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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.

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    • CommentAuthorgoodevans
    • CommentTimeDec 10th 2018
    Many heat pumps don't heat the DHW cylinder to 60 degC and so usually the control panels allow the immersion to be turned on one a week to control Legionella.

    However I observe:
    Many tanks have the immersion heater at a position to heat the top half of the cylinder only.
    In the UK our supply water is chlorinated and potable.
    A blending valve is usually located at the cylinder preventing any sanitising hot water from entering pipes.
    Cold water tanks are often not used in new installations (mains pressure hot water).

    I suppose the questions are.
    For pressurised domestic systems is Legonella control by using the immersion necessary.
    If it is necessary, what is the effectiveness of heating the top half of the cylinder once a week to 60 degC given that the pipes and bottom of the cylinder never reach that temperature.

    According to the NHS it is "very rare to catch Legionnaires' disease in the home" (https://www.nhs.uk/conditions/legionnaires-disease/) - however heat pumps are not that common either so maybe we have problems brewing.
    • CommentAuthortony
    • CommentTimeDec 10th 2018
    Most of North America run cylinders at 50C and I have been using stat set at 47C to for the last 30 years with no problems.condistently heating to 50 has been reported to negate the risk - medical type journal that I can’t find again.
    We heat our unvented hot water tank to 45C using the logic that the system is sealed and the cold water still chlorinated, if only slightly, when it reaches the hot water tank. Therefore there shouldn't be any risk of bugs.
    • CommentAuthorArtiglio
    • CommentTimeDec 11th 2018
    Up until relatively recently , the legionella legislation exempted systems under 300l , apparently this exemption was removed for no real reason and with no evicence to suggest that legionella was a problem in the millions of domestic ( rented) and small business premises it now encompassed.
    The legionella risk assessment industry have had a field day, i’ve seen some that have recommended the replacement of dhw cylinders on the grounds that there are no inspection ports on the existing cylinder and that as such the internal condition cannot be inspected and as a result the risk is increased.
    • CommentAuthortony
    • CommentTimeDec 11th 2018
    • CommentAuthorArtiglio
    • CommentTimeDec 13th 2018

    The comments about dhw cylinders was from being shown a risk assessment that had been undertaken on a low rise block of social housing flats.
    Interestingly the above link shows that a temperature outside the range 20 - 45 is sufficient. Quote

    "Are conditions right for bacteria to flourish e.g. is the temperature of the water between 20 degrees
    Celsius and 45 degrees Celsius?"

    So it would appear that this organisation at least does not subscribe to the need to heat to above 60 degC (periodically) to prevent legionella
    Out of interest what times do you set your heat pumps to top up the hot water?

    Mine is currently set to 5am and 5pm to bring up to 50c. I'm wondering if there is a better timing perhaps in the day when warmer...
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018
    Obviously the best time to use the heat pump is when the outside air is warmest - but before you need it. Providing the cylinder is well insulated I think heating once a day at around 1pm would be optimal - without some fancy weather forecast algorithm.
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018
    Thanks all for your reassurances - I think I will ignore the top up to 60 deg once a week system.
    • CommentTimeDec 13th 2018
    Posted By: Peter_in_HungarySo it would appear that this organisation at least does not subscribe to the need to heat to above 60 degC (periodically) to prevent legionella

    It's a question of how long the high temperature is held, I believe. Boiling water kills things faster than water at 60°C, which kills things faster than water at 45°C etc. Specifically, different proteins denature (i.e. unfold and stop working) at different temperatures, and proteins inside organisms take longer to heat up than those on the surface etc etc. And some will refold and start working again once the temperature drops, while others won't. So it's all very dependent on exactly what bugs and what happens to the temperature. The rules are just safe, simple approximations.
    • CommentAuthorBeau
    • CommentTimeDec 13th 2018
    Our GSHP was set to heat the water to 60C once a week. I was surprised how big a proportion of the electricity the GSHP uses was used on just this cycle. Having done some reading around on the subject I reset it to do the legionella cycle once a month.
    <blockquote><cite>Posted By: goodevans</cite>Obviously the best time to use the heat pump is when the outside air is warmest - but before you need it. Providing the cylinder is well insulated I think heating once a day at around 1pm would be optimal - without some fancy weather forecast algorithm.</blockquote>

    That would make sense, although not sure my tank now set at 47c would last till the next day from one top up.

    Maybe it would be better to heat to 55c once aday...

    Who knows 😂
    • CommentAuthortony
    • CommentTimeDec 13th 2018 edited
    I would consider setting the stat to 47 and letting it keep the cylinder full of hot water all the time. Mine never calls for heat in the night as the cylinder is 4x spray foam insulsted so only cools slowly.

    Minimising the temperature of stored water also minimises heat losses.
    • CommentAuthorgoodevans
    • CommentTimeDec 14th 2018
    yep - tony makes sense.
    • CommentAuthorphiledge
    • CommentTimeDec 14th 2018 edited
    The linked document above states legionella will flourish up to 45 degrees. I'd be wanting to heat way past 45/47 degrees to stop their fun.
    • CommentAuthortony
    • CommentTimeDec 14th 2018
    Constant 50C has been shown to kill em off and I suspect the same at constant 45C probably not tried

    They hate copper too and the dark
    A permanently safe solution would be to fit a thermal store or heat bank where the hot water is mains water heated indirectly by the water within the cylinder. Our heat bank has a heat exchange for the DHW and it's great to have mains pressure, drinkable hot water that has a mixing valve that prevents the water getting too hot - regardless of how hot the tank is (the tank has been upto 90C on sunny days from solar).
    • CommentTimeDec 14th 2018
    The other option is to install a destratification pump that circlulates from top to bottom so the whole tank heats up.
    • CommentAuthorgoodevans
    • CommentTimeDec 14th 2018
    a permanently safe solution would be de stratification tank, 60 degrees weekly heatup - 60 degree pipe flush once a week and blending valves only at the outlets and cold water pipe flushing at least once a week. But It's not going to happen.
    • CommentTimeDec 14th 2018
    We do flush our pipes once a month, but that's as much to do with making sure all the traps still have water in them. Remember it's only inhaled spray that matters for legionnaires, so only showers that aren't used regularly are any concern in a normal house.
    • CommentAuthorbarney
    • CommentTimeDec 14th 2018
    The safer solution would actually be to heat the tank to 70C and hold that temperature for at least 1 hour whilst circulating the water fully through the storage vessel and as much of the distribution pipework as possible - so called pasteurization cycles - weekly would be a reasonable time-cycle - usually undertaken at times of low demand, but some care needed to control outlet temperatures

    Outside of the domestic setting, the above would be a fairly normal regime (usually controlled via the facility building management system)



    60-70degC seems to be the disinfection temperature.

    Or - don't waste your money and space storing lukewarm water that loses its heat :-) Instantaneous water heaters are the way to go.
    • CommentTimeDec 14th 2018
    Domestic systems are fairly different to other systems. They don't have recirculation systems for the tank, and especially not for the distribution pipework as a rule, for reasons of over-engineering, plus waste and consequent costliness.

    Posted By: WillInAberdeenOr - don't waste your money and space storing lukewarm water that loses its heat :-) Instantaneous water heaters are the way to go.

    Agreed except where solar heat is used to heat the stored water, with overnight electricity when solar is insufficent.
    Better for the planet if the solar is exported to displace a coal power station or low efficiency gas station, and a high-efficiency (>100%) gas/lpg water heater is used for hot water.

    The distorted FIT/ET regime is ending soon which paid people to dump publically-subsidised PV into low grade resistive water heaters. Interesting to see if people then switch away from storing hot water in their houses, or if it even becomes more common as people max out their self-consumption.

    But I digress from bugs!
    Lets go back to how legionella started/was found. I came from poorly maintained cooling tower systems - with water being recirculated (for ever!) at 30-ish Centigrade, not being treated, and being blown out of the stack as a plume of fetid water spray.

    Yes there is a tiny, tiny risk of legionella in domestic settings, but it is miniscule. How might you acheive it even? - maybe if you had a long dead leg sitting in a warm enviroment >30C, growing legionella. Then someone takes a shower - so the shower has to have been not in use for quite a while.

    In terms of things to realistically worry about - it is a 1 out of 10.
    • CommentAuthortony
    • CommentTimeDec 14th 2018
    How many people have ever got ill from legionella in a domestic hot water system ?
    • CommentTimeDec 14th 2018
    Posted By: WillInAberdeena high-efficiency (>100%) gas/lpg water heater is used for hot water.

    You know we're going to end up disagreeing, but I'll bite anyway. Link for > 100% efficient gas water heater please?

    And +1 to Tim & Tony.
    Thought you'd like that one :-))

    For example: "up to 107% net thermal efficiency."
    Other brands are available..

    It's all down to the flue temperature. The Net Calorific Value of the gas is defined as: the energy available when the gas is burned and the products are cooled to 15degC without any condensation. However a water heater is exchanging heat with cold mains water which comes at 10ish degC. So the flue can be really cooled, lots of condensation, lots more energy can be recovered, more than 100% of the Net calorific value. Unlike a CH boiler which is condensing again a warmish CH circuit so will have a warmer flue and less condensation.

    Makes great sales pitch, but more seriously it is a more efficient way to use gas than to burn it in a 50% efficient power station, with transmission and standing losses on top before you get hot water out the tap.

    Edit: moving this to a more relevant thread, sorry GE for the sidetrack!
    • CommentAuthortony
    • CommentTimeDec 15th 2018
    ASHP 350% efficient, GSHP 400 to 450% efficient, water source more efficient
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