The increasing cost, both social and monetary, of relying on gas or oil for heat and hot water is driving many families here in the UK to either stop heating their homes or switch to electricity-based sources of heat generation. Many others are seeking out lower carbon alternatives to gas or oil based heat and hot water generation.
I started researching heat pumps over a year ago for a house renovation project; at the time I knew absolutely nothing about heat pumps. I’ve gone from heat pump idiot to heat pump novice, so I’m doing my best to share my ever-so-limited learnings about the great heat pump revolution. The first and biggest question I had when looking at how to build a more sustainable home was how to keep it warm. Heat pumps offer an alternative to fossil fuel based heat and hot water generation as they run off electricity.
I have looked at ground source heat pumps (GSHP) and air source heat pumps (ASHP) as alternatives to gas and oil based boilers. The choice between the two used to be tricky – experts would tell you it depends on location, climate, land, size of the property. The consensus only a few years ago was that GSHP were more efficient and also more expensive. However, there were larger grants available from the government to support installing GSHP. Times, it seems, though, are a-changing. The Boiler Upgrade Scheme (BUS), launched in the UK in April 2022, and in effect until April 2025, provides up to £5,000 towards the installation of an ASHP and £6,000 for a GSHP. However, an ASHP system for a 4 bedroom house will typically cost £12,000-£15,000 (prices accurate as of summer 2023), but a GSHP of similar size will cost upwards of £30,000, due to the additional ground works.
I have read lots of blogs that put the prices lower than that, but the above is what I have been quoted by multiple suppliers and installers, so I don’t believe what I am reading in sales-driven blogs. They are either out of date or wearing rose-tinted glasses. My point here, though, is that the grants being so similar in value £5,000 for ASHP and £6,000 for GSHP, but the cost fully installed being so far apart £12,000 for ASHP and £30,000 for GSHP, means the BUS clearly favors ASHP installations.
Traditional thinking will tell you the decision to go with a GSHP or ASHP system is a complex one. GSHP salespeople will still tell you this. GSHPs were traditionally more efficient than ASHPs. Just a few years ago, a GSHP would give you 4 units of heat output compared to every unit of energy input; whereas, ASHPs were averaging only 3 units of heat output for every single unit of energy input. So, how can we tell how efficient different heat pumps are?
Lucky for us, there is an agreed measure. It is called the Seasonal Coefficient of Performance (SCOP). If you are preparing to speak to heat pump salespeople, get to know this term. They will throw it around like you are supposed to understand what it means. I suspect, though, some less informed salespeople are probably hoping you don’t actually know what a SCOP is. I once picked a salesperson up on a SCOP figure that sounded funny and the vitriol and defensiveness in his response belied his lack of deep understanding of the numbers and also meant he didn’t get the job. But I digress (again).
Seasonal Coefficient of Performance (SCOP)
The SCOP gives a theoretical indication of the anticipated efficiency of a heat pump aggregated over a year using standard climate data across Europe. How is that for a caveated definition? I never heard so many subtle disclaimers! Let me give you the short version: a SCOP measures the total heat energy generated (output) for each unit of energy (electricity) consumed (input). Basically, it tells you how much heat you are going to get out (in kWh) for every electricity unit you input (in kWh). The caveats and disclaimers come from the wide averages used to estimate weather. Heat pump performance depends on how cold it is. The colder it is outside, the harder they have to work.
The main reason for this divergence in ASHP and GSHP efficiency, traditionally, is due to the variation in heating efficiency over the winter season. Ground temperatures do not fluctuate as much as air temperatures. So, as the air gets colder, an ASHP has to work harder to heat the air, reducing its efficiency somewhat. So, when efficiency is measured over the year, ASHP efficiency factors were traditionally lower overall. On average, a GSHP has a SCOP of 3.5-4.5. This means for every unit of electricity you put in, you get 3.5-4.5 units of heat energy out. When ASHP’s were first introduced, they had SCOP ratios of closer to 2. Over the years, however, the technology has improved.
Most ASHPs now have SCOP ratios between 3 and 4, with some product specifications claiming SCOP ratios of 5+. ASHP SCOP ratios are typically given for 35/55 meaning the ratio as measured at an operating water temperature of 35 degrees or 55 degrees. For example, the Mitsubishi Ecodan claims a SCOP of 3.3/4.81 (35/55) but experts will tell you it is more like 3-3.5, on average. The Vaillant aroTherm and NIBE S2125 have slightly higher SCOP ratios of around (5/3.7).
So, GSHPs are still slightly more efficient than ASHPs, but the margin has narrowed to the point that the extra investment required to install a GSHP doesn’t typically pay itself back. I have heard anecdotally that ground source heat pumps are less prone to installation issues, and are better able to sequester warmth for use later, making them more efficient. That being said, the general consensus has shifted in the last 18-24 months. The market has moved distinctly in favor of ASHPs overall.
Cost to Install
The cost of heat pumps (ground or air) is not insignificant. On average, the models we evaluated, including installation, cost between 3 to 5 times more than a gas boiler (including installation). In our situation, the cost of installing a boiler would be significantly higher than usual, because the barns we are renovating don’t have a connection to the gas mains. Under these circumstances, and taking into account a potential grant under the UK government’s boiler upgrade scheme, it became cost competitive to install two air source heating systems, rather than install new gas connections, and new gas boilers.
An ASHP will cost you between £3,500-£5,000 for 11.2kWh, which is about what you need for a large 4 bedroom house. That is just the price of the heat pump itself. You will also need a controller, hot water cylinder, plus installation. All in, you can expect to spend roughly 3 times the cost of the pump itself, or between £12,000 and £15,000 for a large 4 bedroom home.
Heat pumps work best with low flow temperature systems, such as underfloor heating. Many practitioners will tell you that they can work equally well with radiators and other systems, but the laypersons that I have interviewed who have installed systems have almost unanimously stated that they work better with underfloor heating. Expect underfloor heating to add at least another £5,000 to your total investment cost.
Heat pumps are more efficient at lower water flow temperatures, which makes them better suited to work with an UFH system designed to run at 35 degrees, for example, as compared to a radiator system designed to work at 45 degrees. For every degree cooler your flow temperature (the temperature, in Celsius, at which the water is circulating in the heating system) is, you will get a 2.5% increase in efficiency.
That means if you are using a heat pump with an UFH system designed to run at 35 degrees, that will be 25% more efficient than a boiler based system using radiators designed to work at 45 degrees water temperature. Because of this, the combination of radiators and underfloor heating could negatively impact the overall efficiency of your project if the system is not optimized.
For example, if you have underfloor heating designed to work at 35 degrees, and you have a radiator running on the system at 45 degrees, you run the risk that all the water is heated to 45 degrees for the system to function. That would reduce your efficiency. It is for this reason that a project needs to be considered in its entirety. There is a risk the system could be less efficient with a combination of radiators and UFH. However, well designed systems can have radiators and UFH with different manifolds running at different temperatures. It is just an item that needs to be considered in the design phase (and another reason to have help from specialists).
Cost to Run
Now, the thing is, heat pumps aren’t just more expensive to install; they were also traditionally more expensive to run, when relying on electricity and gas provision at historic prices. However, given recent changes in electricity and gas prices, this is no longer the case.
Heat pumps are more efficient than gas boilers – that statistic you will see all over adverts for heat pumps. However, electricity is more expensive than gas, so, even though the heat pump takes less energy to run, the cost of that energy (because it is electricity, not gas) was traditionally higher overall. However, soaring gas prices mean that now heat pumps are marginally less expensive to run than gas boilers.
A heat pump, under ideal conditions, according to sources noted at the end, can achieve up to 500% efficiency (remember that SCOP ratio? This is the same thing.), compared to 90% efficiency of a traditional gas boiler. 400% efficiency means that for every kW of energy input (in the case of a heat pump, electricity kW), the heat pump puts out 4 kW in useful heat. This is because they are transferring heat, not creating it. We will use 400% as the average figure for heat pumps as the 500% measure is what can be achieved under ideal conditions, not what is generally achieved under normal conditions.
Electricity costs, historically, were 4 times more expensive than gas per kWh. So, that traditionally wiped out any benefit from the improved efficiency of the heating system. Let me give you an example, based on peak heat pump output:
Traditional Boiler – input of 1 kW of gas creates 0.9 kW of useful heat and costs 10.3p per kWh based on 2022-3 prices. Average prices over a longer period of time are more like 4p, so it is fair to say UK gas prices have remained relatively high since Russia’s invasion of Ukraine. Prices have backed off slightly in recent months, and large drops in wholesale prices bode well for future retail gas improvements. However, how much of the wholesale price declines will filter through to consumers, and on what timeline, remains to be seen.
Heat Pump – input of 1kWh of electricity creates 4kW of heat and costs 28-34p per kWh based on 2022-3 prices which are currently at or just below the government cap. Average electricity prices in 2021 in the UK were 18p/kWh as a point of reference.
So, if my heat requirement is 10 kW of output, then that will cost me £1.14 in gas input with a traditional boiler, or 85p worth of electricity input with a heat pump under peak conditions. If you require 12,000 kWh of heat to keep your home and hot water warm a year, that would cost you £1,368 in gas with a boiler, or £1,020 in electricity inputs with heat pump, based on average 2022-3 prices as of summer 2023.
Heat pumps require significantly less maintenance than traditional gas boilers, so you can reduce your annual running costs by the cost of an annual boiler service as well. As well, traditional boilers typically last between 10 and 15 years – heat pumps have a lifespan closer to 20 years, meaning that over a long period of time, the cost of replacement should be lower as well.
That being said, there is variation between ASHP and GSHP life spans. There is some risk that ASHP life spans can be reduced by their exposure to the elements (being exposed to weather, rather than underground). Some sources state that GSHPs last longer on average – closer to 20-25 years, and ASHPs are more likely to break down before hitting the 20 year mark.
It is also worth distinguishing between the heat pump in a ground system and the ground loop itself. Installation of the ground loop represents a large portion of the up front investment in a ground source system, and the loops themselves have a lifespan of 60+ years. It is just the pump that needs replacing after 20-25 years.
It is also worth noting that heat pumps and boilers work very differently. Setting a schedule for a boiler is a good way to save gas when you don’t need warming up, like overnight, when you are tucked up under a warm duvet. However, heat pumps work better when the target temperature stays constant because it is harder for the circulating hot water to reach extremely high temperatures. So, it is more effective to set the desired indoor air temperature to a constant, like 18 degrees, and leave it there, letting the heat pump work when it is needed to top up heat loss. This will help to minimize your cost to run.
Heat pumps are more expensive to install than gas boilers, even with help from the boiler upgrade scheme. But, with gas prices rising, they can offer cost savings on your annual energy bills of roughly 25%, or £350 based on average usage at today’s prices.
Given the quotation we received for installation, not including hot water cylinders or underfloor heating, it would take 25 years to pay back the up front investment based on today’s energy prices, or just over 11 years, with a £5,000 grant from the government.
A critical piece of learning from my research and speaking to existing users of heat pumps is that switching to heat pumps is much more cost effective if you are supplementing your electricity consumption with your own renewable source. For more on that, visit this article on solar panel installations.
While gas prices have risen dramatically, prompting many to switch, it is worth considering that electricity prices have also gone through the roof.
At the moment, if you can afford the cost of equipment switching, heat pumps can provide you with modest savings on energy prices throughout the year. I should caveat this by stating, however, that that could change should prices fluctuate in the future.
And, finally, it is worth noting that a GSHP system can also provide passive cooling in summer. GSHP systems can extract heat from your home, and some offer seasonal storage so that heat is stored underground until it is needed. As temperatures rise on average, this feature may become increasingly important.
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