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.
For various reasons it proved difficult to set up the Cyble M-Bus adaptors for the two Itron water meters in the Outbuildings. (There are already three nearly-identical water meters with Cyble M-Bus adaptors in the House, which are working perfectly.)
The first challenge was sourcing the Cyble M-Bus reader heads. The original plan was for the building contractor’s plumber to source these along with the Itron Aquadis+ water meters (which is what happened last time, for the units in the House) but while the plumber had no issue obtaining the meters they struggled with the Cyble units and we all agreed I would source those myself, since I knew which specific ones I needed (and I assumed they would be easy enough to get hold of). The Cyble Pulse-Output reader heads are readily available but for some reason the M-Bus variants are more difficult to source. The best supplier for these in the UK seems to be MWA Technology, who list them on their website as Itron Cyble M-Bus For Aquadis and Woltex but MWA are not geared up to dealing with individual customers and while they were helpful they said they’d have to get their finance team to set me up with an Account – which sounded like a lot of work for both them and me. They suggested various distributors, some of which I’ve used before, but those just pointed me back at MWA. In the end I was successful in sourcing two of the Cyble M-Bus readers from metermarket.co.uk who happened to have a few in stock, though these days they specialise in Electricity meters rather than Water meters.
The second challenge was getting these to report meter readings via M-Bus. I knew I’d have to set the M-Bus addresses for each adaptor and I’d forgotten that these are supplied set to Primary Address 0 (for some reason I remembered them defaulting to Primary Address 1) and I wasn’t sure whether the RS-232 connection to the Relay PW20 M-Bus interface needed any sort of adaptor cable or not.
The PW20 is fitted with a Female DE-91 connector and the USB-to-RS-232 lead (with an FTDI interface chip) has a Male DE-9 connector – so that should all work fine but they both have fixing nuts which clash, preventing the plug from being inserted.
A minor annoyance – the electrical connectors will mate directly – except both sides have the nuts fitted
The nuts are rigidly fixed into the connector on the lead – and they secure the connector to the front of the PW20 (with further nuts inside the case which would be hard to re-attach.) Creative use of two Gender Change adaptors with the locknuts removed seems to work OK. (Ordinarily I use a different FTDI cable with a replacement DE-9 head, which has locking screws rather than locking nuts.)
After trying various cabling options and checking the default Primary Address I was able to program a unique Primary Address (and the meter serial number and initial meter reading) using the Actaris Cyble M-Bus Configuration Tool (CMCT) software. The devices then responded to the normal M-Bus polling code – but with ‘short’ responses, missing any meter readings. Comparing these responses to the ones from the ‘good’ meters in the House, the <Status>10</Status> is the only material difference in the <SlaveInformation> record (the meters in the House report <Status>00</Status>).
Initially I wondered whether the meters needed to ‘see’ a certain amount of water usage in order for a reading to be reported, but even running a couple of hundred litres through one of them didn’t change the response.
There aren’t many functions available in the CMCT software, but one of those is Reset alarms and running that did the trick, with the XML response changing to the expected:
Another thing to note is that by default, the Volume Index in CMCT was formatted as NNNNNNN.N m3 whereas the physical display on the meter shows NNNNN.NNN m3 – and they need to match in order for the readings to be accurate. While it’s not possible to add three decimal places with 7 leading digits (i.e. NNNNNNN.NNN) it is possible to delete two of the leading digits in order to add three decimal places.
Finally, after having had issues with the Cyble reader heads getting dislodged in the past (resulting in them missing some updates from the meters) I wanted to pay more attention to fixing them more securely. I rather expected the new units to come with fixing screws, but they didn’t – but the one screw I had for the meters in the House fitted fine. That’s an M4 x 25mm Cheese Head machine screw, made of Nylon. Note it’s necessary to destructively ‘unscrew’ a plastic piece covering the brass insert that the screw engages with. On the newer meters this projects roughly 5mm above the surface and prevents the Cyble adaptor being fitted until it is removed, whereas on the older meters it’s recessed below the surface and only prevents the screw from being inserted. The removal of this plastic piece is cryptically shown at W2 ‘B’ in the Installation Instructions (attached below).
