Some very basic questions on how a DWH tank works…

10 Dec 2010
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United Kingdom
I think I must be missing something, but what

This is my (mis)understanding…
  • The HSE says that the ALL the water in the tank must be heated to 60c for at least 1hr each day to protect against legionnaires disease.
  • I have never seen a DHW where the coil goes all the way to the bottom, or the immersion heater touches the bottom of the tank.
  • Hot water rises, so only the water above the heat source will be heated.
  • The thermostat is part way up the tank, so turns of the boiler or the immersion heater before the bottom of the tank is heated up.
So how do the tanks in 99% of buildings keep to the HSE rules? (I ask because landlord are now being made to prove that they keep to the HSE rules for legionnaires disease.)
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Just because hot water rises does not mean only water above the coil heats.

Convection will mean the "hottest" water rises to the top, but the hot water already at the top must move down the cylinder while the coil continues to put heat into the cylinder. Thus a cylinder could quite easily heat to the bottom.

The issue comes that chances are it wont be 60* at the bottom unless the cyl stat is up very high (beyond usable most likley) as you are correct the hottest water will be higher in the cylinder. Ie could be 70*c at the top 60* at the coil and 50*c at the bottom when the stat shuts the cylinder down.

There are probably millions of houses where the cylinder stat is set below 60* anyway and legonairs isnt a "massive" problem in this county in this circumstance.

Dont know how landlords etc as you say could prove compliance mind you
Set tank stat to 55 degrees. Water at top of tank will be 65 degrees, enough to kill any legionella. It doesn't grow overnight so stop worrying.
Also make sure your boiler thermostat is turned up quite high.
He beat me to it.
I am not worrying legionella, it is the HSE that I am worrying about! Setting the tank stat to 55 degrees is not an option unless the HSE says it is OK.

Having water at 65 degree at the top does not help, as at some point the hot water will run out, and the water from the bottom of the tank that may never have been heated to anywhere close to even 55c will come out of the shower without passing var a “zone” that is at above 60c for a reasonable length of time.

One landlord had told me he has a 2nd time switch that bypassed the tank thermostat that heats the tank for 2hrs once a weeks in the early hours of the morning with a boiler flow of say 75c that will get very close to the complete tank, including below the coils being at least 55c for long enough to kill legionella.
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It's the inbetween temperature of 25-55 degrees that legionnaires is at its most active.
That is why it is important to lag cold pipes and CWSC's to stop the cold water temperature rising aswell as to prevent freezing.
You need to worry a bit less unless you are the owner of a hotel or tower block.
Nursing homes are very exposed to this risk as the people have low resistance to bacterium
So come on, why don't people get it from electric showers?
It's called convection. The water convection currents mean as water is heated and cools it will circulate around the cylinder. The she want water to be 65 at some point to kill bacteria off as the water heats it rises and as it cools only a few degrees it will fall and more will rise thus heating all of the tank to 65 at some point. If it had to be constantly at 65 why have a Hotwater timer as it would have to be on all the time. Regards running out a properly sized and used Hotwater cylinder should never run out as it would of been sized to the house and user. (If you use a lot f Hotwater make sure it's turned on for
Longer) the same happens what ever you use to heat the water.

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