[Closed] Hot water tank temperature... how hot is hot enough?

I think ours is around 55 - it needs to he hot without scalding. It's been on that temp for years. It's not sat for long though, in a house of 4.

We have an unvented tank that's heated purely by spare PV power.
I have the on-tank Immersion thermostat turned up to the max and the most I ever get is about 61c before the thermostat switches off.

This is really bloody hot and you have to be careful with water from the tap.
For comparison a fairly hot shower is about 40c I believe.

Our hot tub is 40c, and that's 'hot'....

Cylinder stat should be set around 60-65°C. If your taps are too hot at that setting you can get a thermostatic mixer valve to reduce temperature at the outlets.

I read up a bit on this at work recently. Legionella can't survive in water tanks above 47C, so most things are significantly above this to give some safety factor.

I was making a spray corrosion rig and it needed moist air which you create by flowing compressed air through a vessel of warm water. I was initially planning to put it in the warm main chamber (35-37C). Glad I didn't as that would have made a perfect Legionella aerosol machine.....

It's a trade off reaĺly, above 40 degrees the risk of burns increases quickly but as said above around Hugh 30s is ideal temp for nasties to spawn. Really depends on how long water sits in the tank, are there any dead legs in the pipework, taps that rarely get used, as these can be a major issue as well with warm rather than hot water stagnating.

Ours is about 50, there's a knob on the front of the tank that seems to cover both immersion and the gas.

The hotter the tank is the more heat you'll lose as the temperature differential between the water and the surroundings is greater. So it's pointless to keep it that high when you're always going to add cold anyway. So hot enough to kill nasties seems about right.

However don't get this confused with the temperature knob on the boiler. That heats the water that gets pumped through the heater coil in the tank to heat the water you use. According to our instructions that should always be on max for best efficiency. It won't make the hot water taps any hotter.

Before I switched to a combi mine used to be at 75c but that was because I had a crap shower run from the bath taps and couldn't have more than a dribble of water pressure if the hot wasn't properly hot. Made for using the bathroom sink taps (separates at the time) interesting

Just to qualify this...

The hotter the tank is the more heat you’ll lose as the temperature differential between the water and the surroundings is greater. So it’s pointless to keep it that high when you’re always going to add cold anyway.

Yes, but the hotter the water in the tank = more usable water for showers etc from a single tank of hot (by the time it's mixed with cold to get the right temp at the shower).

So it depends on your circumstances really. I want to get the most out of my free PV power so it makes sense to heat the tank to as high as it will safely go so as to get the most showers, etc from a single tank.
(This is in a house with only electric heating, so no boiler option)

Obvs others will have a different set of circumstances as mine are quite niche!

This is whats been happening with the hot water temp over the last 7 days. If it's not sunny then the tank temperature will gradually fall with a typical heat loss over 24hrs being about 8c. Below the temperature chart is a depication of the PV power to you can see the relationship between the two.
The house is a holiday home so there's basically nothing else using power other than a fridge freezer and the wifi.
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so if i have a pretty large hot water cyclinder is there any reason not to set it at about 45 (or whatever is a reasonable tap temp) and once a month ish boost it to 60 to kill off any legionnaire's risk? - this seems more sensible than having it permanently at 55 ish which I do at the moment

Don't understand.... why would this be more sensible?

I think the recommendation is > 65 Deg C once a week for <some period of time I can't remember>

Probably about an hour.

What is legionella?
Legionella bacteria is commonly found in water. The bacteria multiply where temperatures are between 20-45°C and nutrients are available. The bacteria are dormant below 20°C and do not survive above 60°C

The primary method used to control the risk from Legionella is water temperature control.
Water services should be operated at temperatures that prevent Legionella growth:

Hot water storage cylinders (calorifiers) should store water at 60°C or higher

http://www.hse.gov.uk/healthservices/legionella.htm

Don’t understand…. why would this be more sensible?

saving money by not heating water to an unnecessarily high temp all the time

http://www.hse.gov.uk/healthservices/legionella.htm
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dammit it means I even have to turn the cylinder thermostat up! 😉

 

dammit it means I even have to turn the cylinder thermostat up! 😉

No you don't, just take ice cold showers every day.:-)

The two corrosion chamber manufacturers I looked at both run the air humidifier at 50C. I'd say that is a high risk application as it permanently produces a vapour you could inhale directly after the tank. I'd definitely not be setting your tank at 45C, and I'd have thought that wouldn't feel hot enough by the time it reaches a tap.