Heat pumps

What is a heat pump?

Air source heat pumpVery simply a you might think of a heat pump as the opposite of a refrigerator or freezer. It captures heat from outside and moves it into your home. It uses electricity for a process of extracting heat from the air by compressing gas, as opposed to directly creating heat energy itself (like an electric fire) and so can be much more efficient than other electric heating systems. This can mean the amount of heat delivered into your home is much greater than the amount of electricity used to run the system. To put it another way, 1 kilowatt used by a heat pump can give you three or more times the heat that a 1kW electric fire can give you.

The heat generated can be transferred to your house via a water based  or air based system, but in the UK most houses have a central heating system with water filled radiators. Even when outside temperatures are freezing, a heat pump can create a lot of heat with hot water at least up to 60º celsius. [check]

The most common heat pumps use heat extracted from the air or the ground. As ground sourced heat pumps are usually too costly and impractical for retrofitting domestic properties, we will mostly be looking at Air Sourced Heat Pumps (ASHP) working with conventional central heating water filled radiator systems. However we are aware of some ground sourced heat pumps installed in domestic properties in Southampton so we hope to be able to include some case studies of those also.

So will a heat pump save me money?

Not necessarily. Note in first paragraph above the word can. “This can mean the amount of heat delivered into your home is much greater than the amount of electricity used to run the system.” A heat pump can be run very efficiently, it can also be run inefficiently. They run most efficiently at much lower temperatures than conventional central heating systems, so although you could run a heat pump at a high temperature and maybe use smaller radiators, it would most likely be using a lot more electricity.

However by running a heat pump efficiently it will use a lot less than other electric heating and should cost no more than a gas system but energy prices may change all this.

Do heat pumps work with your existing central heating system?

In many cases you can retrofit a heatpump to replace a gas boiler, but in order to gain the efficiency of which they are capable, the temperature of the water needs to be lower than with a conventional gas boiler system. This is why you often hear people say you will need radiators replaced with bigger ones or have more installed.

In order to heat the same room with lower temperature in the system, a larger surface area of radiator is required. Before getting a heat pump you may be able to get an approximate idea of whether you will need to increase radiator size by merely running your boiler at a lower temperature to simulate the type of temperature a heat pump might run at (*typically around 45º ±  10º depending on the efficiency of your house and the external temperature).

*Please be aware though that if your boiler is connected to a hot water cylinder, then the water should periodically be heated to 60º as a precaution against legionnaires disease.

You may possibly also need to have your pipework replaced. If the pipework is sound, then normal 15mm pipes are often fine. Anything smaller  than 15mm though is unlikely to be adequate.

Beware any heat pump installer who does not do a thorough heat loss survey to ascertain that the existing system is adequate. This will include taking measurements of every room and window, noting which rooms have external walls and are particularly exposed and so liable to get cold quickly. See heat pump case studies.

As the most efficient temperature of a heat pump system is lower, it also makes sense to make sure that the fabric of the house is as efficient as possible in regard to heat loss. Obviously poor insulation and/or draughts are not ideal with any heating system. In such cases a conventional boiler can usually still heat the house adequately, albeit inefficiently, whereas a heat pump could either struggle to maintain an appropriate level of heat while running at recommended temperature.

Can anyone get a heat pump?

Sadly no. As we said above, heat pumps are not always viable in many older houses without replacing the entire plumbing system. But also you need to have a suitable outside location for the heat pump. Not only do you need a garden for an ASHP (or possibly a flat roof), but they need to have air space around them. A ground source heat pump will need several deep bore holes in your garden.

If your property is very large you may need two ASHPs, and planning permission will be required.

Also, although heat pumps are much more efficient than conventional heating, there is a limit to the amount of heat they can give out. For this reason we cannot stress enough that the actual fabric of your house must be as efficient as possible in regard to heat retention, ie insulation and draft proofing. And more insulation…

If you are wanting to take advantage of a grant, then In order for MCS certification and the eligibility for a grant, it is necessary for your house to achieve a high energy efficiency rating (EPC certificate at least D rating). Sadly in some conservation areas strict regulations may prohibit or discourage many of the energy saving retrofit solutions such as solar panels and double glazing. Retrofit Southampton intends to lobby both conservation area freeholders and SCC to make their terms and regulations more pragmatic in the light of global climate change and energy crisis.

Weather compensation

As noted above, heat pumps are most efficient at lower temperatures. For this reason they usually have a feature that automatically changes the temperature of the water be optimal in regard to efficiency of operation and ability to make your house warm enough.

So, for example, on a very cold winter day when it is -2º outside you may find that your heat pump must operate at 55º in order to heat the house. However as spring approaches and temperature are more like 10º, then by dropping the system temperature to 45º or less you get much more efficiency from the heat pump.

A typical weather compensation setting would set a low and high system temperature (“set points”) for corresponding high and low external weather temperature.

heat pump weather compensation
Weather compensation set points

In the above readout, the water in your central heating system will be 58º when it is  -10º outside, and 35º when it is a balmy 12º. At 0º it will be somewhere between the two.

How do you know how efficiently it is running?

Heat pump efficiency measurement is called Coefficient of Performance (COP). When measured and averaged over a given period of time it is called the Seasonal Coefficient of Performance (SCOP). So for example if a pump uses 1kW of electricity to create 3kW of heat, it has a COP of 3. Most heat pumps these days have a readout to let you what the COP and SCOP are, either as a display on the system or via an app [check this…].

But how do you know when you have set the set points optimally? There is a sort of rule of thumb (as explained by Freedom Heat Pumps) that if you set your thermostat to demand an extra 1º of heat, then you should expect it to take approximately one hour to do that. This is probably slower than you would expect from a gas boiler system (especially if you have the temperature set high). If you want it faster than that, then you have the option of either setting a higher temperature (resulting in higher cost) or improving the heat loss of your house.

It is recommended to not have the heat pump switch off at night, but to keep it running with a lower thermostat demand (by approx 3º). This means that if you choose your comfortable daytime temperature as 21º, you would set the thermostat to 18º at night. If you want to wake up at 7.30 to a toasty house, you should set the thermostat to 21º at 4:30 am to allow for the three hours warming up time.

More info & links

How to correctly install heat pumps so that they work properly and efficiently

For a very clear and in depth explanation of heat pumps see Energy Saving Trust

A very useful book: 50 things you need to know about heat pumps by Graham Hendra (ABC Heatpumps)
Graham Hendra: Heat pumps and weather compensation

For those of you who already have a heat pump these videos may be very useful: Graham Hendra’s heatpump channel

Videos from Heat Geeks:


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