The cold facts

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Graham Wright of the Heat Pump Association dispels some common confusion surrounding heat pumps and explains why this often underrated technology is becoming a favoured form of heating for the future.

In 2018 the UK continued to commit to more energy efficient building practices. This was underlined in May in a speech by Prime Minister Theresa May, which committed the UK to reduce its building energy usage by half. 

Theresa May’s reference to more energy efficient buildings and smart technologies was certainly welcomed by the Heat Pump Association (HPA) as heat pumps represent one of several environmentally friendly heating solutions for the long term. 

This is a view shared by Claire Perry, Minister of State for Business, Energy and Industrial Strategy, who declared during a debate in Westminster Hall on 10th October 2018, that she would like to see people in rural areas being supported with technologies like heat pumps.

While we acknowledge that a major short-term shift to solely heat pumps would be neither practical nor sensible, it should not be denied that heat pump systems of all genres are capable of significantly reducing carbon emissions. 

Misinformation inevitably has led to concerns in some quarters over whether heat pumps present a suitable option for enough properties for them to be considered a viable long-term heat source, but in my view the case for heat pumps has been made a long time ago. 

We need to ensure that our focus on delivering heat pumps to the domestic market is sustained as it has been in the commercial sector with significant success and recognition in aspects of building design.

 

Energy output

Heat pumps do work better in well insulated buildings, but the same can be said for all heating systems, in the sense that this means less energy output, wherever it is derived from, is required. 

However, if a building needs heating to a particular level then it will need a finite amount of energy (kWh) each year to achieve that, regardless of the source of heat. Hence the most optimal method to provide that amount of energy will need to be sought. 

You can find many examples of low energy, low running cost heat pump systems installed in thermally poor buildings, including Grade 1 listed buildings with no insulation and original leadlight glazing. A badly insulated building may require slightly higher flow temperatures than ideally desirable if existing heat emitters are to be re-used, which may reduce the efficiency slightly.

Take an old Victorian house with no insulation in the cavity and 50mm in the roof void. If the radiators have been selected using a crude rule of thumb it may be they are massively oversized when working at say a 70oC flow (typical of condensing boilers). 

With no additional treatment of the building they could provide enough heat output at a reduced temperature of say 55oC from a heat pump, but that is not the most ideal operating temperature for a heat pump. 

 

 

However it’s discovered (by checking all the radiator outputs at 45oC) that by increasing the size of all of the radiators requiring a flow of say between 45-55oC, the system can run at design conditions at a maximum flow temperature of 45oC and satisfy the heat demand. Hence by uprating certain radiators, the system is more efficient and this will result in lower running costs. 

Of course by insulating the cavity, the overall heat demand will reduce and now the existing heat emitters may be sufficiently sized to provide the new demand at a lower flow temperature that suits heat pumps. This will have the added benefit of reducing the energy demand for heating and hence reduce fuel bills, regardless of the heating system employed.

Another false belief is that heat pumps only work with underfloor heating. In fact, this is a slight misapplication of a correct principle, the principle being that heat pumps work better with as low as possible delivery (load side flow) temperature. 

Underfloor heating can work effectively at flow temperatures of 35-40oC if correctly designed, and the building can be effectively heated with a low temperature source. Natural or fan convector heat emitters will generally need to be quite larger than previously or run at slightly higher temperatures, resulting in small reductions in efficiency.

 

Separating fact from fiction

While there is an element of truth in the belief that air sourced heat pumps aren’t efficient in cold weather, stated alone it is misleading and therefore needs to be put into context. Heat pumps do work more efficiently in higher source temperatures and lower delivery (load side) temperatures, but the heat pump’s performance must be assessed over a year. 

This gives rise to ‘seasonal’ figures, whether Seasonal Coefficient of Performance (SCoP) or Seasonal Performance Factor (SPF). These seasonal figures either predict the performance over a typical year for a typical weather pattern, or actually measure the performance over a season.

I will emphasise here that SCoP and SPF are similar, but not necessarily exactly the same. This is further complicated by the fact that some standards (e.g. the EU RED) use them incorrectly or interchangeably. 

In general, SCoP is the predicted seasonal efficiency of a piece of equipment, whilst SPF is the measured performance of a heat pump system, but which can include or exclude various components such as circulation pumps etc.

Although our damp oceanic climate requires higher than average defrost cycles than a drier environment would, the oceans around us to tend to keep the winter temperature higher than more land-based countries (e.g. central Europe) and this offsets much of the reduced efficiency from defrost cycles – something which may not be represented in standard tests from other European countries.

 

Wet theory

A question we’re often asked at the HPA is whether your average garden pond is able to support a water sourced heat pump. Unfortunately for green conscious lovers of aquatic life, the short answer is no.

Any source needs to have sufficient heat capacity (in this case volume) to deliver the required amount of heat energy until the heat is replenished. A pond 4m in diameter and 0.5m deep is likely to hold just 37kWh of energy, meaning it could supply a heat pump of 8kW for just over 4.5 hours full load. And this is assuming that the heat is replenished in the pond by the next time heat is required.

While there is still work to do to give the wider public a better idea of how heat pumps work and their benefits, I hope this article has gone some way to clear up some of the confusion in the industry surrounding heat pump technology. 

In the meantime, it is crucial that the heating industry as a whole continues to develop technology for a greener future and increase awareness with accurate information on the various technologies available.