Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    • CommentAuthortony
    • CommentTimeJun 15th 2013
    WWHR is is almost non existent in the UK probably due to water regulations making almost impossible to legally.

    I simply leave the hot water in the washing up bowl or sink until it has cooled to room temperature in the winter and on cool days, I have solar hot water so tip it to the drain in the summer, the sane with baths.

    This is simple and free and we could all do it, there is no necessity to recover the heat from warm water back into water so long as it is made use of it is still heat recovery.

    Any container within the heated envelope of a building could be used to recover the heat from baths, showers, washing etc and a simple timed siphon could dump the water when it had cooled, cheap, legal and easy to do.

    I know that in other countries there are heat recovery waste pipes but unless regulations change here which is unlikely, then WWHR will remain a very minority sport, I do more than anyone else I know of of it already at zero cost, and as you all know I have no heting system so it irks me to throw heat away .
    • CommentAuthorTimSmall
    • CommentTimeJun 15th 2013
    I suspect that you have to be a bit careful - leave the water in place for too long (so that the water temp is lower than the air temp maybe?), and you start using up heat from the air to evaporate it. After that point, you only gain that back if you get condensation in your HRV (and only then some of it unless your inside humidity is 100% - which hopefully it isn't).
    • CommentAuthorEd Davies
    • CommentTimeJun 15th 2013 edited
    If condensation is happening in the HRV anyway then you should get substantially all the excess latent heat back even if the inside RH is considerably lower than 100%.

    Under these conditions the exhaust air from the house should exit the HRV at only a degree or two above the outdoor temperature and at 100% RH. The combination of these conditions more-or-less fixes the exhaust absolute humidity. Any excess absolute humidity above that amount entering the HRV from the house should be condensed whatever the inside RH.

    The only caveats on this that I can think of are a) with more latent heat in the exhaust air it'll exit the HRV slightly warmer and so with slightly more water vapour and b) some of the heat will be lost from the HRV but not to the incoming air and c) the excess water vapour might make the HRV run a bit longer or faster with higher associated losses.
    • CommentTimeJun 17th 2013
    Posted By: tonyWWHR is is almost non existent in the UK probably due to water regulations making almost impossible to legally.

    I don't think that's the reason. Have you looked at the price of (WRAS-approved) shower heat recovery devices?

    There are systems that keep the water in a tray or suchlike until it cools. I believe the problem with graywater is that you have to clean such trays because goop solidifies in them, with resultant smell problems as well. Easy if you're doing it in the sink; only a slight hassle if you just keep water in the bath until it goes cold; a major hassle if it's some plumbed-in hidden facility.
    • CommentAuthorSeret
    • CommentTimeJun 18th 2013
    Posted By: tony
    I simply leave the hot water in the washing up bowl or sink until it has cooled to room temperature in the winter and on cool days, I have solar hot water so tip it to the drain in the summer, the sane with baths.

    This is simple and free and we could all do it

    The trouble with this Tony is that much of the heat a bath full of water gives up is evaporative. Fine if you've got MVHR which can control the humidity and still recover the heat, but if you look beyond the confines of forums like this you find that in the wider world MVHR is almost as rare as WWHR. In a conventionally ventilated house cooling bathwater will be a major source of humidity, which means either indoor air quality degrades or additional ventilation will be required, negating any heat gains.

    Any homeowner that's actually trying to be greener is likely to have reduced ventilation down to the point where humidity needs to be controlled, so adding a major source of humidity like baths being left to cool isn't a simple option, it will need to be actively managed (ie: new hardware fitted). I would suggest that when it comes to showers it makes sense to use WWHR to reduce demand, instead of trying to recover the heat in MVHR. Baths you've got no option really, it's got to be left to cool. I'm looking at doing that in a tank when I do an extension in the near future.

    I don't think it's impossible to fit a WWHR system legally, btw. I've got a WRAS-approved system I'm just about to retrofit, it slides in under the bath and in my case doesn't even require cutting any pipes. The systems in the downpipe probably need to be done at build or refurb, but there is at least one easily DIY-able retrofit system (albeit with lower efficiency).
    Seret , have you got a link to the product you'll be using
    • CommentAuthorSeret
    • CommentTimeJun 18th 2013
    • CommentTimeJun 18th 2013
    That seems like a good product but correct me if I am wrong but previous threads on this subject suggested that there must be a double seal/pipe between the waste water and the potable water?
    • CommentAuthortony
    • CommentTimeJun 21st 2013
    It looks to me to break all kinds of water regulations to me.
    • CommentAuthorGaryB
    • CommentTimeJun 22nd 2013
    These WWHR devices are fine if you goal is to improve a SAP rating on a new building, but please treat claimed savings with a healthy degree of scepticism. Savings will be made, but not as much as one would think.

    Some points to consider:

    1. The design of the shower head is the most important element. Handsets are available which will give a good quality shower at 6 l/min flow. Sort this out first. Even replacing the rubber sealing washer with one having a smaller orifice can save 2-3 l/min.

    2. Do not assume that the temperature setting on the shower mixer is the same as what goes down the waste outlet. I have measured this and at a 40 deg C shower delivery temperature, the waste outlet temperature is 33.5 deg C. At a mains water temperature of 10 deg C, 22% of the heat has already been given up to heat the air, the occupant, the shower tray and in latent transfer. This reduces the scope for savings in using the device.

    3. There are pipe losses between the waste outlet and the WWHR unit, plus pipe losses between the water outlet and the cold feed of the cylinder, even if insulated.

    4. The copper WWHR pipe has to be warmed up first, plus the (normally copper) pipe between the unit and the cylinder cold feed. It can take 2-3 litres of flow for these pipes to warm up.

    5. The pre-heated water present in the cold feed once the shower stops is not recovered to the cylinder. OK, in winter, this contributes to space heating, but it is lost heat in summer.

    6. The waste pipe will be coated with biofilm and the normal waste pipe deposits which will affect heat transfer over time. If installed in the vertical this effect will not be as pronounced as for horizontal.

    7. No water is saved using these devices.

    8. Based on a claimed 68% heat recovery, a 5 min shower at 9 l/min with 33.5 deg C waste temp and 10 deg C average supply temp will save around 0.75 kWh per shower. For a 2 person household at say 500 showers per annum this will save around £18.50 pa for a gas combi boiler. A simple payback of 27 years assuming no deterioration in performance (which there will be).

    I rather fear that these devices will encourage people to use 'raindance' shower heads, take longer showers etc. because they believe they are saving more energy than they actually are.
    • CommentTimeJun 22nd 2013
    But with electricity at 0.18p/kWh it is a saving of £67/annum making the payback 7.5 years if the unit is £500.
    Becomes marginal there, but as you rightly say, reduce the usage only saves you money.
    Good post Gary , thanks.
    You posted info on heat loss from primary circuit, DHW storage etc. a while back , cant seem to find , any chance you could point me in the right direction.
    I recall the figures were quite high, so it would seem this might be a better place for people to direct there effort before going for WWHR.
    the unit seret looking at is around £200 (plus fitting and sundries) so ST's electric example might bringthe payback to around 4 years , if 68% heat recovery was achieved?
    I wonder what the EROEI (energy returned on energy invested ) might be , from your figures
    500x0.75=375kWh pa saved , what might the kWh be to get this product in place, cradle to grave etc. ?
    • CommentAuthorSeret
    • CommentTimeJun 22nd 2013 edited
    Posted By: jamesingram
    the unit seret looking at is around £200 (plus fitting and sundries) so ST's electric example might bringthe payback to around 4 years , if 68% heat recovery was achieved?

    Nowhere near it I'm afraid. Between 17-22% depending on how it's installed.
    argh! didn't read that bit :shamed:
    I was at an EcoSquad summit this past Monday and one of the presenters noted that drain water heat recovery units are a pretty bad investment, except in places where a lot of showers are taken (gyms etc.) and are particularly bad if fitted to secondary bathrooms, where the energy saved will never recover the energy (and cost) used to make the device in the first place. Much better to just fit low flow shower heads in the first place.

    Paul in Montreal.
    • CommentAuthorDarylP
    • CommentTimeJun 22nd 2013
    ... the above hits the nail on the head.

    I think they have been dreamed up as an aid to help developers to improve the SAP rating of their otherwise poorly specced designs...:shamed:

    'Waste' of time....:bigsmile:
    • CommentAuthorGaryB
    • CommentTimeJun 22nd 2013
    Posted By: jamesingramGood post Gary , thanks.
    You posted info on heat loss from primary circuit, DHW storage etc. a while back , cant seem to find , any chance you could point me in the right direction.

    If someone can explain how I can post a link to a discussion, I can provide the reference requested by James.
    • CommentAuthorEd Davies
    • CommentTimeJun 22nd 2013 edited
    Link any other link. Here's this discussion's URL copy and pasted from the browser address bar.


    Make sure you post as Text (select that radio button rather than Html in the “Format comment as” bit below the comment entry box).

    For bonus points, you can link to the whole discussion by chopping the “&page=1#Item_16” bit off the end of the URL which makes it go to particular post in the discussion.
    • CommentTimeJun 22nd 2013
    Or just go to the first comment, click on it, then copy the URL.
    • CommentAuthorGaryB
    • CommentTimeJun 23rd 2013 edited
    Thanks Paul - that was 'quick sums in head' calculation. Sums just didn't seem to stack up for these and potential for problems seems large.
    • CommentAuthorSeret
    • CommentTimeJun 23rd 2013 edited
    Posted By: Paul in MontrealMuch better to just fit low flow shower heads in the first place.

    Of course, low flow heads or cheap-as-chips inline flow restricters are a no-brainer IMO but it's not an either/or decision. No reason you can't do both.

    Even with a low-flow head I make the return on investment for fitting even a low-efficiency retrofit unit to be about 12 years. CO2e saving is about 73kg per year, at a cost/benefit of £2.65kg-1 CO2e. That kind of cost efficiency ratio puts it between insulation (£1-£2kg-1) and measures like new boilers (around £5kg-1). Windows are well over £10kg-1 (for SG to DG, worse for DG to DG or DG to TG), by way of comparison. But how many of people would dive in and do those first in a retrofit?

    My figures may be skewed somewhat by the wife's love of very long very hot showers, of course. :wink:

    As for potential problems: of course there are, it's plumbing! I hate plumbing with a passion. Forget hell, mucky pipes are the true domain of Satan.
    • CommentAuthorjamesingram
    • CommentTimeJun 23rd 2013 edited
    Thanks Gary, actually found what I was after here via search on your name.
    This was the bit re primary pipework heat loss, which I found an eye opener

    Posted By: GaryBHmmm, I think I need to clarify why I believe the OP's proposal to be a sound and robust strategy for the particular dwelling.

    Most of the energy used for water heating in the UK doesn't heat the water used by the occupants - it heats the internal space or outside (technically it all ends up outside). The lowest energy form of heating, ignoring solar or heat pump generation, is via electric immersion heating.

    The primary pipework is just an additional source of loss - the water and pipe has to be heated every cycle, the pipework loses heat continuously when the primary circuit is activated and there are continuous additional standing losses by conduction from the cylinder.

    Some example figures from SAP2009 for a 3 bed semi of 85 m2 floor area & a 200 litre cylinder for comparison:

    - Nett hot water use: 1,498 kWh pa (no standing losses)
    - Electrical immersion heating: 1,987 kWh pa (based on the 200 litre cylinder heat loss factor permitted by DECC)
    - Gas combi boiler, 90% efficiency: 2,497 kWh pa
    - Gas system boiler, 90% efficiency, insulated & timed primary circuit: 2,791 kWh pa
    - Gas system boiler, 90% efficiency, insulated but UN-timed primary circuit: 2,852 kWh pa
    - Gas system boiler, 90% efficiency, UN-insulated but timed primary circuit: 3,139 kWh pa
    - Gas system boiler, 90% efficiency, UN-insulated & UN-timed primary circuit: 4,118 kWh pa
    - Gas system boiler, 90% efficiency, 25mm (or 50mm cylinder jacket with gaps): 6,457 kWh pa

    Even connecting up the primary circuit pipes increases the standing losses by 208 kWh pa (assuming pipework is 10mm insulated).
    Obviously if the boiler efficiency is lower or the primary pipework is routed outside the thermal envelope then the losses are much worse and do not contribute to space heating in the winter.

    Posted By: GaryBIf by 'decent' insulation you mean 20mm then the pipe losses drop from 21 W/m to 13 W/m (Engineering Toolbox data) and the losses are still 129 kWh pa. For comparison uninsulated pipe is 56 W/m.

    DECC permitted standing losses for a 200 litre cylinder are 2.26 kWh/day = 825 kWh pa (if the cylinder were maintained at a constant 60 deg C). SAP calcs suggest this is actually around 490 kWh pa, to allow for reduction in loss as the cylinder cools.

    The standing losses due to the primary pipe connections are therefore 26% of the 'base' cylinder standing losses. A similar loss occurs in the DHW flow and cold feed pipe, so total standing losses will be:

    20mm pipe insulation: 129 + 129 + 490 = 748 kWh
    10mm pipe insulation: 208 + 208 + 490 = 906 kWh
    No pipe insulation: 555 + 555 + 490 = 1,600 kWh

    That is why mine are insulated with the equivalent of 50mm insulation...

    For a solar cylinder the primary loop is in the top of the cylinder so wouldn't reduce due to stratification. In the winter most of the standing cylinder losses should indeed contribute to space heating, but are a complete loss in summer. For a rambling Victorian semi the primary circuit is likely to be routed outside of the thermal envelope for most of its length and primary losses will be losses.

    The point is that use of a primary circuit increases overall energy consumption and losses.

    • CommentAuthorSeret
    • CommentTimeJun 24th 2013
    Posted By: tonyIt looks to me to break all kinds of water regulations to me.

    I'm really not that gripped up on the water regs Tony, can you be a bit more specific about why you think it's non-compliant? And more importantly to me and my family, what do you think are the actual risks of that non-compliance?
    • CommentAuthortony
    • CommentTimeJun 24th 2013
    The risks to health are low, the risk of getting fined is likely greater

    The problems start were there to be a loss of pressure in the local mains AND there was a weep inside your heat exchanger and if there was would you know about it. Then waste water could get sucked back down the mains pipe and contaminate someone else's water.

    An air separation layer is needed between the two types of water in the heat exchanger.

    The regulations are more complex than this but that is the gist.
    • CommentAuthorbarney
    • CommentTimeJun 24th 2013
    A shower in a domestic dwelling is generally a Category 3 fluid - a simple backflow device like a double check valve would satisfy WRAS requirements.


    • CommentAuthortony
    • CommentTimeJun 24th 2013
    Yes but what about when mains water is in close proximity to waste water?
    • CommentAuthorbarney
    • CommentTimeJun 24th 2013
    As I said - it's a category 3 fluid - so a double check valve on the supply protects the mains (ie it's a backflow protection device) - it won't protect the householder though.

    It's no different to someone dipping the hoseipe into a barrel full of rainwater with "organic additives" in - the latter being potentially much worse.

    You probably would need a full blown RPZ valve if it was commercial - and a robust risk assessment - but for domestic, no such issues.

    There is probably a greater hygeine risk from hot tubs in the garden to be honest.


The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press