Zero carbon homes are very much on-topic with the COP26 summit approaching in November, but to date rather more discussion has been around standards for newbuilds, downplaying the fact that by most measures 70-80% of the planned “net zero homes of 2050” are already built, 9-inch solid walls, warts and all.  “Decarbonising” them is now exercising government, councils, the RIBA and industry at many levels.

UK Domestic energy

You’ll probably have heard of plans to eliminate natural gas boilers by 2025 – certainly in newbuilds – but my house and yours won’t be so far behind.  Domestic energy use represents about 27% of UK energy consumption, with 85% of that apparently used for heating & hot water – as significant in climate effect as the usual suspects – cars and planes.

Brian’s home in Hammersmith

You may also be familiar with Tom Pakenham’s Passivhaus in Lena Gardens W6 from a few years ago, which sets a formidable standard for whole-house renovation with huge attention to detail to achieve the required standard.

But what if you took an existing Edwardian terraced family home in 2021, and made it zero carbon while living in it, but without gutting the interior, or adding 100+mm of hard insulation to the inside of all the external walls, ripping up the floors to insulate them, and possibly compromising or losing some of the period features we appreciate?  This is what our member Brian Thresh is doing – you may have seen him present the project at London Climate Action week in June.  He shows that it can be less intrusive than we might fear.

Let’s be honest for a moment, there are only so many builders in the world, and so many hours in the day, there is little practical chance of all the country’s millions of homes being laboriously superinsulated in the next few years – after all we’ve been talking about insulation for 40+ years already.  Brian expects to be able to demonstrate that his home is zero carbon, once the annual numbers roll in, through the combination of:

• Replacement of a gas boiler with a substantial 12kW air-source heat-pump
• 6.6kW of solar cells
• Battery storage
• A green & flexible electricity tariff, using a smartmeter

There’s an interesting tension here between expenditure on craftsmanship – a well designed and precisely executed complete refurbishment often with specialist materials – the Passivhaus – and the retrofit of a period home using modern mass-produced technology to achieve a similar net result, but with significantly less upheaval. 

It seems likely that the latter will prevail for the majority because of the numbers described above, with as much of the former as is practicable on the existing housing stock, but how do the finances and carbon footprint compare: Conversion / Running / Whole life?  Much of the public discussion is around carbon, and saving on everyday bills; rather less – a lot less – on Total Cost of Ownership, or Net Present Cost, important for those with shallower pockets. As Brian says, this stuff doesn’t come cheap at the moment.  It will be instructive to assess and compare over the years to come, particularly as technology improves, such that an optimised mix of solutions can be provided for each domestic setting.  Solar cells have already improved significantly, but you may not have owned any yet !

Vaillant Arotherm+ heat pump

We visited Brian’s home recently to see it in operation and can confirm that despite the imposing size of the heat pump, the start-up and running were whisper quiet – a lot quieter than the planes flying over, or indeed the background level of distant road traffic.  The issue for many of us will be finding the space, though superinsulated and smaller homes can obviously manage on something smaller. Brian’s intent was specifically retrofit without massive upheaval.

For those without the space, there’s significant research going on into the use of hydrogen, once the main component of “town gas”, to replace all or most natural gas, as a transitional technology at least, particularly if the hydrogen is “green” not “blue”, though again, blue may be a least-worst transitional option.  Finally there’s research into hybrid combinations of boiler and heat pump, as an optimisation.  This allows a smaller heat pump to be used, with occasional use of the boiler at extremely cold times when heat pumps tend to be less efficient, and to top-up hot water as necessary, presumably 100% hydrogen powered in future, if not immediately. “100% Hydrogen-Ready” boilers are on the way (at minimal extra cost), so if forced into replacement at short notice due to boiler failure, you might do worse than to enquire about their availability – a boiler may not be a totally obsolete expense yet. Meanwhile, the government wrestles with a long-awaited Heat and Buildings Strategy, and its associated sticker-shock, but has released a new Hydrogen Strategy.

Brian tells us that one of the important aspects of his approach is holistic thinking.  Not thinking in silos about the solar electricity, heating, hot water and energy storage.  This involves breaking a few traditional barriers and using the right suppliers who are open to such an approach – which whittles the lists down considerably, as he describes.

Once there were huge incentives to export solar power the the grid via government schemes such as the Feed-in tariff, amazingly approaching 50p/unit just a decade ago.  Though these have long gone, c/o Made-in-China,  solar cells are now less than a quarter of the cost then, microinverters and similar smart technologies have been designed which, cost-effectively and efficiently, allow a mixture of roof positions to be used.  This opens new opportunities to install what might once have once been a suboptimal arrangement, such as panels on a flat roof, which Brian has, and describes in more detail his write-up below.  With a Smartmeter, a suitable supplier and tariff, it also opens the possibilities to blend sources of electricity at different times of day and year to best control costs and carbon.  The same kit is also going to be part of the essential infrastructure for electric vehicle charging, by the way.

That’s not to say that government schemes have all gone, indeed the heat pump is substantially underwritten by the Renewable Heat Incentive with other schemes on the way, probably as part of the upcoming Heat and Buildings Strategy.

Towards Carbon Neutral Urban Housing

One of the more unusual aspects of Brian’s design, largely as a result of the reduction in feed-in tariff, are 4 substantial 2.4kWh battery packs, with a decent endurance of 6,000 charge/discharge cycles & 7 year warranty. Used sensibly, he says that rather than use expensive, albeit green, grid electricity to run the heat pump during the day, he can use either solar or stored electricity from the batteries (which, depending on the time of year have been charged on cheaper overnight electricity, or solar).  The same can be used to heat the water via an immersion heater if required. 

We can’t help thinking that at the stroke of a pen, the government could adjust the feed-in-tariff rules to avoid the need for the added expense, space, complication, import of goods/export of pollution associated with batteries, by effectively using the grid for storage as would have been the case under rules just a decade ago.  Smartmeters facilitate moving any required complexity to the tariff, as some suppliers are starting to do, such that for example, one might be limited to selling only so many units to the grid / day / week at this price or that, and even buying back a certain number at the same price, limiting the financial exposure that frightened government into the reductions, and for which we see these unintended consequences.  Meanwhile, if you have a battery-electric car at home in daytime with off street access, charging it off solar, using its battery for storage, could be more eco/economic. There are also community energy projects, which highlight some of the regulation issues. For more on this subject, listen to Positive Thinking and see Repowering

Its easy to see the many possibilities for optimisations in such a system.  Please read Brian’s write up above for a more detailed description of some of these.  He plans to closely monitor it using the various apps and web services available, optimise usage over the next 9 months or so, and report back when the house has been through a full year of the seasons.  He’s offering tours as part of Open House in September – so you can see it for yourself.