Hot Water Cylinder – Temperature?

paperclap

Hi all,

The thermostat on our hot water cylinder is set to 65°C.

Was wondering if that could be reduced to save a little money?

Is legionnaires really an issue below 60°C?

Thanks!

Ectophile

Temperatures as low as 55C will kill legionnaire's.  It just takes a bit longer.

It doesn't really grow at anything above 50C if you want to reduce the temperature even more.

Section62

paperclap said:

Hi all,

The thermostat on our hot water cylinder is set to 65°C.

Was wondering if that could be reduced to save a little money?

Is legionnaires really an issue below 60°C?

Thanks!

The thermostat sets the temperature at which the heat source will stop adding heat.  Some of the water in the bottom of the cylinder will be colder than that, and the amount of colder water will increase with time, as some of the heat is lost, and as you draw off hot water which is replaced by cold.

As well as considering the potential for Legionnaires, you also need to take into account the effect of heating the cylinder to a lower temperature in terms of getting an adequate amount of hot water out - you won't be getting a constant supply of water at (say) 60C if that is what you set the thermostat at (unless you leave the heat source switched on all day to reheat the water).

Needless to say, as the cylinder cools through the day/night, the volume of water which is in the temperature range where Legionella thrive will be increasing. Which makes it more important that the cylinder contents subsequently get heated to a sufficient temperature to counteract the period where conditions were suitable for Legionnaires to thrive.

Also bear in mind the markings on most cylinder thermostats are a guide, they aren't an accurate indication of the average temperature in the cylinder.

FreeBear

paperclap said: Is legionnaires really an issue below 60°C?

If you have an open header tank exposed to the elements, Legionella may be a risk. With most header tanks being covered and inside a loft, the risk is almost zero. I heat my HW tank to 50-55°C in the winter and have been going down to 45°C this summer. The lower temperature has saved around 20% on gas.

With a combi boiler, your risk of Legionella is even lower, so it is quite safe to run the hot water at a lower temperature - The incoming water will have been treated by your supplier at source, and there is no real risk of contamination before it gets to your house.

Section62

FreeBear said:

paperclap said: Is legionnaires really an issue below 60°C?

If you have an open header tank exposed to the elements, Legionella may be a risk. With most header tanks being covered and inside a loft, the risk is almost zero.

Whilst covering tanks is one way to reduce the risk of water contamination, it doesn't eliminate the risk.  Systems with no header tanks are also potentially vulnerable to legionella.

The risk is small, but shouldn't be forgotten about.

paperclap

So safe to say I can reduce to 50-55°C then?

A 20% saving?! That’s actually a fair amount!

Section62

paperclap said:

So safe to say I can reduce to 50-55°C then?

A 20% saving?! That’s actually a fair amount!

People in practice may find a 20% saving is unlikely because -

1) They'd use more 'hot' water to achieve the temperature they want.

2) They'd run out of 'hot' water sooner each day and need to put the heating back on.

- but it does depend on individual use of hot water, and people's perception of what 'hot' water is.

To illustrate the point, if in 'x' minutes the water in the pipe to (say) the kitchen sink cools by 10 degrees then if the starting temperature was 65 degrees the water coming out of the tap when first turned on after 'x' minutes would be around 55 degrees and feel hot enough to wash something.  If the starting temperature was 55 degrees then the first water coming out of the tap 'x' minutes later would be 45 degrees, which some people would feel was 'lukewarm' and therefore run the tap till it comes through 'hot'.  It isn't an exact science because we are all different.

If you can programme yourself and other household members to believe 45 degree water is hot enough to do the washing up - and also not turn on the cold tap to speed up the time it takes to fill the bowl - then it could work out as the kind of saving you suggest.  But people are not always easy to programme.