DHW and avoiding Legionella

6 views
Skip to first unread message

Mike Holmes

unread,
Jul 26, 2007, 7:21:21 PM7/26/07
to
I am about to convert my standard open vented DHW system with indirect
tank to a HeatBank system following the wiki @

http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank - many thanks
to John Stumbles for that.

While trying to figure out what specification I require for the plate
heat exchanger, I happened upon this article, which suggests hot water
should be *distributed* as well as stored at 60degC.

http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_water_is_vital.html

DHW storage is not an issue with the Heat Bank idea. However, is it
really neccessary to distribute water at this temperature? The same
article suggests that water >45degC can cause serious scalding; I have
been figuring on using a TMV just after the output from the PHE
secondary to mix the hot down to a max of 50degC lets say, rather than
60degC.

As I get about 30l/min flow from th rising main, to be able to
guarantee my DHW is at 60degC, would mean either a very high Heat Bank/
PHE primary temperature ie probably >80degC, or a really high ouput
PHE ie >100kW.

At the end of the day the environment and costs are an issue; the
environment because I don't want to heat the store any hotter than
necc (have read in another thread that heatbanks work well with the
store as low as 70degC); costs because a the higher output PHEs
obviously cost more

Thanks


Mike

John Stumbles

unread,
Jul 27, 2007, 6:05:01 AM7/27/07
to
On Thu, 26 Jul 2007 23:21:21 +0000, Mike Holmes wrote:

> I am about to convert my standard open vented DHW system with indirect
> tank to a HeatBank system following the wiki @
>
> http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank - many thanks
> to John Stumbles for that.
>
> While trying to figure out what specification I require for the plate
> heat exchanger, I happened upon this article, which suggests hot water
> should be *distributed* as well as stored at 60degC.
>
> http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_water_is_vital.html

Seems to be pitched at commercial-type installations. Obviously legionella
is possible in domestic ones too but AFAIK the outbreaks that have occurred
to date have been in larger ones. I don't know what the incidence of
legionella in domestic installations is but if you're concerned about it
(perhaps because you have a long dead leg of pipework feeding a shower)
there's a fairly elegant engineering solution with a heat bank that is to
distribute the primary (from the cylinder) and put a pump + (smaller) PHE +
flow switch etc close to the point of use.

Can't do that with unvented :-)

--
John Stumbles

Procrastinate now!

Jim Alexander

unread,
Jul 27, 2007, 6:12:25 AM7/27/07
to

"Mike Holmes" <mjh-n...@tiscali.co.uk> wrote in message
news:1185492081....@d55g2000hsg.googlegroups.com...

>I am about to convert my standard open vented DHW system with indirect
> tank to a HeatBank system following the wiki @
>
> http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank - many thanks
> to John Stumbles for that.
>
> While trying to figure out what specification I require for the plate
> heat exchanger, I happened upon this article, which suggests hot water
> should be *distributed* as well as stored at 60degC.
>
> http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_water_is_vital.html
>
No, with a heat bank all the hot is heated from a fresh supply of potable
water (unless you live in Gloucester). What is left in the hot pipe
afterwards is no different from any domestic hot water system.

Jim A


David Hansen

unread,
Jul 27, 2007, 8:05:58 AM7/27/07
to
On Thu, 26 Jul 2007 23:21:21 -0000 someone who may be Mike Holmes
<mjh-n...@tiscali.co.uk> wrote this:-

>DHW storage is not an issue with the Heat Bank idea. However, is it
>really neccessary to distribute water at this temperature?

1) it is impossible to avoid legionella. The little nasties are in
the water as it falls from the sky.

2) what should be avoided is a high concentration of the little
nasties mixing with people particularly susceptible to them.

3) at low temperatures they are largely dormant. At high
temperatures they are killed off. Between those they breed nicely
and thus attain a high concentration.

4) people who are not particularly strong are the most susceptible.
That is why the well known outbreaks tend to involve hospitals, care
homes and the like.

5) in a domestic situation the precautions which are taken in larger
buildings are unlikely to be worthwhile.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

Aidan

unread,
Jul 27, 2007, 2:56:02 PM7/27/07
to
On 27 Jul, 00:21, Mike Holmes <mjh-nos...@tiscali.co.uk> wrote:
> I am about to convert my standard open vented DHW system with indirect
> tank to a HeatBank system following the wiki @
>
> http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank- many thanks

> to John Stumbles for that.
>
> While trying to figure out what specification I require for the plate
> heat exchanger, I happened upon this article, which suggests hot water
> should be *distributed* as well as stored at 60degC.
>
> http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_...

BSEE; A free building services trade magazine that's full of adverts
and articles by chubby, middle-aged, managers in ties, who are
usually eager to bring to their readers' attention a major problem and
a SAFETY HAZARD which can only be prevented by the ACME Problem
Preventer; they usually work for, or own, ACME Inc.. It's not helped
by the editorial staff who don't work in the industry and the most
relevant text and illustrations can wind up in the editors' bin. The
articles are just filling around the paid adverts and are trimmed to
suit the available space.

American publishers can usually afford to pay an expert for an
informative article.

It's really just presenting the information in the HSE publication L8,
about preventing legionella in water systems, which is the definitive
Code of Practice on preventing legionella in the UK. L8 is legally
binding on building owners and managers who have a "duty of care"; if
there's an outbreak traced to a commercial building's water systems
and the maintenance regimes have not followed the L8 guidelines, the
managers can get charged with criminal offences.

It doesn't apply to houses, because the houseowners don't have a legal
"duty of care", BUT it is sensible to follow the same guidelines
because legionella will kill you in your own house.

Legionella is in the water supply. Given favourable conditions (nice
temperature and nutrients) it will proliferate. An ideal temperature
is about blood heat, 37 degC.
If you inhale an atomised spray of legionella infested water, you
will get infected . If you're elderly or ill, you'll probably die.

You have to keep the cold water COLD (below 20 degC). If you have a
tank you have to ensure there is at least one water turn-over per day.
Insulate tanks and pipes.

You have to keep the hot water HOT (50 degC or above). Store at 60
degC, 50degC at any hot tap within 60 seconds. If there's a risk of
scalding you need a TMV at the point(s) of use. A TMV on the store
outlet is no good.

Your system will be a combi in effect. There's little hazard if the
water is not allowed to stagnate. I'd distribute at 60 degC since this
will sterilize any bugs in the distribution pipes. Then again I
wouldn't fit a combi.

I know of someone, a very expert plumber, who managed to get an
infection from his own domestic system. He had a second house and used
the shower after the house had been vacant for some weeks. The frost
protection system had been keeping the house and the legionella warm.

Mike Holmes

unread,
Jul 27, 2007, 7:50:16 PM7/27/07
to
Many thanks for the replies

>
> You have to keep the cold water COLD (below 20 degC). If you have a
> tank you have to ensure there is at least one water turn-over per day.
> Insulate tanks and pipes.
>
> You have to keep the hot water HOT (50 degC or above). Store at 60
> degC, 50degC at any hot tap within 60 seconds. If there's a risk of
> scalding you need a TMV at the point(s) of use. A TMV on the store
> outlet is no good.

The idea for the TMV just after the PHE is because if your store is at
75degC say, and you open a tap to give a moderate-low flow rate, then
you're probably going to get water output from the secondary
approaching 70-75degC and - dangerously hot - hence the TMV so you
have a steady maximum temperature in your DHW system - unaffected by
the store if it is at a higher temperature or flow rate through the
PHE. You don't store your DHW, with a heat bank, you store the system
water - check the many threads on heat banks or the wiki link above.

I imagine the best option is to set the TMV to distribute the DHW at
55-60degC. My Hans Grohe shower mixer has a TMV built-in under the
hood, which you can set with a screwdriver, so will probably tune this
down a bit lower.

>
> Your system will be a combi in effect. There's little hazard if the
> water is not allowed to stagnate. I'd distribute at 60 degC since this
> will sterilize any bugs in the distribution pipes. Then again I
> wouldn't fit a combi.
>
> I know of someone, a very expert plumber, who managed to get an
> infection from his own domestic system. He had a second house and used
> the shower after the house had been vacant for some weeks. The frost
> protection system had been keeping the house and the legionella warm.
>
>
>
>
>
> > DHW storage is not an issue with the Heat Bank idea. However, is it
> > really neccessary to distribute water at this temperature? The same
> > article suggests that water >45degC can cause serious scalding; I have
> > been figuring on using a TMV just after the output from the PHE
> > secondary to mix the hot down to a max of 50degC lets say, rather than
> > 60degC.
>
> > As I get about 30l/min flow from th rising main, to be able to
> > guarantee my DHW is at 60degC, would mean either a very high Heat Bank/
> > PHE primary temperature ie probably >80degC, or a really high ouput
> > PHE ie >100kW.
>
> > At the end of the day the environment and costs are an issue; the
> > environment because I don't want to heat the store any hotter than
> > necc (have read in another thread that heatbanks work well with the
> > store as low as 70degC); costs because a the higher output PHEs
> > obviously cost more
>
> > Thanks
>

> > Mike- Hide quoted text -
>
> - Show quoted text -


Aidan

unread,
Jul 28, 2007, 9:32:00 AM7/28/07
to
On 28 Jul, 00:50, Mike Holmes <mjh-nos...@tiscali.co.uk> wrote:
> Many thanks for the replies
>
>
>
> If there's a risk of
> > scalding you need a TMV at the point(s) of use. A TMV on the store
> > outlet is no good.

TMVs at the point if there is a risk of scalding.
TMVs at the store outlet are no good at preventing the isk of
scalding.
You still need to distribute at 60degC ish to avoid legionella in the
pipes.
The TMVs at the outlets are usually set at a maximum of 43degC (I
think) which won't cause scalding to any 'at risk' users.

> The idea for the TMV just after the PHE is because if your store is at
> 75degC say, and you open a tap to give a moderate-low flow rate, then
> you're probably going to get water output from the secondary
> approaching 70-75degC and - dangerously hot - hence the TMV so you
> have a steady maximum temperature in your DHW system - unaffected by
> the store if it is at a higher temperature or flow rate through the
> PHE. You don't store your DHW, with a heat bank, you store the system
> water - check the many threads on heat banks or the wiki link above.

I know. The TMVs are being used for 2 different purposes.

1)Mix to a safe temperature at the store outlet, whilst keeping above
the legionella-friendly temperature.

2) Mix to a non-scalding temperature at the point of use which is
below the legionella temperature.

Andrew Gabriel

unread,
Jul 28, 2007, 10:41:37 AM7/28/07
to
In article <hrjqi.13179$gX5....@newsfe2-win.ntli.net>,

John Stumbles <john.s...@ntlworld.com> writes:
> On Thu, 26 Jul 2007 23:21:21 +0000, Mike Holmes wrote:
>
>> I am about to convert my standard open vented DHW system with indirect
>> tank to a HeatBank system following the wiki @
>>
>> http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank - many thanks
>> to John Stumbles for that.
>>
>> While trying to figure out what specification I require for the plate
>> heat exchanger, I happened upon this article, which suggests hot water
>> should be *distributed* as well as stored at 60degC.
>>
>> http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_water_is_vital.html
>
> Seems to be pitched at commercial-type installations. Obviously legionella
> is possible in domestic ones too but AFAIK the outbreaks that have occurred
> to date have been in larger ones. I don't know what the incidence of
> legionella in domestic installations is

There are none.

This came up here some months back. A few weeks later, I went to
a wedding and found myself sitting next to a bacteriologist or
virologist or immunologist or some such. With this fresh on my
mind, I asked about it. Answer was that we tend to be immune to
the bacteria in our homes as we're continually exposed to them.
The problem starts when you eradicate them, and hence lose your
immunity. That's fine in your home where they no longer exist,
but if you then go into an institution (such as a hospital)
where they are still the norm, or walk past a water-cooled air
handler with them in, you are now susceptable. He's not an
expert on Legionella specifically, but the appearence of it as
a disease over the last few decades does tie up with attempts
to clean our homes of bacteria, i.e. closing down the training
classrooms for our immune systems (as he put it) which is
widely recognised as a cause for some previously harmless
bacteria to have become recognised illnesses.

What would be interesting would be to see if there's any
correlation between those who've caught legionares disease
(and had previously been healthy, not immune suppressed),
and those who've been eradicating their exposure to bacteria
at home (in this case by using water systems which are
designed to minimise Legionella, rather than by explicitly
spraying disinfectant everywhere).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Brian Sharrock

unread,
Jul 28, 2007, 12:15:43 PM7/28/07
to

"Andrew Gabriel" <and...@cucumber.demon.co.uk> wrote in message
news:46ab55a1$0$648$5a6a...@news.aaisp.net.uk...

Tacking onto the end of this thread and directly answering the original
question; I converted to a heatbank about a year ago.
The DPS system I purchased has two discrete thermostatically controlled
loops; one for the 'working fluid' (water) bringing heat from the boiler and
a separate one for the HDW. I 've determined from experiment that -for my
household- the 'working fluid' is best set to 65 C and the DHW set to 55 C.
The 'working fluid' is stored at 65 C - but it's totally contained within
the system and in that sense is not distributed. The DHW
is -instantly-heated by the plate heat exchanger and goes directly to the
tap(s) when-and only when- the tap(s) are opened. I chose 55 C because
it's hot enough for washing but no too hot and when mixed with incoming
mains water at 9 C produce a bath / shower at 42 C quite nicely

HTH

--

Brian


Mike Holmes

unread,
Jul 31, 2007, 8:00:21 AM7/31/07
to
Thanks Brian. Due to the flow rate I get from rising main (c30l/min)
I'll probably have to set my store higher than 65degC (or choose a
bigger PHE - I've been looking at options which have a heat load of
100 and 104kW) to achieve the necessary output.

On the secondary/output side can anyone help with my remaining query.
Here it is: I have decided I will set my TMV (located just after the
PHE secondary output) to 55degC. My question is how do I calculate/
estimate what secondary output temperature I need to have to achieve
55degC where > 1 hot tap/shower is open at the same time? Obviously,
if there was only ever ONE outlet open, the secondary output could be
designed to be 55degC at the designed max flow rate - easy. Equally,
if I had an excessive secondary output temp, then there would prob be
plenty of "headroom", so the TMV may always be mixing with cold, even
if > 1 hot water outlet is open. However, this isn't a very efficient
sitn really as you'd either need a large (expensive) PHE or heat your
store to a really high temp. I would much prefer to "tune" the system
design to be able to design the minimum required PHE secondary output,
at the same time ensuring that my shower doesn't cool down if the 2nd
shower or the kitchen hot tap is turned on.

John

unread,
Jul 31, 2007, 10:58:21 AM7/31/07
to
On 28 Jul, 15:41, and...@cucumber.demon.co.uk (Andrew Gabriel) wrote:
> In article <hrjqi.13179$gX5.11...@newsfe2-win.ntli.net>,

> John Stumbles <john.stumb...@ntlworld.com> writes:
>
>
>
>
>
> > On Thu, 26 Jul 2007 23:21:21 +0000, Mike Holmes wrote:
>
> >> I am about to convert my standard open vented DHW system with indirect
> >> tank to a HeatBank system following the wiki @
>
> >>http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank- many thanks

> >> to John Stumbles for that.
>
> >> While trying to figure out what specification I require for the plate
> >> heat exchanger, I happened upon this article, which suggests hot water
> >> should be *distributed* as well as stored at 60degC.
>
> >>http://www.bsee.co.uk/news/fullstory.php/aid/3430/Providing_safe_hot_...

There are no recorded cases of Legionnaires disease in domestic
premises because they are not investigated. Legionnaires disease is
not reportable as such. However, if there's more than two cases it
becomes an outbreak and is investigated but only as far as commercial
premises are concerned. Private houses are not checked. As always
finding out about these cases can be difficult especially if they're
not in the same area.

I suggest your ?ologist needs to gen up on Legionella. The disease was
discovered in the 1970's. That doesn't mean it didn't exist before
then. As medical awareness grew, more cases have been discovered.
It's believed that most cases are contracted on holiday where
temperatures are higher and water hygiene not is so well established.
The victim also returns home to a system that's been sitting at a nice
temperature for a few weeks so Legionella has had lots of time to
multiply. The first shower releases a lovely aerosol of contaminated
water which they breathe in. By the time any symptoms appear 10 days
later, the system has been flushed many times and so all traces are
gone.

Reply all
Reply to author
Forward
0 new messages