by Rob Reid
8 minute read
I am currently spending considerable time, effort and funds renovating my garden. While digging trenches, laying concrete and getting back ache in the process, I am seriously considering whether I can extract a bit of extra value from my soon to be dug up lawn by using it to satisfy my home’s heating needs. With the launch of the Domestic Renewable Heat Incentive (RHI), and a recent Eunomia study published by DECC showing that impacts of refrigerant leakage from heat pumps is small, the timing seems right to consider this method of heating.
I live in a 100-year old stone cottage with 20″ thick walls, which has a C-Rated Energy Performance Certificate. I am ‘on gas’ with a relatively new combi boiler and I already have a wood burner installed – but I am concerned by the increasing cost of fuel, and about the air quality issues that come from the use of biomass. I’m also conscious that heating my home by burning fuel leads to CO2 emissions, whilst electrical heating will be responsible for less emissions as we decarbonise our generation mix. But would installing a heat pump prove beneficial for me, as well for the planet?
Pump action
Unlike conventional heating which works by burning fuel to generate heat, a heat pump just transfers heat from one place to another. It’s a bit like a fridge in reverse. By gathering heat from a large area, the pump is able to use a relatively low temperature heat source and concentrate it so that it can be used for space heating or domestic hot water.
Although the heat pump uses electricity to run, it uses much less electrical energy than it would take to generate the same amount of heat that it gathers. Efficiency ratings are used to show the amount of heat delivered compared with the electricity used. A high rating indicates good performance, whereas a low rating indicates poor performance.
Like most UK householders who are considering installing this technology, the choice for me is between either a ground source heat pump (GSHP) or air source heat pump (ASHP), although for those with a convenient lake, river or pond there are also water source heat pumps (WSHPs) available.
A GSHP pumps a mixture of water and anti-freeze around a ‘ground loop’ of plastic tubing that is either buried horizontally in a shallow trench or vertically in a borehole. As the fluid passes through the ground loop, it naturally absorbs heat from the ground. The heat pump uses an evaporator, a compressor and a condenser to take the heat from the water mixture and increase its temperature for use in a domestic heating system. The length of the ground loop needed (and therefore the size of the trench or depth of the borehole) can be determined by assessing the peak and annual heat demand for the property.
An ASHP uses an electrically-driven fan to draw air across its evaporator, from which it extracts its heat. The heat is then compressed and condensed to a higher temperature for use in a domestic heating system. Like an air conditioner, it is typically situated on the outside of a property, piping the heat through the wall. Some installations include ducting so that heat can be circulated around the property rather than in the one room which the ASHP is supplying.
Pumps inter pares
Given my situation, I could install either technology. I do not need to consider planning permission if I install a GSHP (or indeed a WSHP) as both are considered permitted developments. Planning permission is required for ASHPs in Wales and Northern Ireland. However, that wouldn’t apply to me – no planning permission is necessary for an ASHP in England and Scotland providing certain conditions related to the size and location of the installation are met.
An ASHP appears far more straightforward to install at the back of the property with minimal disruption. However, an ASHP’s performance gets worse as the air outside gets colder and there is less heat readily available to extract, until it can end up being no more efficient than electric or gas space heating. There is also a concern that ASHPs can freeze up in cold weather, with electricity having to be used to defrost it, which affects performance. By contrast, a GSHP would provide a more constant performance due to the less variable temperature of the ground. I am digging up my garden anyway and I have the space to install ground loops, whether in a trench or vertical borehole – although getting the rigging equipment needed for the latter into place could be tricky given the limited access to the back garden.
Are views about ground source heat pumps entrenched? Photo by Mark Johnson, via Wikimedia Commons.
When weighing up the costs and benefits, it is important to consider the heat source that is being displaced. For example, if direct electric heating, coal or oil is being used, the capital spend on installing a heat pump will lead to big savings, particularly when taking into account RHI payments. However, heat pumps achieve only around the same efficiency as gas central heating, at least providing the heat pump has been set up correctly and is performing well.
Survival of the retrofittest?
So if the disruption is going to be minimal, and I can get the same performance from a heat pump as from my gas boiler, while saving CO2 and benefiting from the RHI, what’s holding me back? Aside from the capital outlay, retrofitting heat pumps in houses has its challenges. Because a heat pump operates at a lower temperature (30°C to 40°C) than central heating systems with radiators (50°C to 60°C), where the heat pump is also supplying hot water, electric immersion heaters are often used to increase the hot water temperature to 60°C. Some fluids in heat pumps produce lower efficiency ratings when operating at higher temperatures. Consequently, some homeowners only use heat pumps for space heating with immersion heating or solar thermal used for hot water.
As a result of their lower operating temperature, they work best when twinned with under-floor heating. Whilst it is possible to retrofit this to an existing building, it is less costly and disruptive if it is designed in when homes are built or refurbished. When fed into a traditional radiator system, heat pumps are less efficient. The heat pump, especially if it isn’t correctly sized, may not able be able to satisfy the peak domestic heating demand. In any case, larger radiators will be needed to compensate for the lower heat output.
The efficiency of the heat pump can be improved further by ensuring that the whole energy performance of the building is brought up to scratch. Any new build that complies with existing building regulations will include thermally efficiency measures such as thick loft insulation, double glazing and cavity wall insulation. Again, older properties may present more problems, making it harder to ensure that the heat pump’s hard work is not being lost through draughty floors, single glazed windows and poorly insulated walls and ceilings.
However, concerns about performance are to some degree allayed by field trials carried out by the Energy Saving Trust. Although performance varied widely, overall GSHPs recorded slightly higher efficiencies than ASHPs. Performance was highly sensitive to installation and commissioning, and some householders reported issues in understanding instructions. Those that were installed and operated effectively performed to a very high standard.
Heated debate
Given the number of factors that come into play when considering a major investment in renewable heat, at this stage, I feel that I still need to gather further information, including an assessment of my heat demand before deciding whether a heat pump is appropriate for me.
First and foremost, I have neighbours and fellow Eunomians that are living in properties where heat pumps have been installed, so I will be speaking with them about their experiences.
I will also consider the Green Deal, as it provides a range of services including an assessment, installation and financing for energy saving improvements in the home with the investment paid back incrementally through utility bills. It has not had the most successful of starts, but DECC recently announced the launch of the Green Deal Home Improvement Fund. This provides financial support towards the cost of installing energy saving measures and Green Deal assessments.
If I do move forward, then I’ll be looking for an expert installer. The Microgeneration Certification Scheme (MCS) is an industry-led and internationally recognised quality assurance scheme. MCS certifies microgeneration products and installers to ensure that products are installed and commissioned to the highest standard. I plan to contact a number of MCS installers to provide quotes for the cost of installing a heat pump, the savings I could make and the payback period taking into account RHI payments.
I plan to report back in late Summer with my research findings including my decision on whether I plan to go ahead with installing a heat pump in my home.
Really interesting article Rob. Obviously the fact that you are already digging up your garden has prompted you to think about a GSHP, but have you also looked into external solid wall insulation? If you have 20” think stone walls, and you were to ‘wrap’ the house, as far as I can see you would then benefit from the thermal mass of the stone, keeping a steady temperature inside your house. This is what we have in our place where we renovated an existing concrete and brick building and externally insulated it. Have you looked into the relative costs and benefits of this?
Thanks Chris, solid wall insulation is a very good suggestion, and it’s certainly something I’ll consider. My initial thoughts are that the timing may not be right. When we moved into the house two years ago we completely gutted the internal rooms – in hindsight, the perfect time for solid wall insulation, and potentially underfloor heating as well. If we are to install insulation externally, we will need to balance the impacts on the aesthetiics of our somewhat unique stonework against thermal performance improvements, which I agree could be significant. I guess this demonstrates how important it is to focus on whole building performance improvements and not just one aspect in isolation. I’ll report back with my more informed view in my next article.