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Heat Pumps: Air source or ground source?

Posted by Aderanti Kudehinbu on 15 November 2016 at 10:30 am

Heat pumps are devices that use a small amount of energy to move heat from one place to another. During cold weather they extract heat from outside sources and pump it indoors. This process can be reversed during summer in order to act as an air conditioning system.

The heat pump circulates a liquid or gas refrigerant. This is cooler than the ambient temperature so absorbs heat. It is then fed through a compressor. Compressing the fluid increases its temperature so that it becomes hotter than its surroundings and can be used to warm up air and water. Pumping heat from one place to another requires relatively little electricity. Consequently heat pumps can provide a good alternative to off-gas oil and electric heating systems provided a property is well insulated. They supply more heat than the amount of electricity they need to run, giving them an efficiency as high as 300% [1].

There are two main types of heat pumps:

1. Air source heat pumps (ASHP):
Air source heat pumps extract energy from the air outside the house. They can
be used to heat a building in a number of ways.

Air to water system: This heats up water then distributes it via a wet central heating system. As the water coming into the radiators will be at a lower temperature than with gas central heating system, often radiators with a larger surface area (eg. double radiators) are installed.

Air to air system: This produces warm air, which is circulated through the house using fans.

2. Ground source heat pumps (GSHP):
Ground source heat pumps extract heat from the ground by pumping water through a series of buried pipes, known as ground loops. The ground loops circulate a mixture of water and anti-freeze. This is warmed up as it passes through the ground which stays relatively warm one metre below the surface in winter. This heat source can then be used to produce hot water, and warm a house via radiators, underfloor or warm air heating systems.

The heat pump performs the same role that a boiler does in a normal central heating system. However the energy comes from the ambient temperature of the ground, rather than using fossil fuels. The heat pump has to be carefully designed so that the size of the pump, the design of the ground loops and the selection of radiators are appropriate to the house.

The temperature of the ground one metre down tends to be at around 8-11oC in the UK. This stays fairly stable over the year, with minor changes which reflect the variation in seasonal air temperature. [2]

Comparing air source vs ground source

Cost
Ground source heat pumps are much more expensive to install. All air source heat pumps are fairly similar, but a ground source heat pump has to be set up differently depending on the location. Some houses will have space for large ground loops, while others will need a vertical system using bore holes. Digging or drilling holes to accommodate the loops will be a major cost so a GSHP install might cost between £9,000 - £16,000 whereas an ASHP might be £6000 - £10,000 according to the SuperHomes article here of 2013. The Energy Saving Trust quote more: £13,000 - £20,000 for GSHP and £7,000 - £11,000 for ASHP. Anecdotally, cheaper deals than both these quoted are possible where just a basic ASHP unit for a small property is needed.

Efficiency
Currently GSHP have the highest Coefficient of Performance (CoP), and Seasonal Performance Factor (SPF). This is because the temperature underground fluctuates less than the air temperature.

Savings
How much you save will depend on the type of fuel your heat pump replaces. Heat pumps have the potential to reduce running costs compared to oil boilers, direct electric heating and LPG or solid fuel fired systems. This can result in substantial savings over their lifetime, despite the high initial cost.

Trials have been conducted by the Energy Saving Trust and Mitsubishi Electric in order to estimate the savings from different heat pumps[3]. It was found that:

  • An air source heat pump with an SPF of  2.45 could give savings of around £150 and 1,400 KgCO2 per year when replacing an oil boiler, or £530 and 5700KgCO2 when replacing electric heating.
  • A ground source heat pump with an SPF of 2.82 is likely to save £300 and 1900 KgCO2 per year if replacing oil, and £685 and 6300KgCO2 if replacing electric heating.  These figures based on average performing pumps, installed in a well-insulated detached building.

Which is easier to install?
The ASHP is much easier to install. It consists of outdoor and indoor units linked through a wall. The GSHP has an indoor unit connected to the ground loop outside. Burying the loop requires a lot of digging or drilling.

Noise level
A typical ASHP system consists of an outdoor unit with a large fan. The fan will make a low level noise while the system is in use. GSHP systems tend to be quieter as pumps tend to make less noise than fans.

Planning permission
The installation of GSHP systems on domestic premises is considered to be a permitted development, so doesn’t usually require planning permission. However this might not be the case with listed buildings or properties in a conservation area.

How it looks
ASHP systems are normally mounted on the wall outside a building. These can detract from the appearance of the property. In comparison the GSHP array is buried underground so has no visual impact on the outside. In this case the heat pump itself is likely to be a large box which will need to be accommodated indoors in a dry, safe place next with a hot water cylinder alongside. You may need to think about whether you have space to accommodate it say in your garage, utility room or extension.

Insulation
Heat pumps are only suitable for very well insulated homes and sizing them correctly is a must. Otherwise your winter heat demand risks exceeding what the heat pump can comfortably deliver. With ground source heat pumps using the ‘boost’ mode in winter is to be avoided as it means extra electricity will be bringing the water up to temperature at significant cost.

In summary, which heat pump you go for will largely depend on space availability and your budget. Maintenance costs should also be taken into account. A GSHP is likely to require less maintenance over its life and will achieve higher efficiency in comparison to an ASHP. It thus has a longer life span. However the air pump is a cheaper and more convenient option.

GSHP and ASHP technologies are both eligible for for the Renewable Heat Incentive.
 

Photo: SuperHomes

References:

1. Energy Saving Trust on heat pumps:  Ground source  Air source

2.    British Geological Survey

3.    Mitsubishi Electric

Further Reading:

Are Heat pumps cost effective?

Green Match

Which? Magazine

 

More information about Heat Pumps on YouGen.

Find a Heat Pump installer

Need help with any Jargon?

If you have a question about anything in the above blog, please ask it in the comments section below.

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Comments

6 comments - read them below or add one

Bean Beanland

Bean BeanlandComment left on: 9 December 2016 at 12:39 pm

Dear NEF Gabby(?),

umm, interesting comments and reference site. The first thing is that the example you pointed me at is an air-to-air heat pump installation. This is a conventional A/C unit. The owner only formally heats one space as far as I can tell and relies on thermal drift for upstairs. Most people would not consider that to be central heating. At 98sqm if the house was at Passivhaus standards it would have a loss rate of around 17W/sqm. This would be a 1.7kW peak load. I do not believe that this house has achieved Passivhaus standards so the peak thermal load is almost certainly higher. The owner discusses a device rated at 0.7kW but what is this rating? Is it the compressor or the fans?

The thrust of the original blog (or so I thought) was a comparison between air-water and ground-water heat pumps. Air-water is a completely different proposition to air-air. The fact that you have cited an air-to-air case suggests some considerable confusion as to what is actually considered to be a renewable technology.

On your own failing example where the house was cold, this suggests to me a failure of design of the system rather than failure of the technology. It's perfectly possible to under-size a gas-fired boiler and achieve the same outcome.

Insulation should always be the starting point. Debate about how to produce what energy you actually need should only really start after the insulation is complete but, in reality, home improvement schedules do not always allow for this perfect scenario.

Finally (for now), I find it tragic that an official on this website that purports to support renewable energy should describe ground-source as scary. The great beauty of ground-source is that there is no visible evidence of the installation on the ground when it's complete. Yes, there is some disruption during installation and this isn't for everyone but to seek to be so negative is a great shame.

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NEF Gabby

NEF GabbyComment left on: 5 December 2016 at 11:31 am

@ Bean Beanland

As Editor of this piece I'm excited that it has provoked your interest and I'd like to put my hands up to any assumptions here which were poorly made. My own experience comes from working in a building heated by a GSHP and visiting SuperHomes with heat pumps and reading the odd EST report over the years and talking to owners along the way.

The National Trust's estate, as you correctly say, is not well insulated. However, it does have a great deal of easily accessible land around its properties without neighbours. This might make very extensive ground loops for a GSHP an inviting possibility. However, if you don’t think inviting diggers or drilling rigs into your garden is a bit ‘scary’ compared to having a plumber pop in a more efficient gas boiler I think you’d be in the minority.

Air source heat pumps really aren’t scary at all in my view, but I’ve found the best examples (if you are looking to replace a fossil fuel boiler and get similar comfort) are in very well insulated homes. I’ve visited a well-insulated home and a poorly insulated home – both with snow on the ground – both with ASHPs, and only one of them met my comfort expectations.

Where the home wasn’t well insulated the owners were spending most of that day in a living room with a supplementary wood stove – elsewhere the house felt ‘cool’. But in the small well-insulated home, the ASHP was providing adequate heat with zero need for supplementary heat sources, just by trickling out the warm air 24/7. The owner has also saved big time on his bills. See the Q&A on his SuperHome page for more: bit.ly/1bv8QNR. I’d be very interested in what you make of his experience and advice re: insulating first...

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Bean Beanland

Bean BeanlandComment left on: 2 December 2016 at 9:17 am

Sadly, this article is littered with misconceptions and inaccuracies. It is a shame that YouGen do not get blogs peer reviewed before they are posted.

If anyone is considering the installation of a heat pump system, ground- or air-, I strongly suggest that they do some research and find one or more installers or consultants with excellent track records and take considered advice before making a decision. In places, the article seems to suggest that heat pumps are in some way so much more "complicated" than fossil fuel systems and this implies that they are rather scary. This is absolutely not the case. Part of the problem is that most of us have become so complacent about traditional fossil fuels systems that we assume that they perform as the manufacturer intended. This is just not true. Most domestic fossil fuel boilers rarely achieve condensing mode and so the efficiencies claimed by the manufacturers literally go out the window.

The writer claims that the exemplar figures are based on "average performing pumps, installed in a well-insulated detached building." I question where this concept comes from because, as far as I am aware, no such study exists.

Further on in the article, the writer claims that "Heat pumps are only suitable for very well insulated homes". This is also factually incorrect. Even the National Trust is investing heavily in heat pump systems. I think it's pretty obvious to everyone that a vast majority of their Estate is Listed and thermally woeful, and yet, their investment in heat pumps is delivering excellent results, again and again.

It is true that there are examples of poorly performing heat pumps, many of which are well publicised. But again, this needs to be considered against the backdrop of complacency with respect to poorly performing fossil fuel and biomass boilers. The key is to work hard to find an installer or consultant that you can trust. This cannot be done over the phone or by email. Ask for case studies. Ask to speak to previous clients. Do not cut corners with specifications. If you invest wisely then a heat pump system will serve you well for many years.

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marshman

marshmanComment left on: 30 November 2016 at 8:03 pm

I had a GSHP installed almost exactly 12 months ago. Large detectched house in exposed position, well insulated, existingunderfloor heating. Fantastic!  In 12 months it has used just under £250 worth of electricity (2450kWh) for heating and hot water. (and some of that electricity comes from the PV during the day). It keeps the whole house warm 24 hours a day 7 days a week. The last few days when it has been really cold (-5 deg C) last night I have had to tweak the settings as the house is too warm!

 

One thing not mentioned in the comparison between air and ground source is that when the outsdie temperature is like it is now the air source is trying to extract heat from air at -5 deg C. The in comming water from the dground loops of my system is at 11 deg C. Last winter it never dropped below 9 deg C. This puts the air source heat pumpo at a massive disadvantage just when you need heat the most.

 

Maximum power draw is only 4kW so even if we have a long power cut -as we did after Storm Angud I can run the heating from the generator.

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allensp

allenspComment left on: 30 November 2016 at 7:12 pm

The latest decision to phase out the use of HCFC gases (the refrigerant) because it causes greater green house warming than methane and CO2 would seem to negate any efficiency savings of these systems! And yes they ALWAYS leak!

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RobertPalgrave

RobertPalgraveComment left on: 30 November 2016 at 5:51 pm

A key factor you don't comment on is the highly positive carbon impact of using an electrically powered heat pump.

De-carbonisation of the UK power grid is proceeding fairly well, and the target suggested by Committee on Climate Change is that by 2030 it should be reducing to below 100g CO2e/kWh.

With today's  grid carbon intensity of around 370 g CO2e/kWh. a heat pump with SPF of 3 or higher is already producing heat with a lower carbon footprint than gas, oil and coal. (see Parliamentary PostNote 523 at http://researchbriefings.parliament.uk/ResearchBriefing/Summary/POST-PN-0523)

That will only get better over time as we have more low carbon power generation on the grid. Whereas gas heating will never improve.

If you have solar PV installed, it can provide power for the heat pump on cold but sunny days. This week (last few days of November), my solar has produced about 12kWh per day, and my heat pump has used about 16kWh per day

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