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Thread: EvoHome ( and general ) Hot Water Temperature Thoughts?

  1. #1
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    Default EvoHome ( and general ) Hot Water Temperature Thoughts?

    I've recently added the hot water kit to my EvoHome, to control an unvented cylinder that has twin coils. The boiler is currently feeding both coils in parallel ( the idea was the twin coil gives me the option of solar-thermal in future without having to replace the cylinder again. ) It's a Worcester Bosch cylinder that has several temperature sensor locations - currently I've put the EvoHome insertion sensor in at the lowest level, fractionally above the primary heating coil, theory being it will "see" the temperature drop quickly and hence all water above will be at or above the temperature band set on the EvoHome ( assuming the water is switched on. )

    Several thoughts/questions spring to mind though. The default hot water temperature on the EvoHome was 50C, and the "eco" ( which is the recommended setting ) on the boiler is 60C - it obviously makes sense to have the boiler about 10C higher than the EvoHome so the water is capable of being heated to the set point fairly quickly, so that makes sense and clearly Honeywell and Worcester Bosch have a similar idea of what the setting should be. However, legionella will survive at 50C, so that seems too cool from that point of view. In fact, with the default differential setting, the water will regularly get to 40C, a temperature at which the bacteria can actually thrive. On the flip side, if it's such a danger, why do both manufacturers have this as the default or recommended setting? Is it because the tank is sealed so there's less risk? Is that even true?

    In my case, stratification I assume will mean the top of the cylinder is hotter than the bottom, and this is a 250L tank which is nearly 2m tall, so maybe by quite a bit. I could obviously check that by measuring with another probe near the top, but I'm still not sure I really want bacteria living happily at the bottom of the cylinder! For now, to feel a bit safer whilst not wasting too much energy, I've set the EvoHome to 55C ( differential 7C ) and the boiler to 65C, but it still seems odd to me that the default settings are as they are if it's such a risk. ( There's a lot of discussion about this in various places on the net, and of course the environmental conditions under which legionella live and grow is well known, but most stuff that I see on this specific topic of unvented cylinders seems to be speculation paraded as fact, with no reason for the defaults being cited that make sense. )

    My second thought after the install is, why bother with a hot water schedule at all? My tank loses around 0.4C an hour when the water isn't used, with the only real significant drops happening when a shower is taken. Given the fact you presumably always want the water heated during a shower, and that's the most common time the boiler will need to fire anyway, having a schedule seems to make no difference, other than add the possibility that if you shower during the day when the water would normally be off, you risk having a nice comfortable environment for the aforementioned bacteria.....

  2. #2
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    Some great questions, you've obviously given this quite a bit of thought...

    Quote Originally Posted by sidepipe View Post
    I've recently added the hot water kit to my EvoHome, to control an unvented cylinder that has twin coils. The boiler is currently feeding both coils in parallel ( the idea was the twin coil gives me the option of solar-thermal in future without having to replace the cylinder again. ) It's a Worcester Bosch cylinder that has several temperature sensor locations - currently I've put the EvoHome insertion sensor in at the lowest level, fractionally above the primary heating coil, theory being it will "see" the temperature drop quickly and hence all water above will be at or above the temperature band set on the EvoHome ( assuming the water is switched on. )
    The typically recommended position is about 1/3rd of the way up, which is usually near the top of the indirect heating loop, so it sounds like you have yours in the right place.
    Several thoughts/questions spring to mind though. The default hot water temperature on the EvoHome was 50C, and the "eco" ( which is the recommended setting ) on the boiler is 60C - it obviously makes sense to have the boiler about 10C higher than the EvoHome so the water is capable of being heated to the set point fairly quickly, so that makes sense and clearly Honeywell and Worcester Bosch have a similar idea of what the setting should be.
    That's strange - I'm pretty sure the default hot water temperature on Evohome is 60C as I run mine at 54C and remember turning it down, not up...

    Having the boiler flow temperature only 10 degrees higher than the desired hot water temperature is a bit marginal because if the hot water can't quite get up to temperature the boiler will never shut off... a 10 degree differential is also quite slow to heat. Many systems are installed such that the flow temperature is boosted when there is a hot water demand - have a look and see if your boiler has this option. If you use Opentherm it should happen automatically however if you use BDR91's then one way to do it on a boiler that has a boost input would be to wire the orange wire switch from the hot water zone valve to the boost input on the boiler. So when the hot water zone valve closes the switch closes and the boiler goes to its hot water heating flow temperature instead of its central heating flow temperature.

    Of course for this to work you can't already be using the heating and hot water zone valve orange wires wired in parallel to fire the boiler. If the boiler will fire from the boost input alone you could wire the hot water zone valve orange wire to the boost input and the one from the heating zone valve orange wire to the normal thermostat input of the boiler. That way it will fire either way but only boost the flow temp for hot water.

    If your boiler can't work this way then you'd need a third BDR91 configured as a boiler relay. It all depends what inputs your boiler has and how it is currently wired up to the Evohome.
    However, legionella will survive at 50C, so that seems too cool from that point of view. In fact, with the default differential setting, the water will regularly get to 40C, a temperature at which the bacteria can actually thrive. On the flip side, if it's such a danger, why do both manufacturers have this as the default or recommended setting? Is it because the tank is sealed so there's less risk? Is that even true?
    Putting aside that Honeywell defaults to 60C as far as I know, I think the issue is that legionella likes stagnant water, which is an issue in things like commercial building evaporation cooling towers...as it gives it time to grow. In a hot water cylinder in a residential property you're typically emptying the cylinder every day or two so any given body of water isn't staying there for long and it is constantly being flushed through with fresh cold tap water.

    50C is still too low though, and I find 60C uncomfortably hot at the tap so I've compromised with mine by setting it to 54C with a 5C differential.
    In my case, stratification I assume will mean the top of the cylinder is hotter than the bottom, and this is a 250L tank which is nearly 2m tall, so maybe by quite a bit. I could obviously check that by measuring with another probe near the top, but I'm still not sure I really want bacteria living happily at the bottom of the cylinder!
    The top of the cylinder will be hotter than the bottom in between reheat cycles while you're using hot water and drawing cold water in the bottom, but once a reheat cycle is underway and especially towards the end of it there will be strong convection currents circulating the water past the element to the top and back down the sides, so there will be almost no stratification while that convection is occurring.

    Put simply, the temperature measured at your sensor will never be higher than the actual temperature at the top where you draw off hot water, but will often be lower due to cold water entering.
    For now, to feel a bit safer whilst not wasting too much energy, I've set the EvoHome to 55C ( differential 7C ) and the boiler to 65C, but it still seems odd to me that the default settings are as they are if it's such a risk. ( There's a lot of discussion about this in various places on the net, and of course the environmental conditions under which legionella live and grow is well known, but most stuff that I see on this specific topic of unvented cylinders seems to be speculation paraded as fact, with no reason for the defaults being cited that make sense. )
    If you're still concerned about legionella there is only really one sure-fire solution - run the hot water cylinder at a higher temperature like 60-65C and then use automatic thermostatic mixing valves at each of your hot taps to bring the temperature at the tap down to about 48-50C. I almost went this route in my system but the added cost and complexity to install put me off as the valves are somewhat pricey, especially compact ones that can fit in a bathroom sink pedestal. (Although ironically the pedestal that put me off the mixer valves is now gone, replaced with a cabinet with plenty of room for plumbing...)

    They have two inputs, one for hot and one for cold (with the cold input teeing off the cold tap's supply) and one output that goes to the hot tap. The valve automatically mixes the right amount of cold water into the flow to reach the target hot water tap temperature, and uses a temperature sensor to do this automatically.

    There's a number advantage of using these:

    1) Legionella cant grow as you can run your cylinder at a higher temperature that would normally be too hot at the tap.
    2) No scalding at hot taps because your hot water temperature is regulated at the tap to whatever you set on the mixer valve
    3) As your tank gradually cools you won't notice because it will adjust the blending to reach the same target temperature - so your tank dropping from 65C to 60C won't make any difference to the 50C you set at the tap
    4) With a small cylinder you can effectively get "more" hot water, because the hotter the cylinder is the more cold you have to blend in to reach the same temperature, so less percentage of the water came from the cylinder. Good if you have a small cylinder like we do.
    5) Temperature loss in different pipe length runs doesn't affect the temperature at different taps - each tap can be set independently, for example bathroom sink and/or bathtub not set as hot as a kitchen sink for washing dishes...you can set each hot tap to be whatever temperature you like.


    My second thought after the install is, why bother with a hot water schedule at all? My tank loses around 0.4C an hour when the water isn't used, with the only real significant drops happening when a shower is taken. Given the fact you presumably always want the water heated during a shower, and that's the most common time the boiler will need to fire anyway, having a schedule seems to make no difference, other than add the possibility that if you shower during the day when the water would normally be off, you risk having a nice comfortable environment for the aforementioned bacteria.....
    Your cylinder might only lose 0.4C an hour but that's not true of all of them... ours is a small and somewhat older 100 litre cylinder and if I left it scheduled on 24/7 then it would go through about 6 reheat cycles a day even if we didn't use any hot water.

    So clearly its wasteful in this situation to leave it going when we're out all day or on days when we're away. It doesn't take long to heat up again so I schedule it for times when the house will be occupied.
    Last edited by DBMandrake; 14th November 2018 at 04:59 PM.

  3. #3
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    Thanks for taking the time to reply with such detailed info!

    Quote Originally Posted by DBMandrake View Post
    That's strange - I'm pretty sure the default hot water temperature on Evohome is 60C as I run mine at 54C and remember turning it down, not up...
    Well, I've only recently installed the water kit so wouldn't have adjusted any settings previously. I've had EvoHome for quite a while running the heating, so it's possible I guess, but unlikely. Another possibility would be they've changed the default with newer software updates I suppose.

    Having the boiler flow temperature only 10 degrees higher than the desired hot water temperature is a bit marginal because if the hot water can't quite get up to temperature the boiler will never shut off... a 10 degree differential is also quite slow to heat.
    Mine does heat up pretty quickly, with only a slight tail-off as it nears the set temperature... maybe because of the better insulation? Takes about 15 minutes to go from 45 to 55.

    Many systems are installed such that the flow temperature is boosted when there is a hot water demand - have a look and see if your boiler has this option.
    ...
    An interesting thought, and in fact the boiler I have has a separate control for hot water temperature. Unfortunately, it only has any effect when there's an optional internal diverter valve fitted, in which case there are separate demand signals for heating and hot water. This wasn't possible in my installation because the cylinder and CH flow are some distance from the boiler, so would've been more work. AFAICT the DHW demand connection does nothing if the diverter isn't installer - would've been nice if it simply changed the flow temp to that set for DHW.

    If you're still concerned about legionella there is only really one sure-fire solution - run the hot water cylinder at a higher temperature like 60-65C and then use automatic thermostatic mixing valves at each of your hot taps to bring the temperature at the tap down to about 48-50C. I almost went this route in my system but the added cost and complexity to install put me off as the valves are somewhat pricey, especially compact ones that can fit in a bathroom sink pedestal. (Although ironically the pedestal that put me off the mixer valves is now gone, replaced with a cabinet with plenty of room for plumbing...)
    Like you say, an expensive solution. The old unvented cylinder I had used a mixer valve on the DHW flow from the cylinder, so a similar idea but much simpler and cheaper ( without the advantage of individual tap control. ) To be honest for me having the water at 60 degrees wouldn't be much of an issue.

    Your cylinder might only lose 0.4C an hour but that's not true of all of them... ours is a small and somewhat older 100 litre cylinder and if I left it scheduled on 24/7 then it would go through about 6 reheat cycles a day even if we didn't use any hot water.

    So clearly its wasteful in this situation to leave it going when we're out all day or on days when we're away. It doesn't take long to heat up again so I schedule it for times when the house will be occupied.
    Yep... I don't know what our old cylinder was like but I suspect it was much worse too. Certainly when the house isn't occupied for more than a day it makes sense ( but that's what away mode's for ) I think with a very low loss it might make sense for me to leave the DHW on unless I'm away... at least during the daytime.

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    Quote Originally Posted by sidepipe View Post
    Like you say, an expensive solution. The old unvented cylinder I had used a mixer valve on the DHW flow from the cylinder, so a similar idea but much simpler and cheaper ( without the advantage of individual tap control. ) To be honest for me having the water at 60 degrees wouldn't be much of an issue.
    The new OSO cylinders do this to allow you to store the HW at a much higher temperature yet not have scolding hot water coming from your taps. It is something I have considered doing myself on our MegaFlo which is only 170L (A cylinder we inherited with the house). The issue being our new showers now drain 90l of the cylinder in about 8 minutes when using the soaker. Being able to store it at a higher temperature than 55oc would give us more Hot Water. The cylinder reheats in about 20 minutes from a 50% draw off so isn't terrible but can be irritating sometimes.
    Last edited by mtmcgavock; 14th November 2018 at 08:20 PM.

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    Quote Originally Posted by mtmcgavock View Post
    The new OSO cylinders do this to allow you to store the HW at a much higher temperature yet not have scolding hot water coming from your taps. It is something I have considered doing myself on our MegaFlo which is only 170L (A cylinder we inherited with the house). The issue being our new showers now drain 90l of the cylinder in about 8 minutes when using the soaker. Being able to store it at a higher temperature than 55oc would give us more Hot Water. The cylinder reheats in about 20 minutes from a 50% draw off so isn't terrible but can be irritating sometimes.
    Actually that set me off on another train of thought - an increase of 5C doesn't sound a lot, but actually has a big impact. The obvious one is of course that it will be longer before the water becomes "too cold," because ( assuming the rate of loss of temperature with flow is the same, which is approximately true in this small range ) it will take longer for the water to drop below a "usable" level. However, also if you're using a mixer, you actually draw less hot water to get the same output temperature.

    For us, the vast majority of hot water use is for showers - in fact our use otherwise is almost zero ( since we have a hot water tap in the kitchen anyway. ) It's quite possible that the lower hot water draw of raising the cylinder temp by that 5C more than compensates for the additional cylinder/pipework loss and lower boiler efficiency.

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