Hot Water Differential setting

[killa47]

Historically, all my previous house cylinders/boilers were set for DHW at 60C (both with and without the two kids) and thus far we are all still alive and well, So I think I will stick at the 60C.
My understanding was that legionella is more likely to survive (if a correct term) at 55C and below rather than occur at 60C. So I presume if DHW is heated and re-heated to 60C thrice daily (or more), this should be safe.
No doubt a contentious subject but open to further advice from all re this.

[killa47]

We are lucky, no kids in the house, but when the young ones come the water temperature has not been a problem. The showers are thermostatically controlled and other taps are mixers apart from a loo and the utility room. Having a shower can drop the water temperature in the tank to about 35 but it is back up to 60 rapidly. Overnight it drops to around 45 to 50. I never used to monitor the hot water before Evohome as there was no way of doing it. Now that I see what is happening as mentioned earlier I have knocked an hour off its on time and I think I may be able to knock another hour off which would give me 2 hours on in the morning and one in the evening.

Agree with everything G4RHL says - my evohome DHW matches his temps and timings almost exactly.

[Rameses]

That's the theory, but the problem with setting your cylinder to 60 degrees is that is a scalding temperature, far too hot for safety, especially if there are children in the house and all your taps aren't mixer taps. Set it lower and the risk of legionnaire's goes up. As we have a thermosyphon hot water cylinder with no real control over the temperature our hot water depends on the boiler temperature - with a central heating temperature of 70 degrees the hot water in the cylinder is about 60 degrees, which is too hot at the tap.

I've been thinking about what to do for a while now and have come to the conclusion that the best and probably only (but certainly not cheapest) solution is automatic thermostatic mixing valves installed near the taps, rather than trying to add temperature control to the existing antiquated hot water cylinder:

http://www.rwc.co.uk/products/thermostatic-mixing-valves-tmv

This way you get the best of both worlds - you can run your hot water cylinder hot enough to kill legionella (60ish) but regulate the maximum hot water temperature at the taps to the recommended maximum of about 46-48 degrees. In fact you can set a different maximum temperature at each tap (for us kitchen sink, bathroom sink and bathtub) that is appropriate to its use - for example the bathroom sink that is being used to wash hands and faces and would get used a lot by children doesn't need to be nearly as hot as the kitchen sink that will be washing dishes.

Another advantage is that because it regulates the temperature on the fly by mixing in cold water, variations in temperature of your hot water cylinder from say 55-70 degrees won't be noticed at the tap, so you don't need super accurate control of the cylinder temperature. Also you increase the effective storage size of your cylinder a bit by running it at a higher than delivery temperature then automatically mixing in a proportion of the cold water supply to achieve the desired temperature.

All - you are all right - which is why evohome allows you to change / suit

- default is 55 with +-10 which in theory allows for 65-45 range.
- this allows for hot water to be 'around' and still give opportunity to hit the 60c required to reduce chance of legionella
- scalding is an issue (unless as DBM says you can get localized temp control. http://www.honeywelluk.com/products/...Mixing-Valves/ ) Therefore being above 60+ can have its disadvantages
- the thermal curve of your hot water tank is important here (including placement of sensors and which sensors)
- the smaller the differential the more your boiler will be on to maintain that 'band' - and less savings to be made


I find personally in my house 57 with +-7 works for me. Allowing for balance of achieving hot enough water to purge the system, whilst the insulation I have on my tank is good enough I never go below 50c (even with quick shower top up). And this is plenty hot - without kicking the boiler off too often.

Find what suits you - but the default is the balance of all the above.

[killa47]

Also - I should add that our boiler and DHW is in the house garage extension (uninsulated) and built circa 1925 with high ceiling. Our DHW temperature losses between on demand will be higher than users with boiler or cylinder sited inside their house. This has a bearing on temp settings and timings.

[G4RHL]

...and remember Some Like It Hot!

[paulockenden]

The +/- aspect of the differential is interesting. I find my system uses the minus part, but never goes over the set point. So +/- 5 degrees doesn't actually give a 10 degree band, as you'd expect. The water temp is maintained within 5 degrees.

[roydonaldson]

My understanding was that if it was set to 60C with a 10C differential, then it would maintain 50-60C. ie. it is a lower threshold setting and not a +/- setting. And in practice, this is what I see in my hot water based upon using these settings.

[rotor]

I'd also like to point out that the actual water temperature is probably a lot hotter than what the sensor is telling you. My Evohome says that my hot water is at 52, but if I measure the temperature of the hot water coming out of the tap, it is ~61 degrees. I am using the insertion probe, but there is no thermal paste in the cavity, so I'm guessing the tip of the probe isn't fully in contact with the metal, hence not getting an accurate reading.

The reason I want to point this out, is that based on my readings of this forum, I was planning to increase the Evohome setting to 60, but fortunately I didn't, as I would've been heating the water to 70+ degrees!

As an aside, I am also using a 7 degree differential. 10 was definitely too much, and 5 seems too little. Heating the hot water competes with heating the house, which I'm guessing is why Honeywell chose 10 as the default, to stop having to continually top up the hot water.

[killa47]

Agree with paulockenden and roydonaldosn, that's how mine appears to be working - turns off at 60C and triggers back on temp lower than 55C.

[Rameses]

Interesting fact (I just learned)

@ 20c - legionella survive but dormant
@ 35-46c - legionella Perfect growth conditions
@ 50-54c - legionella lives but cannot multiply
@ 55c legionella dies within 5-6 hours
@ 60c legionella dies within 32 mins
@ 65c Legionella dies within 2 mins