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Case Study - Air Source Heat Pump

Reducing Energy Usage

I have been asked by Hambledon Greening to give an outline of how we have reduced energy usage at our house and provide some suggestions arising from our experience. I am not providing technical details of the equipment installed or the economic side because the equipment has improved greatly since we started with insulation and solar panels (PV) in January 2010 and the financial incentives offered by the government have changed several times since then. In general terms, the incentives have reduced commensurately to the reduction in the cost of installation, but the policies have changed detrimentally and suddenly on occasion.


The three elements

The key components in reducing fossil fuel energy consumption are generally considered to be:

  1. Reduction of heat loss

  2. Generating one’s own green energy

  3. Replacement of inefficient fossil fuel heating systems with low or no carbon systems.

1 Reduction of Heat Loss

Generally, the starting point to reducing energy usage and costs will be the reduction of heat loss from a home. Both government schemes that we used, for the PV and the heat pump, required that an energy survey be completed and the home meet sensible requirements for insulation. We have had our home retrofitted to improve insulation and air tightness. This was probably our most cost effective step.

During the Winchester Festival in 2019 we visited two homes in Winchester, one built to Passiv Haus standards and the other being extensively adapted to meet these requirements. See . In essence, a Passiv Haus has very high levels of insulation and is close to airtight. This enables it to be ventilated by a heat exchanger system, but have no heating system because the heat generated by the occupants and the appliances is enough to heat the home, even in mid-winter. We were told that the additional building cost of a new house was approximately 10%. Our conclusion was that if we could build a new house it would be a Passiv Haus, but that converting a house would be unlikely to be feasible unless it was going to be completely refurbished. However, many of the principles of the Passiv Haus can usefully be applied to an existing house to reduce its energy consumption.

A number of useful online resources exist, for example and various governmental websites. These will provide information on the hierarchy of cost effectiveness of measures such as roof insulation, cavity wall insulation, reducing drafts (which are generally a major factor in heat loss in houses), double/triple glazing etc.own energy.

2. Generating one's own green energy

We installed PV in 2010 when there was the highest “feed in tariff”. This essentially provided us with a payment for every unit of electricity that we produce. The rate is increased by the RPI measure of inflation. It has already repaid the cost of the installation and continues. This repayment is despite the cost of installation being much higher at that time and the panels being fitted on a flat roof. It is also more shaded than I had anticipated by the roof line until noon and, in winter, by a large tree in the late afternoon.

At the time, no subsidy was available for solar water heating, but this should be considered, particularly if it could be combined with a heat pump.

It should be noted that even a passiv haus will use electricity for its appliances, heating water and the heat exchanger. Therefore, PV may still be worth considering even if the financial side of supplying electricity to the grid might not be sufficient incentive on its own.

At the time I researched wind turbines, they seemed to be too obtrusive and insufficiently cost effective in a domestic context. If you are lucky enough to have your own stream or river, this might be capable of generating electricity cost effectively.

It should be noted that most of the electricity used in the house will not be supplied by the PV as most use is in the winter, at the night or cooking before or after peak generation. Instead, the grid acts like a battery.

3. Replacement of inefficient fossil fuel heating systems with low or no carbon systems




Low and no carbon heating systems divide generally into those burning renewable fuel and those using a heat pump. I consider that the first does in fact increase carbon emissions for a variety of reasons, including the transport of it. It also significantly impairs air quality, to the detriment of the health of all of us. I would therefore dismiss this and focus on heat pumps.

We installed an air source heat pump in January 2020. We had considered it in 2010, but decided not to do so at the time because of the noise produced by the units available, the need to substantially upgrade our radiator system and the government deciding not to proceed with the subsidy system that was proposed.


Ten years later, the equation had changed

  1. The government had introduced the Renewable Heating Incentive (RHI) under which we receive quarterly payments to reimburse a significant portion of the cost of installation over a period. I believe there is a consultation currently on how this could be “improved”.

  2. The heat pump system that we had installed uses a unit like a large air conditioning unit outside the house and a second pump inside the house that boosts the temperature of the water to 60oc. As the radiator system had been over specified and the increased insulation reduced the heating requirement from that envisaged at installation, the radiator system has needed no changes to cope with the coldest days we have encountered.

  3. The latest air source heat pumps are substantially quieter. However, the inside unit is as noticeable as the boiler it replaced and should probably have been fitted on rubber bushes, an option mentioned in the installation manual. Although it is fitted immediately under the head of our bed, it soon was not noticed.  


Consideration should be given to alternatives to using air as a source as other options use less electricity. These include ground source, using a bore hole or horizontal pipes buried over a large area of ground, or water source, using a stream, lake or the sea. Ground source will generally have a higher installation cost and horizontal pipes will generally not be feasible unless there is a large horizontal area that can be excavated.


Our energy saving efforts have been well rewarded financially and have substantially reduced our outgoings and exposure to inflation, particularly exposure to increases in the cost of oil.

We have also gained space by removing the oil tank and thereby removed the risk of environmentally damaging oil spills. Our neighbour’s huge tree falling on it was a particular worry.

The PV has required no repairs or maintenance and the heat pump is promised to have very low maintenance costs.

There are frequent improvements to equipment and changes to the government incentives to it. The optimal solution will therefore change over time. The industry is still highly fragmented, so that finding the right solution is made more difficult because different installers tend to be tied to one or a few suppliers of equipment. Lists of certified installers are available on line. It is important to obtain information from several to be able to identify the most appropriate solution and to identify an installer that you can have confidence in. One issue is that the house has to have a substantial survey to establish if it is suitable and what will be required to satisfy the RHI requirement. Particularly given the shortage of installers, many will require an upfront payment for this survey. However, paying this may effectively give the installer exclusivity. I also found that some of the advertised installers were “introducing” others to do the work. They are probably best avoided as a local installer is likely to have a reputation to maintain.

John Russell July 2021

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