Hot Water Cylinders and Legionella

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#1673094 17-Nov-2016 19:53

See this post. The Building code and Act link has changed to https://www.building.govt.nz/managing-buildings/managing-your-bwof/legionellosis/

timmmay

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#1673133 17-Nov-2016 21:12

Some important / interesting parts of the document you linked to seems to be

"In order to comply with the New Zealand Building Code 1992 (currently under review), stored hot water in residential dwellings is required to be held at temperatures of 60ºC or higher". At 50 degrees it won't multiply but it won't be killed either. (p5)
"Hot water systems in households are a source of infection to which most people will be exposed in their everyday life. Lower hot water temperatures (< 48.8°C) have been shown to be significantly associated with the presence of L. pneumophila in residential water systems (Lee et al 1988)"

Based on this, heating water overnight and letting the temperature decline during the day doesn't seem to comply with best practice. Anyone using hot water could be exposed to the bacteria, depending on what the temperature falls to. So using a timer to heat hot water at night, while saving you money, could prove fatal. Using a timer to avoid periods of high load would seem to be acceptable, but letting hot water temperature decline below 55 degrees would seem to be a mistake, according to that document.

Has anyone else who's read the document come to any other conclusion?

Scott3

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#1673194 17-Nov-2016 21:55

Rikkitic:

Ideally everyone should have a tempering valve but not everyone can have one. We live in an old farmhouse fed with spring water from a gravity tank. When the new cylinder was installed, the plumber had to get dispensation from council authorities to leave out the tempering valve, because our water pressure was too low and otherwise the hot water wouldn't have been able to reach the second floor. This isn't a problem with the cylinder because we rarely have the power on and when we do the thermostat keeps it from getting too hot. The problem is the wetback. Sometimes we actually hear the water boiling and then we have to open the taps to let the steam escape. Fortunately we are aware of the danger and no children use our showers.

I know you are already aware, but your install is seriously dangerous. "Steam coming out of the taps..."

Even on electric the thermostat will heat water hotter than the recommended 55 Deg at the tap (to kill legionnaires)

Uncontrolled heating (i.e. Solar, wetback) can boil your cylinder (which in itself shouldn't be an issue as long as it is a low pressure atmosphere vented one), but that water unless cooled by a tempering valve can cause serious burns. It's off the scale of the below chart:

Water ºC Major Burn in

49 degrees 5 minutes
52 degrees 2 minutes
54 degrees 30 seconds
57 degrees 10 seconds
60 degrees less than 5 seconds
63 degrees less than 3 seconds
66 degrees 1.5 seconds
68 degrees 1 second

I understand that you know this, and are super careful (Always turning cold water on first, and off last in mixers etc.), but it only takes a guest or tradesperson to wash their hands in hot water and they will end up with significant burns.

Most rural NZ houses have a pressurising pump. I would recommend getting one, and a normal hot water setup with a tempering valve. As a positive side effect to better safety, you will have much better water pressure for showers etc.

gzt

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#1673217 17-Nov-2016 22:55

Downsizing the wetback is an option. I've noticed for many wetback sizing is bring a full cylinder to temperature in one hour or something like that. Unnecessary for many installations, and misdirecting heat that could be used for space heating. Maybe something low tech like a brick in front of the bladder will tone it down a bit.

Disclaimer: not a plumber.

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#1673232 17-Nov-2016 23:17

tchart:
timmmay:

You don't want to adjust the cylinder temperature, you want a mixing thingy (valve?) after the cylinder to mix in variable amounts of cold water. This ensures the bugs are killed but reduces the temperature at the tap. This was done by default for my new relocated cylinder a few years ago.

Hey @timmmay all our taps have mixer "thingys" (it a newish build ~6 months old). Problem is the water is so friggin hot its dangerous. We usually have to turn the mixer to 3/4 cold to get a decent temperature for the bath, dishes etc.

Get the plumber back to have a look. Might be a stuck or never adjusted tempering valve. Playing with it might unstick it, but a plumber may take it apart to check for debris from the install.

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#1673252 18-Nov-2016 00:09

timmmay:
Has anyone else who's read the document come to any other conclusion?

I read the part in the first section that said at 60 degrees legionella is killed in 32 mins and at 66 degrees is was killed in 2 min. It seems to me an hour a day at 60+ should be safe.
The question then becomes how long does it take for your cylinder to reach 60 degrees if this is the case

timmmay

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#1673275 18-Nov-2016 06:51

KrazyKid:
timmmay:

Has anyone else who's read the document come to any other conclusion?

I read the part in the first section that said at 60 degrees legionella is killed in 32 mins and at 66 degrees is was killed in 2 min. It seems to me an hour a day at 60+ should be safe.
The question then becomes how long does it take for your cylinder to reach 60 degrees if this is the case

The problem with "one hour a day" is if you heat until 7am and the cylinder is 65 degrees, have three showers, if you say use 1/2 of the hot water in the tank and the mains supply is 10 degrees then your cylinder is around 35 degrees for the rest of the day. That means if anyone uses hot water during the day there could be bacteria present.

I guess some experimentation could be done. Keep the cylinder on off-peak heating but measure the temperature a couple of days to see where it falls. If it's still 60 degrees at 5pm on a typical then yes it'll be safe enough on that typical day, but days of high demand increase risk.

tdgeek

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#1673278 18-Nov-2016 06:58

timmmay:

KrazyKid:
timmmay:

Has anyone else who's read the document come to any other conclusion?

I read the part in the first section that said at 60 degrees legionella is killed in 32 mins and at 66 degrees is was killed in 2 min. It seems to me an hour a day at 60+ should be safe.
The question then becomes how long does it take for your cylinder to reach 60 degrees if this is the case

The problem with "one hour a day" is if you heat until 7am and the cylinder is 65 degrees, have three showers, if you say use 1/2 of the hot water in the tank and the mains supply is 10 degrees then your cylinder is around 35 degrees for the rest of the day. That means if anyone uses hot water during the day there could be bacteria present.

I guess some experimentation could be done. Keep the cylinder on off-peak heating but measure the temperature a couple of days to see where it falls. If it's still 60 degrees at 5pm on a typical then yes it'll be safe enough on that typical day, but days of high demand increase risk.

Or, if the heat is enough to kill the bacteria, what happens when cold water comes in? That must have dormant bacteria? How long does it take for them to become active and be a risk? Given that if any one of us turns heat off to save power or wait for the solar to top it up. The top of the cylinder might be safe as thats been 60, the bottom might be 30. Couple of hours to activate the bacteria? A day? A couple of days?

richms

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#1673290 18-Nov-2016 07:11

I was told that it takes multiple days to allow it to grow, and it is only really an issue if you have the stuff in the incoming water like from an untreated supply, and that the chances of anything growing in a sealed system supplied by a chlorinated city supply are extremely remote.

Richard rich.ms

timmmay

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#1673291 18-Nov-2016 07:15

tdgeek:

Or, if the heat is enough to kill the bacteria, what happens when cold water comes in? That must have dormant bacteria? How long does it take for them to become active and be a risk? Given that if any one of us turns heat off to save power or wait for the solar to top it up. The top of the cylinder might be safe as thats been 60, the bottom might be 30. Couple of hours to activate the bacteria? A day? A couple of days?

It'd be safest to assume it is active immediately that the temperature rises.

Another mitigating factor is chlorine kills the bacteria, and water to Wellington (but not lower hutt) is chlorinated. This reduces the risk significantly, and heating it makes it 100% safe. That makes me a bit more comfortable not heating 100% of the time.

I changed my timer to heat a lot more last night : 1am to 7am, 10am to 4pm. We don't use much water in the evening so that can wait until 1am. Though we've had the water heating mostly overnight for a few months at least with no issues so far.

timmmay

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#1673292 18-Nov-2016 07:16

richms:

I was told that it takes multiple days to allow it to grow, and it is only really an issue if you have the stuff in the incoming water like from an untreated supply, and that the chances of anything growing in a sealed system supplied by a chlorinated city supply are extremely remote.

Told by who? Do you have a reference?

tdgeek

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#1673293 18-Nov-2016 07:17

richms:

I was told that it takes multiple days to allow it to grow, and it is only really an issue if you have the stuff in the incoming water like from an untreated supply, and that the chances of anything growing in a sealed system supplied by a chlorinated city supply are extremely remote.

Phew, thanks, this thread was worrying me. My bottom cylinder is often low. The top has been at 60, may still be, may be 50's, which is fine. Solar whips it up in the morning. But any below 60's for me is temporary, a day max.

richms

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#1673311 18-Nov-2016 07:45

timmmay:

richms:

I was told that it takes multiple days to allow it to grow, and it is only really an issue if you have the stuff in the incoming water like from an untreated supply, and that the chances of anything growing in a sealed system supplied by a chlorinated city supply are extremely remote.

Told by who? Do you have a reference?

Was discussing it with someone who was selling solar optimizers for hot water heaters last year. Cant recall too much more about it.

The outcome was that a weekly 60 degree cycle was all that was needed to prevent issues.

Richard rich.ms

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#1673328 18-Nov-2016 08:06

Not sure I trust salesmen with my families health.

tchart

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#1673418 18-Nov-2016 10:30

tchart:

timmmay:

You don't want to adjust the cylinder temperature, you want a mixing thingy (valve?) after the cylinder to mix in variable amounts of cold water. This ensures the bugs are killed but reduces the temperature at the tap. This was done by default for my new relocated cylinder a few years ago.

Hey @timmmay all our taps have mixer "thingys" (it a newish build ~6 months old). Problem is the water is so friggin hot its dangerous. We usually have to turn the mixer to 3/4 cold to get a decent temperature for the bath, dishes etc.

Cylinder was set to 70 degrees! Have dialed it back to 60.

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