Can I get my hot water from an air source heat pump?

The short answer to this question is ‘no’ although it can make a significant contribution depending on the configuration of your system.

On initial consideration, it might be hard to understand why this should be the case.  Most heat pumps are designed to heat water (to run your central heating system) to 45-50°C and domestic hot water is used at 38-42°C.  However, unlike a combi boiler, a domestic heat pump cannot produce hot water quick enough to provide an adequate flow of water to baths, taps, and showers. Also, most domestic heat pumps are not designed to be able to provide hot water at the same time as providing space heating, with the demand for hot water taking priority.

One way to get around this problem is to store hot water in a cylinder until it is needed.  However, care is needed when storing water between 20 and 45°C due to the risk of legionella, particularly if the water remains in the cylinder for long periods of time.  Heating the water in the cylinder to 60°C for at least 30mins or 55°C for at least 5-6 hours is the conventional way to kill legionella bacteria in domestic properties, although for safety and simplicity, the usual advice is to keep the cylinder thermostat set to 60°C.

The simplest way to achieve 60°C is to use an electric immersion heater, fitted into the top of the hot water cylinder.  For maximum efficiency (unless you are having a hybrid system installed) it is likely that the existing hot water cylinder will need to be replaced with one that has a much larger coil capacity, to compensate for the fact that a heat pump will deliver water at a lower temperature to the cylinder than a conventional boiler would do.  The greater surface area of the larger coils aids the transfer of heat from them to the water in the cylinder.

If you don’t have space for a hot water cylinder (with a diameter of 55cm or more) then there are other options available.  These include:

A hybrid system

A hybrid system is designed so that the heat pump provides your space heating needs and a boiler (which could be your existing gas or oil boiler) provides your hot water.  Another reason for having a hybrid system is to cope with a home with a high heat demand.  This is because the largest size of heat pump that can be installed on a standard (single phase) domestic electrical supply is 14kW (although an upgrade to a three-phase supply may be possible, at a cost)

Further information and advice on hybrid heat pumps can be found on the Energy Saving Trust website

A heat battery

A heat battery can store spare heat (or electricity) in a material when it changes phase from a solid to a liquid.  These materials are referred to as phase change materials.  When hot water is needed the phase change material change back to a solid, releasing heat. As heat batteries are generally smaller and lighter than a cylinder or thermal store they can be a good option if space is limited.

Instantaneous water heaters

An example of an instantaneous water heater is an electric shower – it uses electricity to produce hot water only as you needed it.  Instantaneous water heaters can also be fitted to supply a single tap and can be economical because the hot water is only generated as it is needed.  However, an instantaneous water heater is not as well suited to providing hot water to a bath, due to the volume and flow speed needed for a bath.

And potentially, a high-temperature heat pump,

A high-temperature heat pump can produce heat between 60 and 80°C by using a different refrigerant to a more conventional heat pump.  However, this type of heat pump is more expensive to buy and run.  High-temperature heat pumps are reported to provide 2.5 units of heat for every unit of electricity consumed; a conventional heat pump is more likely to produce between 3 and 4 units of heat for each unit of electricity consumed.

Summary

This blog is intended as a general guide only to the options available for the delivery of your hot water supply if you have an air source heat pump installed.  The sizing and design of a heating system (whether or not this includes an air source heat pump) is a matter for a professional installer who will be able to discuss the options through with you.  To find one go to the Microgeneration Certification Scheme website.

If the idea of enhancing your home’s energy efficiency through strategic retrofitting and delving into the realm of low-carbon alternatives intrigues you, then our SuperHomes project could be a perfect match. Our community members are at the forefront of redefining sustainable living. For further details, feel free to visit www.superhomes.org.uk