however, are clearly compensated for in operation. Together with the boiler, both storage tanks require space, but there is always the noticeably more expensive option of a combination storage tank.
