Category: air source heat pump

Eco features: Air source heat pump + Solar PV + Whole house insulation + Air tightness + MHVR + Compost Toilet

The insulation and airtightness measures

The features

• Two wood fibre insulating sheets (Steico Protect boards), one 6cm and one 8cm have been fixed to the external walls and rendered over with specialist lime render (Lime Green Prep-Bond WP render)
• 14cm wood fibre ‘wool’ insulation (Steico Flex) was built into the wall of the new dormers.
• Air-tightness measures were designed into the structure of the house, including Intello Air-Tight membrane, Tescon Vana airtight tape and Blowerproof ‘liquid brush’ (an airtight paint).
• A distributed mechanical ventilation and heat recovery (MVHR) system has been designed into the house, with two units set into external walls (one upstairs, one downstairs), and intelligent fans between different rooms.

The MVHR kit

• They used the Free Air system by Blu Martin, distributed in the UK by Paul Home Heat. 
• The MVHR cost: Total purchase cost £6k (not including installation). When running at full pelt (which it very rarely has to) the MVHR system uses about 200W energy – i.e. about the same amount as 3 old-fashioned incandescent light bulbs.
• Outcome: a reasonably airtight house that is warm & airy throughout the year.

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The Solar Panels

The Air Source Heat Pump

• The Kit: Mitsubishi Ecodan Ultra Quiet PUHZ-W85VHA2 air source heat pump plus Mitsubishi 210 litre hot water tank, under floor heating throughout the downstairs, 4 radiators upstairs. 
• The total cost: £11K plus VAT (includes all fixtures and fitting)
• Outcomes: The heat pump serves underfloor heating, 4 radiators and their hot water requirements.
• Funding: They receive payments from the Government’s Renewable Heat Incentive scheme that come to approximately £650 per year

Watch the video to find out more:

Gas disconnection

Rachel and Adam had their property disconnected from the gas main. This cost around £2k, and had to be done by engineers from SGN. Without doing this they would have been liable to keep paying a standing charge for gas, even though they had already had the gas supply cut off at the meter on their property.

Composting Toilet

The Kit: A Separett Villa 9000 urine separating toilet (£700) in the garage, and a Biolan Eco 220L composter (£820) in the garden. 

“The toilet needs emptying once a week into the composter, which we find no more difficult or unpleasant than dealing with nappies – not a favourite job, but not a problem.

As well as getting great quality compost, it means we deal with all our toilet waste on-site. Our waste does not have to be piped away and purified using both mechanical & chemical processes, which add significantly to pollution & environmental degradation. Please read ‘The Humanure Handbook’ for more info.

By not flushing the toilet, we make significant savings on our metred water bills.”

Adam

Water saving

As well as having the compost toilet system, Rachel and Adam try to use as little water as possible:

“We reuse bath/shower water, pre-soak washing up in old water etc. We also have a water-butt system for the garden which holds c 1000 litres rainwater. This has not run out at all since installation, including in the dry summer of 2021, so we have not needed any tap water to keep our veg beds happy.

All these measures save a considerable amount of water.  Our metered water use comes to approx. 65 m3 per year, instead of the average for a 3 person household of 135 m3 per year. At current rates, we pay £320 per year for all our water costs, rather than the average of £580.

Our  metered water use comes to about 65 m3 per annum.

Average water use for a detached UK house with 3 people is approx. 135 m3 per annum.”

Energy usage and costs

All their energy comes from electricity so there are no gas bills.

• Total annual electricity use is 4600kWh = approx. £820 + £96 standing charge = £916
• Total energy generated by solar PV is 2080 kWh which brings in approx. £80 generation tariff + £114 deemed export tariff. So the income from FIT (feed in tariff) payment = £194
• They also receive quarterly payments (for up to 7 years) from the government’s Renewable Heat Incentive scheme which total £640 per year.

So total annual energy payments = approx. £920
total annual receipts for FIT and RHI = approx. £835
which is a net annual expenditure of £85.

This energy use includes their house, heating, hot water and their electric car.

For reference, the average annual energy use for a family of 3 in a medium sized detached house is 12,500 kWh for gas and 3,100 kWh for electricity, with a total annual bill of £1163 (figures from Octopus Energy).

The table below shows their energy use across the year.

Month	Energy use (KWh)	Energy generated (KWh)
Oct 20	394	117
Nov 20	354	59
Dec 20	512	41
Jan 21	586	46
Feb 21	484	84
Mar 21	431	196
Apr 21	391	277
May 21	449	319
Jun 21	431	285
Jul 21	334	319
Aug 21	217	245
Sep 21	298	172

Thanks to Adam for providing all this useful data.

Overall outcome: A warm, airy comfortable house that costs very little to heat, doesn’t use much water, and produces high quality compost for growing veg.

ADam and Rachel

Rachel’s Dos and Don’ts – Lessons learned from their whole house project

Thank you Rachel, for this very thorough and insightful list. It’s well worth a read if you are thinking of starting on a retrofit project.

Don’t

• Don’t do it ! Why put yourself through it!

Do

• Trust yourself: While the detail of designing and building houses is complicated, a lot of the broad ideas aren’t. Everyone is learning along the way and can and will make some mistakes
• If you want to do an eco project make sure your architects really understand green issues – one thing we did get right! And try to make sure that their green priorities are the same as yours (no oil products / only local products as far as possible etc)
• Try to find an architect and builder who have worked together. If you’re lucky enough to achieve this then get the builder’s input as soon as plans start to be drawn up so they can advise on costings before you have elaborate gold-plated plans
• If you’re doing major renovation consider pulling the whole thing down – especially if it’s detached. Maybe not the greenest thing in some ways but it means you can do everything as you want from the beginning and you’ll save a fortune on VAT.
• Consider how to project manage it. If you’re not a builder but know a builder you can trust, who you’re not using, who is willing to act as a consultant and do inspections every week or two, this will be well worth the money. This is probably the best thing as they’ll understand the practical challenges better than an architect but also be on your side and not want you to be ripped off
• If you can project manage it, consider hiring specialists for jobs which need specialist skills such
  as plumbing, electrics, carpentry. The downside is everyone will blame everyone else for why it’s
  hard for them to do their jobs in the way they would like but we found working directly with
  specialists was much easier than depending on general builders and their subcontractors. It may take
  longer but save money.
• Use specialist installers for marmoleum and MVHR
• Think twice before installing marmoleum – it’s hard to find experienced installers and extra hassle putting it over under-floor heating
• If you have bought a property and are paying rent elsewhere is it worth moving in while you wait
  to get planning permission and find builders you’re happy with? This can take the financial pressure
  off getting everything done as quickly as possible. It may be cheaper to do this and move out for 6
  months later and get your stuff put into storage
• Liaise with an airtightness specialist early on, get them to build things into the plans with your
  architect and check your builders are happy to consult with them in problem areas such as making
  holes in the house! Airtightness is as important as insulation but seems to be less well understood.
• Make sure you withhold payment or part-payment until jobs are complete – you want to incentivise people coming to finish off the job
• If possible seek advice on maximising solar potential before design and structural calculations are done
• Check where the solar inverter and heat pumps are going to go before / during the design stage (not after the house has been built as we did!) – Well we did this but it still didn’t work out as planned!
• Specify Suppliers and precise products – if we did it again we’d use the Green Building Store for all external doors and windows– it’s much cheaper but has a long lead-in time so you need to order early. It’s better to over-specify then you can negotiate with the builders if they have a reason to want to change anything
• Specify U and G values for windows / doors – and pay to get an estimate of airtightness so you can hold the builders to that
• If you want to have a compost toilet inside an airtight house it’s possible but you need advice from a specialist on how to do this
• Consider how much you want to be dependent on Wi-Fi for appliances like thermostats and burglar / fire alarms, and weigh that up against having more cables coming in and out of your house which can impact airtightness
• Read up on principles of green building. Even if you don’t understand the detail or the application, understanding the concepts will be invaluable.
• Ask your builders to work to full-size hard copy plans and have them on display at the site. Some contractor’s staff tended to look at plans on their phones and we suspect that’s the reason they missed some important details.
• Visit the site often. Don’t be pushy but don’t be shy of talking to the builders in details.

“Money spent getting a job done well is never wasted – the costs of repairing poor work can be high.
It’s probably worth getting a PHPP calculation done (this tells you how close to PassivHaus you
can get, taking into account airtightness and insulation etc) at the design stage, even if you’re not
aiming to build to PassivHaus / Enerphit standard. That will give you leverage if the builders fail to
build to a high enough green standard.”

Rachel

What we got right

• Architects whose design we mostly trusted and who understood what we wanted
• Happy with most of the materials they recommended – steico wood fibre insulation, lime render
• Happy with Earthborn paints

But despite that fairly short list, we’ve got a very energy efficient, warm house. Even if it could
have been more energy efficient, more airtight, with more solar, it’s still pretty damn good.
Now we’ve been here for a year and we’re settling into the house and how it works. In the middle
of November, we haven’t had to turn on the under-floor heating. Our energy use is around 1/3 of the
UK average for a house of this size (and that includes charging our electric car) and we’re
generating a good proportion of our own electricity. Our water use is tiny and we have a great
source of compost for the garden 😉

The house project has been fascinating at times, utterly hellish at times, and all-consuming for
nearly two years. But we’ve now got a wonderful home which is truly our own.