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Using an air source heat pump (ASHP) to reduce your carbon footprint - guide

Using an air source heat pump (ASHP) to reduce your carbon footprint - guide
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Heat Pumps and the impact on your Carbon Footprint

 

Are Heat Pumps value for money?

 

Suppose you had £10,000 to spend on an energy-related installation for your house. How might you apportion the fund across the following three categories?

  1. Reducing heat-loss / improving insulation
  2. Installing electricity micro-generation such as PV Solar Panels and possibly a Home Storage Battery too
  3. Installing a Heat pump, either ground-source (GSHP) or air-source (ASHP)

There isn’t one single ‘correct’ answer, and the expenditure will be apportioned differently according to the current state of the property.

Over the past 25-years the public has tended to put most of the money into Solar Panels (option 2), with the rest being used in additional loft or wall insulation (1). But this choice has been biased due to the Feed-In Tariff which paid money to households generating electricity, even if most of that was consumed by the home itself instead of being exported to the Grid.

The FIT has now gone. The replacement Smart Export Guarantee only pays for power actually sent back to the Distribution Grid, and at a much lower level. OVO pays 4p per kWh at the time of writing, regardless of what time of day the electricity is offered. Microgeneration is now less financially viable.

 

Two guidelines dominate the decision-making process:

a: additional insulation is more viable than both micro-generation and heat-pumps in terms of both energy-efficiency and cost

b: a heat-pump requires a house with an Energy Efficiency Rating of A or B

 

In the UK, Energy Efficiency is calculated using the Standard Assessment Procedure to create a SAP-score between 1 to 100.

 

The two types of Heat Pump

 

If your house can’t achieve a SAP-score above 80, then there’s no point considering a heat-pump.

It costs about £160 for a qualified surveyor to calculate your SAP.  The average SAP score for a house in the UK is 54.


 If your house has a sufficiently high energy rating, and you’re considering a heat pump, first note that there are two main types.

The Ground-Source Heat Pump (GSHP) uses a substantial length of pipework as a ‘collector’ to absorb heat from the surrounding soil. This requires excavation to at least 1.5m depth, and preferably 2-3m.

 

 

There are two sub-categories of GSHP in which the collector may be

  • in water; typically river or pond
  • in a bore-hole; drilled into the bedrock to a depth around 30-100m

In either case, you may need an extraction-license from your local water company, even if the water itself isn’t being removed.

 

The other main type is an Air-Source Heat Pump (ASHP) in which the collector is reduced to a large finned radiator. A fan is used to suck air through the collector, thereby extracting heat from the surrounding air.

 

 How does a Heat-pump work?

 

The principle is very similar to a fridge in that a heat-pump uses a compressor and a fluid which passes easily between its liquid and gaseous forms. This is based on Charles’ Law †

When its pressure remains constant, the volume occupied by a gas is directly proportional to its absolute temperature.

Thus as a gas is compressed (reducing the volume), its temperature will rise.

component parts of an ASHP

The refrigerant enters the collector (the radiator grill) as a fluid and picks up heat from the air-stream. As this passes through the compressor, the increased pressure causes the temperature to rise further. Within the heat exchanger unit, that energy passes into the surrounding water, causing the refrigerant to condense, and the cycle repeats.

It takes electrical energy to operate the compressor, but an optimised GSHP system might achieve a Coefficient of Performance (COP) of 4. That means it delivers 4kW of heat output for every 1kW consumed.

 

 

As with other forms of renewable / zero-carbon energy, a Heat pump operates slowly over a long period of time. It is most efficient when the heat is stored for use later, and when lower temperatures can be utilised:

 

As the UK moves towards Time Of Use tariffs, it will become more expensive to use electricity during the early-evening peak period from 5pm onwards. Sufficient energy needs to be stored to avoid importing electricity by then.

There are two main approaches to ensure that a Heat pump site can operate for several hours during peak-demand without drawing power from the National Grid:

Although the diagrams show the thermal store delivering space-heating via radiators, it would achieve greater efficiency if underfloor heating was installed due to the lower temperature required.

The greater the output temperature which the Heat pump must attain, the lower is its efficiency (COP).

The battery storage option is easier to install and occupies significantly less space, but note that the capacity required is similar to that of a small electric car!

Costs can be reduced by retaining partial gas-heating together with a hybrid ASHP. This approach was successfully trialed in the 2-year Freedom Project, which concluded in Spring 2017.

©️Western Power Distribution and used with permission

Freedom was a partnership project between Western Power Distribution, Wales and West Utilities, and funded by the Welsh Government.

 

What permissions are required?

 

1: The regional Distribution Network Operator (DNO) must be notified of all grid-connections of devices requiring 16A or more.

Most DNOs use the standard ENA Application Form. The Electricity Networks Association represents all UK DNOs and provides stability and harmonisation of practices.

Here’s the link to the ENA Forms for applicants installing Heat Pumps in SE England. There are two levels of application depending on whether the installation fulfills the criteria for the fast-track Smart Connect process. This only works for heat pumps that are already approved and listed on the ENA’s databases.

Sites which require connections for two such devices may be refused or requested to pay for local grid enhancement. The most common devices in question would be a house with an existing EV charger point which now wants a Heat pump or air conditioning system.

See Obtaining permissions from your DNO for more comprehensive explanation.

 

2: Building Regulations. Installation of Heat pumps falls within at least two categories of the Building Act:

Part-L1B, Conservation of fuel and power in existing buildings

Part-P, Electrical Safety

Many installers will be registered under the Competent Persons scheme, meaning that they may undertake the work due to their qualifications and membership of the appropriate trade body. However, they are still required to notify the Local Authority with oversight of Building Regulations, who can insist on their own inspection.

 

If you are actively considering installation of a Heat pump then you should also read the Tutorial on insulating pipes.

The pipe insulation guide contains photographs and information derived from participants in the Zero Carbon HeatingTrial, funded by BEIS. It’s very practical and draws on first-hand experience.


40 replies

Userlevel 7

Some great feedback this, @Jeffus - I’m gonna make sure we pass it on to the team as I agree, maintenance costs will be another important factor to consider.

 

Where have you seen these eye-watering running cost examples then?

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Just wanted to pop back here to highlight this recent guide from our content team on Heat Pump costs and benefits.

 

Considering a Heat Pump in future? - We’d love to hear your thoughts on this introductory guide (which is a great compliment to @Transparent’s more detailed guide above!) :smiley:

Looks useful

Would it be possible to include something on annual service costs in the ovo guide. I am sure all heat pumps will need an annual service to comply with the warranty? Is this more expensive than a gas boiler? 

Also would it be possible to include some examples of repair bill ranges for heat pumps, e.g. compressor failure, leaks, electrical controller failure etc. 

Just to give an idea of running costs. I have seen some eye watering examples but these may not be typical. 

Userlevel 7

Just wanted to pop back here to highlight this recent guide from our content team on Heat Pump costs and benefits.

 

Considering a Heat Pump in future? - We’d love to hear your thoughts on this introductory guide (which is a great compliment to @Transparent’s more detailed guide above!) :smiley:

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Well I think we can assume that the yellow segments are a greater proportion of each ring!

VAT will rise proportionally of course because it’s a percentage.

The other three colours will have shrunk. Green & purple are ‘fixed’ in contracts with Ofgem.

I wonder how OVO views its Operating Costs in blue. I suspect they’ll want to retain their profit margin to a large extent in order to give them a substantial buffer against market fluctuations.

I think we also ought to consider where the extra money is going to. Unlike the USA we’ve become heavily reliant on purchasing energy from countries who aren’t friendly towards us. So we tend to suffer more when compared against global averages.

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Adrian Letts wrote:

Removing levies from low carbon electricity is necessary to help households make the switch from gas boilers to climate friendly electric heat pumps.

We discussed this issue in the session with Simon Maine in August. See the bit about the carbon tax here for which I supplied this diagram:

 

 

Has the diagram changed much with the increase in gas costs etc both for domestic gas usage and the impact of gas prices on electricity generation?

I wonder what the diagrams will look like this time next year. 

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Adrian Letts wrote:

Removing levies from low carbon electricity is necessary to help households make the switch from gas boilers to climate friendly electric heat pumps.

We discussed this issue in the session with Simon Maine in August. See the bit about the carbon tax here for which I supplied this diagram:

 

Just a couple of months ago we were believing that the ‘Environmental and Social Obligation’ element would become a new Carbon Tax. Instead it seems the Government proposes shifting it onto gas.

Since then we’ve seen Insulate Britain emerge as an offshoot from XR and begin taking disruptive protest action.

Regardless of the nature of their protest, I have sympathy with their underlying grievance. Today’s news doesn’t alter this at all. If the mechanism of an ‘Environmental and Social Obligation’ didn’t resolve the poor insulation in our housing stock when it was levied on electricity, it will make no difference when moved to gas.

The vast majority of our housing is still not in a state where it could replace gas boilers with (electric-only) Heat pumps.

And we will still be building those houses for another 8 years anyway because they’ve been pre-registered to avoid the current Building Regulations requirements. :rage:

 

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@Tim_OVO Thanks for the info from the CEO.

Definitely a positive step forward.

There needs to be a clear rationale to explain to the public the reason for the change. And there needs to be support for low/middle income households (grants / low cost loans etc.)

If the policy is successful in moving people from gas to electricity and the policy costs stay the same then there will be a revenue gap in the future. It’s how this shortfall is addressed:

  • Could increase unit charge of gas further (won't be popular for those unable to transfer and will leave those last to make the switch paying very high prices)
  • Could switch costs back to electricity later (won't be popular unless this rationale is explained)
  • Could move costs to general taxation

There’s an analogy with shifting fuel duty when the switch from petrol/diesel to EVs happens. It’s all about the framing and the deemed fairness of the policy decisions.

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@Tim_OVO Thanks Tim, interesting. Lucky old us if this turns out to be the case, but I fear for the millions of households who cannot afford to change their technology. What on earth will happen to them? :( 

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Agreed - wasn’t there a slogan a while ago that ‘the polluter pays’ ?

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Our CEO, Adrian Letts, shares his thoughts on the Government’s plans to shift green surcharges from electricity bills onto gas bills:

 

I was pleased to see that the Government is planning to shift green surcharges from electricity bills onto gas bills in the FT this morning. 

Removing levies from low carbon electricity is necessary to help households make the switch from gas boilers to climate friendly electric heat pumps. We need to make clean heat the cheaper choice and show consumers that a zero carbon home is ultimately a more valuable home. 

The focus now must be on ensuring we effect this transition while protecting vulnerable consumers. 

You can find out more by reading Options to Reform Energy Bills - research we commissioned with Public First earlier this year. 

 

 

Is this a win for anyone who’s making / has already made the move to heat pumps @juliamc @nealmurphy @Bev @Heatherd …..? 

 

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Erm @knight - were you involved with the Festival of Debate, organised in May’21 by Community Energy England based in Sheffield?

As a couple of months has passed since that event, it would be worthwhile contacting the coordinator, Emma Bridge, because she’s on your doorstep! She will know if the event has triggered further community-based projects to undertake actual energy initiatives rather than discussions.

I wasn’t and I didn’t know about it. Thanks for the heads-up though. I may look to see if anything has happened. It would be great if it did.

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Erm @knight - were you involved with the Festival of Debate, organised in May’21 by Community Energy England based in Sheffield?

As a couple of months has passed since that event, it would be worthwhile contacting the coordinator, Emma Bridge, because she’s on your doorstep! She will know if the event has triggered further community-based projects to undertake actual energy initiatives rather than discussions.

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I would suggest that the answers for the North of England are likely to be found in the Trial and Demonstration Projects already being undertaken elsewhere by Community Energy Groups.

Two energy strategies are worthwhile further investigation.

 

1: The first is Borehole Thermal Energy Storage, which is being Trialed by the Owen Square Community team in Easton (Bristol). This captures solar energy which is then stored below ground using a technique pioneered by ICAX.

Owen Square is a series of roads with terraced houses and little ground space. So its tactics are a fair analogue of the northern housing which @knight refers to.

 

2: The second is to use borehole-inserted Ground Source Heat Pumps (GSHP). These use heat collector tubes inserted vertically into the ground rather than spread horizontally.

GSHP technology has a higher efficiency than air-sourced, but suffers from the problem of pipes freezing if too much heat is extracted during a cold winter. The trick to this is to find locations where underground water-flows continuously past the collector-pipes, thereby replenishing the body of heat available to be harvested.

I don’t yet know if anyone has tried sinking a borehole GSHP system into abandoned and flooded mine workings. But if not, there’s probably grant-money available to try it @knight !

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A really important point raised about the suitability of using an air-source heat pump in some house types, @knight 

 

It’s certainly not a simple path to zero carbon heating - not sure if you’ve seen it already but there’s been an interesting discussion going on here about the implications of using hydrogen heating systems over here -

 

 

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No gardens, high-density, overshadowed properties, no room to put the required kit. And probably the biggest problem. most of the housing stock will be nowhere near the required minimum efficiency level.

Has anyone ever tried putting a heat pump system into a 2-up-2-down terrace house?

I call out the North as that is where I live but the Midlands and some areas in the South have the same issues. But the North also has the issue that temperatures and weather can be noticeably harsher. I assume that an air-sourced heat pump would be less efficient the colder the ambient temperature gets?

From the BBC GCSE revision guide:

 

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Thanks for popping up here in this topic, @knight  - You’ll need to qualify that observation however

What is it about the existing housing stock in the North of England which makes it pointless to consider the use of Heat pumps?

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All very interesting. But I’m afraid totally pointless for the majority of existing housing stock in the UK. Especially here in the the North of England.

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and in case you missed it @Tim_OVO - that link is already embedded in the lead-article at the top of this page :wink:

Userlevel 7

In case you missed it, we’ve made a guide on heat pump pipe insulation, here: 

 

 

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Are you feeling lucky? :sun_with_face:

Always @Transparent the sun always shines in Surrey.

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OK @juliamc … so let me throw another bit of maths into my answer.

 

Assumptions:

It’s a cold day in mid-January with some periods of peak sunlight, but the sun will be low in the sky of course. At 51.5°N (SE England) the sun’s elevation peaks at just 45° and sunset is 16:20.

You want to halve the size of the tank to 600l by adding a further 15°C to the stored water. At 17:00 it will be at 70°C, falling to 40°C over 5 hours.

By noon your heat-pump is supplying at the maximum 55° for which it has been optimised.

Your solar-thermal panel registers 68°C on the roof at noon when the sun is out, dropping to 50°C within 30sec of cloud coverage. You have a 6°C hysteresis setting. Ie the solar-pump operates ON when the temperature of the collector exceeds that of the tank by 6°.

It switches OFF again when ΔTemp falls to 3°C. otherwise the losses in the pipework mean that you will sending warm water out of the tank to the roof.

You have a standard 3kW electric immersion heater.

 

Calculations:

If you were to use the immersion heater, you could definitely raise the tank temperature from 55°C to 70°C by 17:00 hrs. That’s a 15°C temperature rise.

Energy required = Specific Heat Capacity  x  ΔTemp  x  mass in grams
where the Specific Heat Capacity of water is 4.184J/gram/°C

As the tank is now just 600l capacity, its mass is 600Kg or 600,000g

4.184 x 15 x 600,000 = 37,656,000 Joules

As before, 1kWh = 3,600,000J

So the power we require is  37,656,000 / 3,600,000  = 10.46kWh

Your immersion heater is 3kW, so it can deliver that power in 3½hours

You have from noon to 13:30 to extract something usable from the solar-thermal. Without that, you will need to switch on the immersion heater in order to reach the required 70°C by 17:00hrs.

That would cost you about £1.90 including a reasonable proportion of your standing charge.

 

Are you feeling lucky? :sun_with_face:

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@Transparent yes I do understand that:rolling_eyes: , but why would you want to ? (Though I understood the thermal store to have a temperature gradient, so in fact the lower level would be less than 70 deg.) I’m suggesting you could supplement the heat pump/radiator setup with the solar thermal etc. This is all about getting over the high demand peak of 5pm - 10pm isn’t it ?

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@juliamcwrote:

The thermal store would be better (and smaller) containing water of a higher temperature - such as could be collected from a solar thermal array or the immersion run at times when renewables are available. Then the radiator system could draw off heat via a coil set in the thermal store.

The moment you consider operating a thermal store at a higher temperature than the Heat pump delivers, you will thereafter only be able to provide that heat from your Solar Thermal or immersion heater.

As Messrs Flanders and Swann so aptly explained the Second Law of Thermodynamics

 

You cannot pass heat into a hot water cylinder at 70°C by feeding it from your Heat pump that has a maximum output of 55°C :nerd:

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Yes @Jess_OVO that figure of 4.184J/gram/°C for the Specific Heat Capacity of water is a constant.

Compare this with the Specific Heat Capacity of (Ethylene) Glycol at 2.433J/g/°C. We commonly add Glycol to water in order to lower the freezing point, but it carries only 58% of the heat energy compared with water.

I use a 4:1 ratio of water to Glycol in the fluid mix which circulates to my external solar-thermal panel. So that 20% dilution has reduced the Heat Capacity to 3.834J/g/°C.

As a result the circulation pump will operate slightly faster in order to transfer the same quantity of energy to my Thermal Store.

The manufacturer’s recommendation was to use a 40% mix of Glycol, reducing the Heat Capacity yet further. But as I’m based in the West Country it’s unlikely that my thermal solar collector will face temperatures low enough to warrant that (-23.5°C)!

freezing point of a water/glycol mix

 

The Specific Heat Capacity of water is not much affected by the level of salts dissolved in it. You’d need to be using water more concentrated than the sea before it might become a factor in the calculations.

 

If you did want a 1200l storage tank, but chose to put it outside, then you would certainly need to increase the level of insulation.

Think of the insulation thickness as a gradient between the water temperature and the outside air.

 

Each type of insulation has a stated resistance to the passing of heat. This Thermal Resistance is called the R-value, measured in °C.m²/W (degrees Celsius per square meter per watt).

The higher the R-value, the better is the insulation.

If you want a higher R-value:

  • increase the insulation thickness
  • or change the material to a better insulator

Many self-builders specify a “plant room” for their house, possibly within a basement. This is usually unheated, but contains all the large tanks, pumps, battery storage and ventilation equipment. As its temperature will still be significantly higher than the external air in winter, it offers a compromise for the levels of insulation required.

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The lower the temperature of the water, the larger the tank needed to hold the energy you wish to store.

@Transparent I see.

Would this work ?  Don’t use the water directly from the thermal store in the radiators (which has to be at the lower temp to match the heat pump output). The thermal store would be better (and smaller) containing water of a higher temperature - such as could be collected from a solar thermal array or the immersion run at times when renewables are available. Then the radiator system could draw off heat via a coil set in the thermal store.

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