29 Mar 2010

Part G and the Water Calculator

One of the big changes coming through in the next round of building regulations is the amendments to Part G, which is due to come into effect shortly (6/4/10). Part G deals with water and it's been one of the sleepier areas of the regs to date. But this is all set to change when people designing new homes have to get their heads around the dreaded Water Efficiency Calculator for the first time.

At the moment we, as a nation, consume something like 150lts/day of potable water each in our homes. I don’t know where this figure comes from, and it certainly doesn’t relate to any home I have ever lived in recently (my personal average seems to be around 250lts/day). Moves are afoot to lower this figure — the targets are all in the Code for Sustainable Homes — and the latest version of Part G brings a figure into the equation for the first time, that figure being 125lts/person/day.

This doesn’t mean that you will only be allowed to consume 125lts/person/day, but that you must install equipment which theoretically — as if is you just happened to be the average consumer — means that you could go on living your life normally and only consume 125lts/day rather than the 150lts/day that you are currently consuming.

And what that means, of course, is that whereas up till now you just plumbed in a shower or a bath because it looked good and promised eternal happiness, now you have to perform a calculation and show it to building control who will check that you are in fact allowed to have this bit of kit.

You can find the Water Efficiency Calculator online here.

In order to get it to work, you will have to know the flow rates of your showers and taps, the capacity of your bath and the flushing volumes of your loos. You have to fill them into the calculator to work out your notional usage. And the total has to come in at under 125lts/person/day or else.

The two things that are likely to have greatest effect on these calculations are showers and baths. A shower with a very average flow rate (say under 20lts/min) is likely to find it difficult to pass the new hurdle rate. So the hunt will be on for low flow showers which still do the business, whatever that might be.

This topic came up at a seminar I attended at Ecobuild chaired by Nick Grant (or not! see Nick's post below). A woman from an “upmarket housebuilder” asked what she should do as the current batch of low-flow showers were simply unacceptable to her clients. It was suggested that the Nordic Eco shower was a possibility, and that Hansgrohe had a 6lt shower that was OK. Wolseley Sustainable Building Center promotes Mira’s Eco handset which, it says, gives a good 6lt shower. But the woman seemed unconvinced. I suggested to her that they could have swimming pools instead, as theses weren't included in the water calculator, to which she replied “Oh, they already have, but no baths and feeble showers.”

What else is new in Part G?
• G3 is being extended to include thermostatic taps on baths (max temp 48°C) because 93% of home scalding accidents occur in baths. CLG claim that a cost benefit analysis had been carried out to do this — i.e. it costs more to treat scalds than it does to fix thermostatic taps. The decision to exclude other taps (i.e. basins) was made because of the fewer number of accidents though, it was noted that in Scotland, a wider range of taps now has to be thermostatic.

• The safety features in G3, which have up till now just covered unvented hot water systems, are being extended to other water storage systems such as ordinary cylinders and thermal stores. This was controversial as it was challenged by a number of European manufacturers who sell all manner of water storage gear without all the safety paraphernalia that is required in the British market. Essentially, the kit must be modified for the UK market and must include at least two additional safety features, being temperature and pressure valves.


  1. Hi Mark
    The new "Water Efficiency calculator for new Dwellings" is not that easy to navigate around.
    With that in mind I have desined a Windows Based piece of Software to take out all the effort.
    Google for Part G Calculator - top link

    Dr R D Saunders MBEng, BEng(Hons), PhD
    M.D. Innervision Design

  2. Hi Mark

    Thanks for the mention but unless you are referring to last year's EcoBuild I don't think I was chairing the session you mentioned. Was good to have your input in the hot water one though.

    The calculator approach is inherently flawed (on many levels) but cannot easily be dropped as it was voted on by 'Stakeholders' and became policy. There is a critique on my website which led to a review for CLG. Unfortunately the report of the review has not been published as I would rather reference that.

    Many of us find it really draining to have to put so much effort into debunking seriously flawed policies as you regularly flag up on your blog. We would rather get on with designing more sustainable buildings (and renovations) but have to spend huge amounts of time and effort (usually unpaid) arguing against supposedly green policies that are making things worse.

    This is another good example.

    In the hope of providing a positive alternative Judith Thornton and I (with lots of help) produced the AECB water standards. if you follow these you will also tick the Part G box.

    I tried not to comment as this subject gets me particularly heated!

    Will go and have a lie down now.

  3. Is there any talk of minimun house sizes or room sizes?

    I juts have a vision of smaller and smaller houses with smaller bathrooms, for alot more money.


  4. Hi,

    Just found your blog - will be doing a lot more reading but ..

    I currently live in Belgium where by law you have to install a 5000litre rainwater tank in all new houses and use it for loos and washing machines etc. Sounds a lot more eco-friendly than the bath police you describe in your artice!

  5. Hi Mark,

    Just a quick note of thanks, I found this article really useful, we've just started a project on site and this is our first project where part G2 has had an effect, so its enabled me to answer my clients questions, clearly and concise.


    Dave Cornett
    SNOW architects
    RIBA Chartered Architects
    Liverpool - Wirral - Cheshire - Manchester