Hot water and buffer tanks for heat pumps

One might be forgiven for thinking that water tanks would be an uncontroversial subject. Yet when it comes to heat pumps, there has been some history that makes it worth reviewing the subject in detail.

Hot water tank selection for heat pumps

The difference between heat pumps and standard boilers that causes confusion when it comes to selecting a tank, is the seasonal variance in performance of a heat pump. A boiler produces the same output year round. Heat pumps produce more heat when the conditions are favourable; for heating, this means when there is more environmental heat to harvest. An air source heat pump that produces 5kW of heat in winter might produce 12kW in summer. If your water tanks is sized for a 5kW heater, but the heat pump produces 12kW, then what happens is that the capacity of the coil is filled with 5kW; the heat pump gets energy 7kW returned to it, which makes it run inefficiently or even trip out. This constant starting and tripping is bad for your pump and means it won’t run properly if it is needed. Although this is less of a problem with ground source heat pumps (as ground and water temperatures tend to remain more stable year round), the same rule also applies.

When sizing a water tank for a heat pump 0.25 metres squared of coil for every kW of output. This is far more than is required for a standard boiler. Just to make this more confusing, some manufacturers use the minimum (winter output) rating the name for a heat pump. At Stiebel Eltron we always use the maximum output to prevent confusion on this issue.

In the early days of the market installers would sometime use over-sized standard tanks to cope with this problem. But this has a number of issues, the first of which is hygiene. Water in a tank must be pasteurized regularly to 60⁰c (65⁰c for commercial premises) to prevent the build up of harmful bacteria such as legionella. If you oversize your tank then this becomes expensive and carbon hungry, because it requires an electric immersion heater to run to bring the temperatures up to this high level. Not to mention that in the UK, with some of the smallest homes in the world, space itself is at a premium. Because of this inefficiency MCS regulations are now in place to prevent this practice, meaning that installations with over-sized tanks may find themselves ineligible for government incentives.
For this reason, new installations of heat pumps are often paired with a tank made specifically for heat pump use.

Tanks designed specifically for heat pumps either have an over-sized coil for the volume of water held, or are thermal stores, like our very popular SBS range. Instead of having a coil that sits in the water in the tank heating it directly, thermal store cylinders have a “pipe” that runs through the centre of the tank. This pipe is in fact a high surface area corrugated stainless steel heat exchanger. The water in the tank is simply a heat store, used to heat water that runs through the heat exchanger and off to taps and radiators. This stored water in the tank is never in contact with the drinking water and thus the need to pasteurize it is removed. Moreover, as this type of tank has no coil to overload, it can cope with any size of heat pump.


However changing a hot water tank is a challenge in the retrofit market. It is both costly and disruptive to remove a tank that is functioning well and replace it with another. To answer this need At Stiebel Eltron we have a new heat pump product that has the unique facility to be used with standard tanks, making it ideal for retrofit situations. Our new WPL range has a setting to limit the output from 5-15kW to match an existing cylinder. Alternatively you can use one of our high temperature heat pumps, which also have this facility and have the added advantage that they can achieve the temperatures required to pasteurize water with using an immersion heater.

We have a very nifty tank just launched with an oversized coils called the Hydrotower. This is very space saving and has an integral buffer tank, which leads me neatly into considering why you might need a buffer tank...

Buffer tanks for heat pumps

Heat pumps need a minimum amount of flow going through the system to work properly and therefore have a set minimum of heat that they must generate when in operation. Take the example where a customer is heating just one bedroom at night, with a ground source heat pump. The heat pump needs to be on, but it may not be able to expend all of the heat it generates, heating one room. Moreover, even if enough energy was being used to heat the room to keep the pump ticking over, a thermostat will shut off the emitters when the room comes up to temperature. Once the heating circuit closes, the integral thermostat in the heat pump will stop the pump as it will have no way of getting rid of the heat it is producing. This cycling on and off is not preferable when it occurs more frequently than every seven minutes, because it can affect the longevity of the system and so pumps are usually installed with a minimum 20 minute cycle. The issue is that this 20 minute delay before the heating restarts might be long enough to create a perceptible drop in temperature, causing a undesirable fluctuation in room comfort. Buffer tanks are sometimes therefore needed in addition to hot water tanks to allow the pump to continue working until the temperature in the room drops enough for the heating to come back on or to allow the system to provide heating efficiently its minimum capacity.

Whether or not a project will demand a buffer tank is dependent on things like the kind of emitter being used, the size of the heat pump and the exact configuration of the system. All Stiebel Eltron heat pumps have controls that allow them to be operated without a buffer tank; but if space can be found for one I would always recommend including it in the system design.

For air source heat pumps the need for a buffer is even greater. The heat exchanger in air source pumps runs much colder than ambient temperatures, and as a result it becomes frosted with ice. Air source pumps must periodically work in reverse to defrost the exchanger. A buffer tank provides a source of heat to steal for this purpose, without robbing heat from the property being heated. Without this dedicated store of heat, an air source machine might start to take heat from a house, actually reducing the temperature of the property in a downward cycle.

In summary, tanks and buffers must be considered carefully as part of the system design for a heat pump. Choose tanks that are specially designed for use with heat pumps or a heat pump that is especially designed to run with standard tanks. And, if you can, find a place for that buffer.