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Thread: A thought on water temperature control

  1. #1
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    Default A thought on water temperature control

    You need to raise the water temperature to 60 degrees for some few minutes to ensure that you kill Legionella.

    So from an economy point of view could Honeywell consider allowing you to set temperatures with their hot water control. I would like to set the temperature to 60 degrees from 0600 - 07:00 then run at say, 5 degrees until the off point?

    Worth having this feature?

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    Quote Originally Posted by basiluk View Post
    You need to raise the water temperature to 60 degrees for some few minutes to ensure that you kill Legionella.

    So from an economy point of view could Honeywell consider allowing you to set temperatures with their hot water control. I would like to set the temperature to 60 degrees from 0600 - 07:00 then run at say, 5 degrees until the off point?

    Worth having this feature?
    I'm guessing you mean 50oC, as 5oC is the off point.

    But I don't understand what that feature would achieve. The water in your DHW cylinder falls in temperature for one of two reasons:

    1. The water cools naturally, losing heat through the insulation of the cylinder; or
    2. You extract hot water from the cylinder which is replaced by cold water entering at the bottom.

    For (2), if you are worried about Legionella, you would surely want to reheat the new incoming water to 60oC anyway. And for (1), you can allow a deadband up to 10oC in the hot water differential settings on Evohome, such that the cylinder would drop to 50oC before initiating a reheat.
    Last edited by Edinburgh2000; 8th May 2018 at 08:04 AM.

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    Quote Originally Posted by basiluk View Post
    You need to raise the water temperature to 60 degrees for some few minutes to ensure that you kill Legionella.

    So from an economy point of view could Honeywell consider allowing you to set temperatures with their hot water control. I would like to set the temperature to 60 degrees from 0600 - 07:00 then run at say, 5 degrees until the off point?

    Worth having this feature?
    Based on 4 years of using Evohome, it is unlikely that we will see much in the way of change to the present controller. My guess is that a new controller will not be long in coming. Whether it has the raft of improvements proposed by many over the years, only time will tell.

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    Quote Originally Posted by basiluk View Post
    You need to raise the water temperature to 60 degrees for some few minutes to ensure that you kill Legionella.

    So from an economy point of view could Honeywell consider allowing you to set temperatures with their hot water control. I would like to set the temperature to 60 degrees from 0600 - 07:00 then run at say, 5 degrees until the off point?

    Worth having this feature?
    Thinking about this some more. A lot of HW controls now have built into their firmware a 60C cylinder heat once a week.

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    Quote Originally Posted by HenGus View Post
    Thinking about this some more. A lot of HW controls now have built into their firmware a 60C cylinder heat once a week.
    Once a week is enough to prevent Legionella ?

    For anyone genuinely concerned about Legionella who also doesn't like scalding hot water, I suggest that you look into installing thermostatic mixing valves at all your hot taps.

    They let you maintain your hot water at a higher temperature, say 65C, and automatically blend cold water into the flow to the hot tap to reach a locally preset temperature, say 48C.

    As they actually measure the temperature of the water just before it comes out the tap and adjust automatically they compensate for any changes in the hot water temperature out of the cylinder due to long pipe runs at different flow rates, variations in cylinder temperature etc. Because they are at each hot tap you can set a different temperature at different locations. For example cooler at a handbasin that children will be using for hand washing, hotter in a kitchen sink.

    I almost installed some thermostatic mixing valves before I converted my system to S-Plan, in the end I didn't mainly because a valve to fit in a pedestal hand basin was expensive and difficult to fit, (I only found one suitable valve that wouldn't have required me to install it below the floor) ironically that hand basin has now been replaced with a cabinet type where there is plenty of room...

    Without something like this there is always a trade off between the possibility of Legionella and excessively high hot tap temperature.
    Last edited by DBMandrake; 8th May 2018 at 01:45 PM.

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    Quote Originally Posted by basiluk View Post
    You need to raise the water temperature to 60 degrees for some few minutes to ensure that you kill Legionella.

    So from an economy point of view could Honeywell consider allowing you to set temperatures with their hot water control. I would like to set the temperature to 60 degrees from 0600 - 07:00 then run at say, 5 degrees until the off point?

    Worth having this feature?
    I am not sure I see the need for this. You want the temperature up to 60c to ensure legionella is killed. That’s fine. But then a function that keeps it at 50c (I assume you meant 50c) for the rest of the one hour the heating is on. Firstly it will take a short time to get it up to 60 so that is part of the hour gone, secondly I assume people will be drawing off hot water in that hour, having showers, getting washed. The replacement cold water will drop the temperature below 50 so the boiler will switch on anyway. That is another chunk of that hour gone by. Thus, by the time nobody is using hot water, all have washed, showered or what have you, and other washing devices have been operated, your hour has gone by and there never was a need to have a 50c fall back setting.

    My hot water is set for one hour in the morning and one hour at night at 60c. The boiler does not have to be on long to get the temperature back up to 60c. During the day it drops, drops below 50c but we have no issues with that. Sorry but I cannot see the need for such a function unless you were wanting the hot water on all day.

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    Maybe similar to allowing each ON time on the CH schedule to have a different temperatures, the controller could ask what temperature you want the HW for each ON time. Its doable. But whether Honeywell see the need, is a different question. This I think is an issue only for people who need the HW tank to be heated through the day.

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    The risk of legionella in the home is very low. As well as temperature it also needs nutrients and oxygen. And growth happens in stagnant or extremely slow moving water.

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    Quote Originally Posted by G4RHL View Post
    I am not sure I see the need for this. You want the temperature up to 60c to ensure legionella is killed. That’s fine. But then a function that keeps it at 50c (I assume you meant 50c) for the rest of the one hour the heating is on. Firstly it will take a short time to get it up to 60 so that is part of the hour gone, secondly I assume people will be drawing off hot water in that hour, having showers, getting washed. The replacement cold water will drop the temperature below 50 so the boiler will switch on anyway. That is another chunk of that hour gone by. Thus, by the time nobody is using hot water, all have washed, showered or what have you, and other washing devices have been operated, your hour has gone by and there never was a need to have a 50c fall back setting.
    It's worse than that. Due to stratification if you heat the cylinder up to 60C initially, that will heat the entire cylinder to 60C. If you then drop the temperature set point to 50C for the rest of the day, as soon as you draw in some cold water at the bottom (near the sensor) the system will reheat until it detects 50C where the sensor is, however the top part of the cylinder will still be putting out 60C water as that 60C water won't be mixing with the 50C water below it.

    So if the drop in temperature from 60C to 50C is caused by hot water drawoff not gradual loss of heat through the cylinder walls, you are still actually getting scalding 60C water from the tap until most of that initial 60C parcel of water is used up and replaced by the 50C water below it.

    Stratification effects are one reason why having multiple hot water temperatures or a hot water temperature schedule doesn't really make sense. The only use I can see for multiple hot water temperatures is if you specifically wanted to boost the temperature shortly before using the entire cylinder to run a bath, so you could get a bit more bath water from a small cylinder. Because after you did that the entire cylinder would be at a low temperature and would reheat correctly to the new lower temperature setpoint.

    But as a means for avoiding Legionella it just means that for the majority of time that you were scheduled to 50C you would still be getting scalding 60C water anyway, so why bother.

    The real solution to this that isn't just a compromise between scalding and Legionella is automatic thermostatic mixer valves as I mentioned earlier. Personally like Paul I think the risks are low as long as you're well above 50C and you regularly use the hot water. I run mine at 54C with a 5C differential (active all day when people are home, but off at night and during the work day) as a compromise between scalding and Legionella risk.
    Last edited by DBMandrake; 10th May 2018 at 09:03 AM.

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