Heat pumps for hard to treat homes

Lots of UK homes are hard to treat with heat pumps, right? Well, no not really. What we have really historically suffered from in the UK is a lack of UK specific products. In the gap between growing demand and the delivery of new UK specific products, many UK properties – and particularly older homes - have been branded as “hard to treat” because they don’t fit the profile of the market for which existing heat pumps had been designed. I’m not keen on the label “hard to treat” because it sounds like the homes themselves were not good candidates for greener sources of heat. In fact, all they need is a different approach, which combines both new technical solutions and cultural change in the way we operate our heating.

The European market for heat pumps has historically been driven principally by France, Germany and Sweden. In each country the housing stock, electricity supplies, average winter temperatures, longevity of a green heating agenda and the cost of fossil fuels differ from our own. Fossil fuel heating in the UK is relatively much cheaper than elsewhere in Europe and greener heating options were until recently considered for those with an “alternative”, dare I say, hippyish, outlook on life or for the techno-loving rich . They were not, in short, taken seriously as an option for the mainstream consumer. It is only in the last five years or so that the UK has really begun to push thinking and offer incentives on alternatives to fossil fuel heating, which have made them a viable option for most consumers. In contrast special lower electricity prices and tariffs for running heat pumps have been long established in Germany.

This means that until recently heat pump products typically available were not always a good fit with the average UK home, because average homes were not heat pump customers and demand was low.

Incentive schemes, a growing green conscience, the recent rise in fossil fuel costs and increasingly stringent standards for lower carbon emissions in building regulations have meant that the UK market has nonetheless developed rapidly. Investment from heat pump manufacturers to create products that meet this demand has followed. There are a large number of so-called hard to treat homes in the UK simply because we differ from the European market in three main respects:

  1. Firstly we have a comparatively large number of old houses (over 40% are pre- 1945 compared to percentages in the 20s for France, Germany and Sweden for example) . These need more power to heat them than equivalent sized new homes.
  2. Secondly, we have a temperate maritime climate which means that we operate our heating differently than northern continentals (in short we turn it on and off and they leave it continuously on in winter because it is too cold to turn it off).
  3. But perhaps one of the most difficult problems we face with installing heat pumps in the UK is that most homes have single phase electrical supply , unlike most of the rest of Europe, which has three phase.

So let’s look at these differences in more detail.

Single phase supply

Single phase supply limits the amount of power a heat pump can deliver for heating and hot water production. Traditional boilers have a dedicated supply of energy (a gas supply or oil tank running to your house) , the primary use for which is always heating. They only use electricity for the control circuitry, which represents a tiny electrical load. The electric motors that run a heat pump have to compete on the circuit with the other large appliances that require electricity and we therefore have to ensure that demand by a heat pump is limited to ensure that it does not disrupt the supply to other appliances (or in other words, blow the main fuse), or indeed, other properties. With a three phase supply every home has three lines of electrical energy linked up to their heat pump and the load is shared between the three lines. In single phase systems the three lines generated by the turbine at the power station are not run into each house. Instead different properties are supplied by just one of the lines. This, of course, means a lot less wiring and a cheaper system to install, but does mean that there is more risk of the supply being unbalanced by single, power-greedy users.

This is why in the UK there are requirements for heat pump installers to notify a network operator before installing a heat pump, so that they can monitor demand on the network.
The difference between single phase and three phase supply means that products designed for the European market are very often not suitable for UK domestic supply. As a result, there has until recently been a dearth of products suitable for domestic use in the UK. Five or six years ago you would have struggled to find a single phase heat pump for the UK. But with the burgeoning interest and government commitment to greener sources of heating, and the continuing rise in fossil fuel bills, manufacturers have begun to respond to demand.

Stiebel Eltron is committed to supplying products designed for the UK market. So obviously a range of single phase products is the first and most obvious answer and Stiebel Eltron UK now has a wide range (see table below).

kW required MCS approved (RHI ready) single phase options from Stiebel Eltron
5 Ground source: WPC 5(s), WPF 5(s) Air source: WPL10AC(s)
7 Ground source: WPC 7(s), WPF 7(s) Air source: WPL13(s)
10 Ground source: WPC 10(s), WPF 10(s), WPF10M(s) Air Source: WPL15A(s) (coming summer 2013)
12 Air source: WPL18(s), WPL15A(s) (coming summer 2013)
14 Ground source: WPF 14 SET(s) Air source: WPL13(s), WPL25A(s) (coming summer 2013)
16 Air source: WPL25A(s) (coming summer 2013)
17 Ground source: WPF 17 SET(s) Air source: WPL33HT(s)
20 Ground source: WPF 20 SET(s)


Intermittent heating patterns

Another reason that UK homes can be “hard to treat” is that we are used to running our fossil fuel systems intermittently. We turn them off when we go out to work and expect them to be powerful enough to heat up our home quickly when we get back. For this reason fossil fuel boilers are often oversized for the property treated. On the other hand, the golden rule for heat pumps is that the lower the water temperature flowing to your heating system, the more efficient they are. This is why the heat pumps are often paired with underfloor heating or fanned radiators, which are suitable for use with low flow temperatures. However this type of emitter can be expensive and difficult to fit in retrofit situations (harking back to our other issue of older housing stock). Obviously slow response times are not an issue when you turn the heating on at the beginning of the winter and leave it on day and night until the end. In Sweden for example, they have on average 70 days below freezing in the south of the country, rising to 180 days nearer the arctic circle. In these conditions it is far more efficient to maintain a constant temperature inside, as the temperature differential with the outside is so great. Moreover inside temperatures would drop so quickly and take so long to raise again if the heating were turned off that it becomes untenable.
Obviously in our milder climate there are times when it may be more efficient not to run the heating, and we can stand the chill for a bit when we return home, as long as the heating does respond within a reasonable period.

The first and most obvious solution to this issue is simply common sense and education of the client. Whereas efficiency is always paramount, and I would never advocate running normal heat pumps at high temperatures, there is of course a sweet spot where running a pump slightly outside optimum conditions still maintains a good return on investment and reduces carbon footprint. Strike a balance with the client early on in the design process between efficiency and response times and a solution can be found .

Bear in mind that clients don’t always require all areas of the property to have the same response times. Accommodating a slower response in sleeping areas is often not a problem as long as living areas heat up quickly. So you can save on installation costs by only upgrading the heating system in certain areas. All Stiebel Eltron heat pumps have the ability to control different areas of a property at different temperatures so you can create areas with a faster response.

A new product that increases the sweet spot between response time and efficiency even further is our higher water temperature heat pump. It is designed to give temperatures and response times and comfort with return on investment.

For more reading on the relative efficiency of heat pumps using different heat emitters see the MCS heat emitter guide.

Older housing stock

Another issue with older homes is that can be prohibitively expensive to raise the insulation value of the building. This makes it harder to raise and maintain comfort temperatures compared to newer homes. This problem can really only be solved with power. To have reasonable response times and reach a reasonable level of comfort the heat pump for an older home needs to have a higher kilowatt output for the size of project than a newer home of a similar size (which of course would also be true of a fossil fuel system). This posed an engineering challenge on our single phase supply to reduce the start up current of a more powerful electrical motor so that it didn’t overload the system. These high power and single phase heat pumps were a little while in coming – but they are now available and the range of suitable pumps continues to grow.

Reason why UK homes are hard to treat with heat pumps UK has a lot of older, difficult to insulate properties (e.g. those with solid walls, listed wooden windows etc) UK has a single phase electric supply. Historically most heat pumps were designed for European three phase supply UK has a culture of intermittent heating using oversized fossil fuel boilers for fast response The emitter sizes in UK homes are very small because they are designed for high flow temperatures. Replacing the whole system can be impractical and expensive.
Solution Using high kilowatt heat pumps designed for the UK market. Single phase heat pumps designed for the UK market. Consider a hybrid system Cultural change by educating customers on different heating patterns and agreeing a balance between speed, power and efficiency. Choosing high power heat pumps. Consider a hybrid system. Running heat pumps with higher flow temperatures that can work with traditional radiators. Again consultation and agreement with the client on the balance between upfront cost and operational efficiency at design stage is key. Consider a hybrid system.
Products designed to help WPL 33HT(s), WPL 15/25 A(s), WPF14/17/20 kW SET (s) WPL 33HT(s), WPL 15/25A(s), WPL 13 (s), WPL 18 (s), WPL 10 AC(s), WPF Range, WPC Range WPL33HT(s), WPL15/25A(s), WPF14/17/20 kW SET (s) All Stiebel Eltron heat pumps have weather compensation and have the facility to operate with a boiler in a hybrid system. All STE heat pumps can run part of the property at a higher temperature than other areas. This means the customer doesn’t always have to find the money to upgrade the emitters in the whole house. WPL33HT(s), WPL15/25A(s)


In short, I would encourage any installer evaluating a property to think again about the possibility of a heat pump. New products are constantly changing the boundaries of the possible and the viable. As a manufacturer we welcome the calls asking if it is possible to do something – and it helps us to know what products you see a need for that you don’t yet have. So instead of thinking “hard to treat”, let’s start asking, “is it possible?”. And we look forward to hearing from you soon!