Six most frequently asked questions: Air Source Heat Pumps


Heat pump technology works in a very different way to boiler-based heating systems. This means that when it is being retrofitted into a customer’s home, it is not a like-for-like replacement.

Mark Wilkins, Technologies and Training Director at Vaillant, answers the six most commonly asked questions from installers who want to know more about how to retrofit air source heat pumps.

1. Is a heat pump sized in the same way as a boiler?

All heating systems follow the same sizing principle. A correct heat loss calculation should be carried out, whether you’re installing a new boiler or a heat pump. Heat emitters (radiators) should be sized correctly, flow rates should be calculated, and domestic hot water comfort should be covered, to ensure the new heating system you’re installing will perform efficiently and will provide your customer with the optimum heating and comfort required.

It is important that the output of a heating system matches closely with the heat loss of the property in question. An undersized heating system will not heat the building on cold days and an oversized one leads to short-cycling of the boiler or a heat pump, which significantly reduces system efficiency, as well as being more expensive to purchase.

2. What information is needed to calculate heat loss?

The formulae used for heat-loss calculations require a number of variables. So, to calculate correct heat loss, not only do you need to know the size of the property, but also other information such as its location, what it’s made from, as well as door and window types. In addition to the property’s overall dimensions, the size of each room is also required to ensure that the radiator and pipework are sized correctly. This way, all the rooms in the customer’s home will have the correct flow rates running through the pipes to achieve the desired temperatures. All of this information will need to be obtained when carrying out a survey at the customer’s site.

3. Is an immersion heater needed for the cylinder?

Because most heat pumps work more efficiently with lower temperature systems, a back-up heater (direct electric immersion heater) may be required to provide legionella protection. However, if using the Vaillant aroTHERM plus heat pump, an immersion heater won’t be necessary.

The aroTHERM plus uses the refrigerant R290, which allows a high flow temperature of 75°C from the heat pump for the legionella protection cycle, without the need for a back-up heater. The hot water storage temperature can then be set in the same way as with a traditional boiler heating system.

4. What if there is an existing hot water cylinder at the property? Can it be used with a heat pump?

A standard boiler cylinder doesn’t have a sufficient sized coil inside to work with a heat pump which typically operates at a lower flow temperature. Although the Vaillant aroTHERM plus can operate with a flow temperature of up to 75 degrees, a larger coil is needed to ensure faster re-heat times and for the heat pump to run efficiently.

5. Can existing pipework and radiators be used when installing an ASHP system?

Lower flow temperatures and lower temperature differentials for heat pump heating systems mean that more water going through the pipes is needed to ensure sufficient heat transfer. Therefore, you will need to calculate the pipe size needed when designing a system to maintain correct flow rates. In addition, pipe runs would also depend on where the heat pump and hot water cylinder are to be installed.

In order to ensure a room stays warm, heating emitters need to be appropriately sized for the space in question.

In short, while carrying out a site survey, installers need to assess whether the existing infrastructure is suitable for a new heat pump system, and what renovation may be required.

6. What training should I take in order to install heat pumps?

Currently, for property owners to access the Government’s Renewable Heat Incentive (RHI) funding, which is available for heat pump installations, these have to be carried out by a MCS (Microgeneration Certification Scheme) certified installer.

As part of the requirements to become MCS certified, installers are expected to have manufacturer-specific training. As a manufacturer committed to supporting installers, we offer a range of modules to help installers take their first steps into low carbon heating. We are also on hand to give guidance to installers on live jobs by helping with system designs and answering queries when they are on site.

To find out more how Vaillant is supporting installers and to learn more about their range of heat pumps and energy efficient boilers, visit