Rainwater Harvesting – Summary after 4 Months of Operation

The rainwater harvesting system went live at the start of December 2025, replacing the use of mains water for the toilets, the outside taps and the washing machine – and literally halving the mains water consumption in the process. The rainwater does need to be pumped out of the underground tank but a dedicated electricity meter shows the pump and its controller consume just 1 kWh of electricity per 1,000 litres of water delivered – which costs £0.05 compared with £2.61 for importing 1,000 litres of mains water.

The water saving is no surprise, since the ‘always needs to be mains water’ and ‘potential future rainwater’ consumption have been metered separately since the House was built. The low electricity consumption was largely expected by making careful choices when selecting the water pump and its controller (and by including a pressure vessel which gets ‘charged up’ whenever the pump runs, avoiding the need for the pump to start more than a few times a day) – but it is good to have this confirmed.

Back when I was specifying the systems for the House, the consensus in the low energy building community was heavily influenced by a 2010 report by the UK Environment Agency which concluded that domestic rainwater harvesting systems were 40% more carbon intensive than using mains water, with a split of 52% ‘operational’ carbon and 48% embedded. That ‘operational’ carbon was due to the electricity consumption of the rainwater pump – especially for ‘direct’ systems which need the pump for every rainwater draw-off (rather than periodically filling a cistern which then supplies water by gravity). Despite this report, I went ahead and installed parallel pipework systems in the house, for potentially splitting out rainwater usage from mains water usage. To a large extent, this was based on the expectation that the carbon intensity of the electricity supply would reduce over time.

More recent research has shown that things have changed and that rainwater harvesting systems are now typically beneficial from both a carbon- and a cost-saving standpoint. However, it still makes sense to reduce the embedded carbon – for example by avoiding tanks which need to be bedded in concrete – and to choose energy-efficient control systems.

My experience is that the water looks as clear as mains water with no hint of any smell. There is no sign of any algal or bacterial growth where the water gets stored at room temperature in toilet cisterns. The mains water here isn’t especially hard but the rainwater is completely soft so there’s no limescale build-up and washing detergents are more effective. I had wondered if it might be necessary to retrofit an inline filter for the washing machine in particular but based on current experience that is absolutely not necessary.

The filter on the inlet to the underground tank has been getting quite dirty with fine dust coming off the new clay roof tiles (a distinctive ‘Staffordshire Blue’ colour) and needed cleaning ever month or so, but that’s now reducing.

While the saving on the mains water bill is the main appeal, there are wider benefits too:

  • Provided the tank isn’t already full, hundreds of litres of rainfall will get stored in the tank, reducing the run-off from the site and helping mitigate the risk of flooding in the wider area
  • Less demand is placed on the water supply and delivery network
  • The tank holds an on-site reserve of water for firefighting purposes, which is useful since the nearest fire hydrant is literally miles away

There’s some more detail in the Technical Article here.

Hornbeam Hedge Planting

For the ‘farm’ hedges around the boundary of the site, I’m happiest with Hawthorn – which is what the boundary hedges were originally and still mostly are now, just with a few (big) gaps. Some of the Hawthorn has given way to Blackthorn, which isn’t quite as good but provides an interesting contrast. (The main problem with Blackthorn is that it spreads by suckering and the whole hedge tends to grow wider over the years. It also responds less well to hedge-laying than Hawthorn.)

For other hedges within the site, especially near the buildings, I generally prefer Hornbeam. While it’s a UK native and happy to tolerate wet ground (much better than Beech which looks very similar) it can be kept trimmed into a tidy, formal shape.

The only sensible way to plant a hedge more than a few metres long is using bare-root plants but those really need to go into the ground while they’re still dormant – which ideally means before the end of March (and even then the plants will need to have been kept in cold storage). The existing Hornbeam hedge to the South of the House lost the leaves it had kept over winter and started to bud around mid-March, indicating that new bare-root hedging would likely also be coming out of dormancy at a similar time.

Ever since the Outbuildings got completed, with the ground around them looking rather flat and bare, the plan has been to plant a hedge along the back of the parking area to the North of the Garages, to act as a wind-break and to help separate that area from the planting beds further North. That was waiting on the fence along the Eastern boundary being installed, which happened at the start of March.

What should have been a fairly quick job of preparing the ground turned into a much bigger task when it became clear the ground had been heavily compacted by all the building plant traffic. When it gets compacted, the ground here turns into a solid layer which is almost waterproof – resulting in a thin layer of mud on top of an impermeable layer below. That’s no good for planting into so the compacted layer needed breaking up (generally with a pickaxe) and forking over. To make matters worse, under the compacted layer there was a lot of demolition rubble from the old barn buildings (which had been there since the House was built) – many barrow loads of full-sized bricks and half-sized concrete blocks – so that had to come out to check the ground wasn’t compacted below it. In some places that meant digging down nearly a full metre, to expose the original topsoil.

With the made-up ground consisting mostly of subsoil, rather than topsoil, it needed plenty of organic matter adding – of which there is a plentiful supply from several years of composting the hay cut every August from the wildflower meadow. The main problem with that is judging how far all of the disturbed ground and compost will settle, so that the hedge ends up level with the adjacent parking area. Ideally I’d leave it to settle for a month or so but then that would really be too late to be planting bare-root hedging.

Previously I’ve bought bare-rooted Hawthorn and Hornbeam from a local supplier but they were out of stock of Hornbeam for this season so I used mail-order supplier Best 4 Hedging based in Chorley, Lancashire, who were also a bit cheaper. I’d hoped that by placing an order on a Monday I’d avoid the risk of the plants being stuck in-transit over a (potentially warm) weekend but they ended up shipping on the Friday and I wasn’t expecting them until the Monday but I was pleasantly surprised by DPD delivering on the Saturday.

The plants were just starting to sprout leaves – which helped to confirm none of them were dead. They were very similar to what I’d planted before, with the expected variation in heights and stem thicknesses; quite possibly they had come from the same nursery as before. I kept them in buckets of water in the dark for a couple of days while finishing the preparation work and then planted them over a couple more days. The hedge is 24m long and I’d planned on 5 plants per metre (40cm spacing in two staggered rows 40cm apart) for 120 in total. It always feels like vandalism to cut off the top 1/3 of each one after planting, but this is important:

  • To encourage more shoots to sprout from the base, for leaf cover closer to the ground
  • To reduce the number of leaves needing water from the root system while that recovers from the shock of being transplanted
  • To reduce the effect of wind loading while the plants get settled – especially since I don’t bother staking them

It’s quite hard to get a good photo of a line of brown sticks against a brown background; I’ll take further photos once they get some decent leaf-cover.

120 bare-root Hornbeam plants forming a hedge by the Garages, with a mulch layer to help retain moisture

With the rainwater harvesting system feeding two outside taps by the garages it proved easy to water in the new plants using a hose and spray nozzle. While the cost of 1,000 litres of metered mains water is only about £2.50 it’s good not to have to give any thought to the water usage. I was also careful to thoroughly soak the compost mulch covering several times, in the hope that by being wet that would be less likely to blow away in the 40mph winds forecast for a few days after planting.