EvoHome ( and general ) Hot Water Temperature Thoughts?
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Some great questions, you've obviously given this quite a bit of thought...
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.Originally Posted by sidepipe
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...Several thoughts/questions spring to mind though. The default hot water temperature on the EvoHome was 50°C, and the "eco" ( which is the recommended setting ) on the boiler is 60°C - it obviously makes sense to have the boiler about 10°C 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.
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.
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.However, legionella will survive at 50°C, so that seems too cool from that point of view. In fact, with the default differential setting, the water will regularly get to 40°C, 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?
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.
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.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!
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.
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...)For now, to feel a bit safer whilst not wasting too much energy, I've set the EvoHome to 55°C ( differential 7°C ) and the boiler to 65°C, 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. )
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.
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.My second thought after the install is, why bother with a hot water schedule at all? My tank loses around 0.4°C 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.....
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.
) 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.
