Hot Water Cylinders and Legionella

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#1673437 18-Nov-2016 10:54

I don't think that all these "facts" have been mentioned, but I apologise if my cursory reading means I'm repeating what others have said.

Legionella can survive anywhere in the water system so even if the hot water reservoir is heated to 60C there can be pockets of infection further on in the pipes and, most commonly, showerheads. What this means is that we have our cylinder water temperature higher than 60C and we normally run hot water at temperatures above 60C to start our showers.

Finally, tankless flash hot water heaters have also been employed to reduce Legionella. These devices eliminate the problem of thermal stratification in conventional hot water heaters, but they do not eliminate Legionella in biofilms on the pipe walls, in shower heads, etc. The closer they are to the outlet, the more effective they are.

http://www.waterandhealth.org/legionella-control-in-institutional-water-systems/

It is common that the

... level of chlorine helps suppress the proliferation of Legionella, it is insufficient to inactivate the bacteria.

Chlorination is not as effective when it gets to a hot water cylinder. Higher temperatures reduce the effectiveness of chlorination so you should not rely on chlorination of potable water to kill the legionella.

The addition of lime to soften potable water provides food for legionella.

Rust and other scale can also be food for legionella. So old or degraded systems are a higher risk.

Low temperatures are also protective against legionella. Normally, below 20C is the upper limit for protection and much of New Zealand has ground temperatures that are generally well below this.

Most Lower Hutt water users might need to be careful. Although we get very pure water that is not chlorinated it has had legionella food (lime) added. At my place the water usually enters my hot water cylinder at ground temperature which is usually below 20C so that is probably the most protective environmental factor.

For most of us, the risk of legionella from potting mix is probably a higher risk.

(Edited to correct grammar)

timmmay

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#1673447 18-Nov-2016 10:54

Something that support the "one hour" idea from above, from the 2014 building code water supplies section.

My conclusion here is if it's ok for solar hot water, it's likely ok for standard hot water cylinders. So heat the hot water to at least 60 degrees at the bottom of the tank at least once a day. If I had an easy control I'd actually set the temperature a bit higher, because the additional cost of that is relatively trivial at $25/year, but my savings through using a timer are around $400 and a higher temperature mitigates risk a little more.

However this conflicts with the Ministry of Health recommendations linked to earlier.

Building Code Text

6.14.3 Legionella bacteria Irrespective of whether a mixing device is installed, the storage water heater control thermostat shall be set at a temperature of not less than 60°C to prevent the growth of Legionella bacteria.

3.5 Protection from Legionella bacteria

3.5.1 To prevent the growth of Legionella bacteria, solar water heaters must either: a) have a continuously energised heating element fitted within 55% of the bottom of the water tank (by volume) and a thermostat set to 60°C or higher, or b) be controlled so that the water above the element is heated to 60°C once a day, and the element is in the bottom 20% of the water tank (by volume) and no more than 150 mm from the bottom of the tank, or c) be controlled so that all of the stored water is heated to 60°C or higher, once a week for not less than 1 hour. The temperature must be measured by a probe in the bottom 20% of the water tank (by volume) and no more than 150 mm from the bottom of the water tank. For open loop systems the stored water includes the water in the solar collector and water must be circulated through the collector during the heating period.

3.5.2 Where the solar water heater stores potable water and is used as a pre-heater for an instantaneous water heater, either: a) the hot water storage tank connected to the solar collector must be fitted with supplementary heating and a controller operating to meet the conditions outlined in Paragraph 3.5.1, or b) the instantaneous water heater must heat all water passing through it to not less than 70°C.

KrazyKid

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#1673535 18-Nov-2016 12:16

I was reading up on ripple control on the theory that those who had implemented this had considered this issue.

I only reviewed Vector and their policy states that the may turn off your hot water heating for no more than 5 hours a day if required.
I assume this would mostly likely be split into two periods to cover the morning and evening peaks.

Perhaps this can be taken as a conservative maximum safe period for power savings.
Ripple control has been around for years and their has no large reports of legionella from hot water that I am aware of (just a few isolated cases)

It is a good point about the bacteria surviving in pipes.

Shoes2468

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#1673879 18-Nov-2016 21:30

KrazyKid:
I was reading up on ripple control on the theory that those who had implemented this had considered this issue.

I only reviewed Vector and their policy states that the may turn off your hot water heating for no more than 5 hours a day if required.
I assume this would mostly likely be split into two periods to cover the morning and evening peaks.

Perhaps this can be taken as a conservative maximum safe period for power savings.
Ripple control has been around for years and their has no large reports of legionella from hot water that I am aware of (just a few isolated cases)

It is a good point about the bacteria surviving in pipes.

ripple control is used to switch off hot water cylinders for a lot more than that here in Canterbury, you can have the hot water cylinder on at nights only via ripple control goes from 11pm to 7am or there about a with a minimum of 10 hours a night or something like that. So you don't always get the full 11pm to 7am.

Goosey

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#1673958 19-Nov-2016 08:30

+1 for night store i.e. ripple control switches on the cylinder at say 11pmish and switches off say 6amish (times vary depending on loads).

You can ask a sparky to install an overide switch incase your half way through the day and need a boost of hot.

Aredwood

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#1674225 19-Nov-2016 16:16

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.

You will actually get thermal stratification in a hot water cylinder. So if you heat the whole cylinder to 60deg, then use 1/2 the cylinder. You will have 1/2 a cylinder of 60deg water, and 1/2 a cylinder of cold water. Not a full cylinder of warm water. This is why cycling a heat source on and off is not much of an legonella risk. As long as you do manage to heat the whole cylinder to 60 deg during a heating cycle.

Also tempering valves are a legonella risk. As they are most often installed close to a hot water cylinder. Quite often with the cold pipe feeding the tempering valve joined quite close to the cold inlet to the cylinder. Meaning this cold pipe actually remains constantly warm, And it only ever has a small flow rate through it as you don't need much cold water to cut the temp from 60 to 55. Perfect breeding ground for legonella. This risk hasn't been addressed by the plumbing codes yet.

I still come across recirculating hot water mains that run at low temperatures as well. They were installed after the rules requiring tempering valves came out. But it was only later that they were identified as a risk. I have removed a few tempering valves from them. As the risk of scalding is preferable to the risk of legionella. The average person can easily tell if the water coming out of their taps is too hot. They can't tell if there is legionella in it.

[forgot to add]

The 55deg outlet temp from the tempering valve won't be hot enough to reliably kill any legionella that might be infecting the tempering valve cold supply. And some brands of tempering valve are factory set to 50 deg.

timmmay

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#1674227 19-Nov-2016 16:23

So @Aredwood what do you think, is it safe to heat water just between 1am and 7am each day? Right now I'm heating most of the time, as Ministry of Health says one thing (heat all the time) but the building code seems more relaxed.

We have a tempering valve and definitely need it, before we had that dialed down it felt too hot - modern cylinder on default settings.

Aredwood

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#1674233 19-Nov-2016 16:40

@timmmay Yes no problem with what you are doing. (heating between 1am and 7am) Assuming that time is enough for your cylinder to fully heat. (Best to calculate on the assumption that your cylinder is completely cold at the start of the heating cycle)

Check the cold inlet pipe to your tempering valve. Does it stay fully cold? Or does heat leakage from the cylinder keep it warm? Another risk if your cylinder is in a roof space. As the heat there can warm up cold water pipes enough that they can become a legionella risk as well.

timmmay

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#1674275 19-Nov-2016 18:40

@Aredwood it gets up to 40 degrees up in the roof space with the hot water cylinder in summer, perfect breeding temperature, but it's still chlorinated so probably keeps it down a lot. Cylinder heats in 3 hours. No matter what we do that feeder pipe will be low flow, middling temperature. I'd have to check it on a regular day.

bleater

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#1856049 31-Aug-2017 17:04

Aredwood:

You will actually get thermal stratification in a hot water cylinder. So if you heat the whole cylinder to 60deg, then use 1/2 the cylinder. You will have 1/2 a cylinder of 60deg water, and 1/2 a cylinder of cold water. Not a full cylinder of warm water. This is why cycling a heat source on and off is not much of an legonella risk. As long as you do manage to heat the whole cylinder to 60 deg during a heating cycle.

Also tempering valves are a legonella risk. As they are most often installed close to a hot water cylinder. Quite often with the cold pipe feeding the tempering valve joined quite close to the cold inlet to the cylinder. Meaning this cold pipe actually remains constantly warm, And it only ever has a small flow rate through it as you don't need much cold water to cut the temp from 60 to 55. Perfect breeding ground for legonella. This risk hasn't been addressed by the plumbing codes yet.

I still come across recirculating hot water mains that run at low temperatures as well. They were installed after the rules requiring tempering valves came out. But it was only later that they were identified as a risk. I have removed a few tempering valves from them. As the risk of scalding is preferable to the risk of legionella. The average person can easily tell if the water coming out of their taps is too hot. They can't tell if there is legionella in it.

[forgot to add]

The 55deg outlet temp from the tempering valve won't be hot enough to reliably kill any legionella that might be infecting the tempering valve cold supply. And some brands of tempering valve are factory set to 50 deg.

I'm having an old low-pressure cylinder replaced with a mains-pressure system, and found out that the plumber is going to install a tempering valve. I asked his specifically about whether the introduction of a tempering valve would increase the risk of legionella growth in the system, and he was dismissive, and insisted that he was obliged by the building code to install the tempering valve.

This Stuff story seems to indicate that some people are coming to harm from legionella originating in tempering valves: http://www.stuff.co.nz/dominion-post/news/72781724/tests-show-legionnaires-bug-could-be-present-in-many-hot-water-systems

It seems the risk is high enough that this specific risk from tempering valves ought to be addressed in the building code. Any thoughts?

tdgeek

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#1856091 31-Aug-2017 18:33

Im glad this thread appeared as Ive finally got my solar HW fully sorted. Ill footnote that at the bottom as @aredwood and others may be interested.

I have my bottom element off 24/7, relying on solar to top it up, which it does over a day at this time of year, unless its a good sun day. The top element come son at 7am to 8am and 9pm to 10pm (My EK free hour of power). The last few days in ChCh haven't been great, the Collector temp is 11, the bottom cylinder temp is 22 right now, the top is 66 (I just boosted for half an hour)

This time of the year solar wont heat the while cylinder to 60+ unless its a good sun day, so the bottom is often low. Right now its 22, so essentially close to stopping legionella. The one week rule to 60 and not less than an hour, does that mean that its safe to be at breeding temps for a week? Given that my bottom half this time of year will always be low or 40's, but the water we use will be over 60 at the top of the cylinder, is that ok?

Footnote for @aredwood

So, after getting a lot of great info from you when we moved here with solar HW, and Marcus at Apricus, I assumed all was ok. I cycled the bottom element as required, after solar had helped. The other week I concluded that the top element never works. Got a plumber in, element ok, relay ok, but thermostat was faulty, so that was fixed. The timers, that I wasn't that aware of until then were reset by me and all works. Then it doesnt again. The issue was that these controllers are Repsol a German company., and they locate the thermostats different to what we do, hence that "extra" setting. Its a confusing setting, because how we interpret the thermostat setting differs to Germany who have a need for it. Mine was not set to default (If temp under 40 use timers, if over 45 dont use timers) Even default isn't ideal. The setting the previous owner had was way different, stopping the relays at times. So Marcus said to set that setting to 60 and 66, so now if the temp is under 60, timers work, if over 66, they dont. Its just a manual way to allow this setting to operate but also to be ignored so that it doesnt turn the relay off when I want it to come on. The good thing is I have learnt exactly how this all works, and why, so happy with that.

Rikkitic

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#1856097 31-Aug-2017 18:47

I don't know what the building code regulations are, but when we had our hot water cylinder replaced (no solar but wetback) the plumber rang someone to make sure it was okay not to install a tempering valve. The reason was our water pressure (gravity feed from a hillside holding tank) is too low and with a tempering valve the water could not have made it to the second-floor bathroom. So we do not have a tempering valve, but do have council approval, and we have to use common sense when running the shower because sometimes when a fire has been burning in warmer weather steam comes out of our hot water system. There is a lot to be said for common sense.

Plesse igmore amd axxept applogies in adbance fir anu typos

Aredwood

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#1856107 31-Aug-2017 19:10

bleater:

I'm having an old low-pressure cylinder replaced with a mains-pressure system, and found out that the plumber is going to install a tempering valve. I asked his specifically about whether the introduction of a tempering valve would increase the risk of legionella growth in the system, and he was dismissive, and insisted that he was obliged by the building code to install the tempering valve.

This Stuff story seems to indicate that some people are coming to harm from legionella originating in tempering valves: http://www.stuff.co.nz/dominion-post/news/72781724/tests-show-legionnaires-bug-could-be-present-in-many-hot-water-systems

It seems the risk is high enough that this specific risk from tempering valves ought to be addressed in the building code. Any thoughts?

There is no mandatory requirement for tempering valves. But there is a requirement to limit water temp to "sanitary fixtures used for personal hygiene" (fixtures that are installed in bathrooms). Another way of complying if you have single lever mixers, is alot of them have adjustable stop rings that prevent you from turning the handle to full hot. Adjust these to limit the water temp. Also there is no max temp for kitchen or laundry taps. I have piped out some houses so the kitchen gets supplied with hotter water directly from the cylinder. This works really well if there are young kids in the house, as the tempering valve can be turned right down to supply only 45-50deg water to the bathroom.

I use this method alot in commercial kitchens. Auckland council have a rule requiring min 60deg to commercial sinks. So you have to set the hot water thermostat to 70deg to allow for pipe losses. And then use the stop ring method to limit the temp to the handwash basin in the kitchen. Often you end up with a sink and basin right next to eachover, with the sink being supplied with far hotter water.

Tempering valves are regarded as an "acceptable solution" So the council is happy if they are there. Even if they introduce secondary risks. Also tempering valves are often unreliable. RMC recommend that their tempering valves are replaced every 5 years. Unless you have a testing program implemented. How many households, let alone commercial operators do this? Probably 0. I have come across heaps of stuffed tempering valves (all brands). And there is also a certain generation of Robertshaw cylinder thermostats, which fail in a way that they only heat the cylinder to approx 45deg.

My own house I do alot of things that I would never do in a customers house. One of them is solar hot water on my cylinder with a tempering valve. 80deg out all of the taps during summer. And when I use my boiler system, it can also get the cylinder up to 80deg. And often my cylinder is only using manual boost control. So often only 45deg or so out of the taps as well. And Im on rainwater - so no chlorine added. But both me and the flatmates are in good health, and are neither elderly or young kids. So I can take more risks myself than what I would be comfortable for customers to take. Also if I was to ever sell my house, I would install a tempering valve before selling it.

timmmay

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#1856114 31-Aug-2017 19:17

tdgeek:

Im glad this thread appeared as Ive finally got my solar HW fully sorted. Ill footnote that at the bottom as @aredwood and others may be interested.

I have my bottom element off 24/7, relying on solar to top it up, which it does over a day at this time of year, unless its a good sun day. The top element come son at 7am to 8am and 9pm to 10pm (My EK free hour of power). The last few days in ChCh haven't been great, the Collector temp is 11, the bottom cylinder temp is 22 right now, the top is 66 (I just boosted for half an hour)

This time of the year solar wont heat the while cylinder to 60+ unless its a good sun day, so the bottom is often low. Right now its 22, so essentially close to stopping legionella. The one week rule to 60 and not less than an hour, does that mean that its safe to be at breeding temps for a week? Given that my bottom half this time of year will always be low or 40's, but the water we use will be over 60 at the top of the cylinder, is that ok?

I'd be careful with that. From what I can read the center of your cylinder probably sits around 40 degrees, which is perfect breeding temperature for the bacteria. If the water is chlorinated there that helps. There's always new water coming in, but you don't know if you ever do a full change.

Personally I want my cylinder fully heated for an hour every day. It's not that expensive, and you don't want to gamble with your life to save a few dollars. It's a nasty disease.

tdgeek

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#1856128 31-Aug-2017 20:12

timmmay:

tdgeek:

Im glad this thread appeared as Ive finally got my solar HW fully sorted. Ill footnote that at the bottom as @aredwood and others may be interested.

I have my bottom element off 24/7, relying on solar to top it up, which it does over a day at this time of year, unless its a good sun day. The top element come son at 7am to 8am and 9pm to 10pm (My EK free hour of power). The last few days in ChCh haven't been great, the Collector temp is 11, the bottom cylinder temp is 22 right now, the top is 66 (I just boosted for half an hour)

This time of the year solar wont heat the while cylinder to 60+ unless its a good sun day, so the bottom is often low. Right now its 22, so essentially close to stopping legionella. The one week rule to 60 and not less than an hour, does that mean that its safe to be at breeding temps for a week? Given that my bottom half this time of year will always be low or 40's, but the water we use will be over 60 at the top of the cylinder, is that ok?

I'd be careful with that. From what I can read the center of your cylinder probably sits around 40 degrees, which is perfect breeding temperature for the bacteria. If the water is chlorinated there that helps. There's always new water coming in, but you don't know if you ever do a full change.

Personally I want my cylinder fully heated for an hour every day. It's not that expensive, and you don't want to gamble with your life to save a few dollars. It's a nasty disease.

Thats where my thoughts are heading. This is the third day in a row thats been cloudy and cool. Yesterday solar did a reasonable job as it as cloudy but a little bright. What Im thinking is to turn on the bottom element when needed in the morning to partially heat the bottom 2/3, as the solar will take care of the rest. More so when its like this week, and less when its going to be a reasonably good sun day. Not at all if a good sun day, plus allowing for usage.

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