Secondary return on the hot water?

451 views
Skip to first unread message

ARWadsworth

unread,
Apr 13, 2012, 3:10:27 PM4/13/12
to
Is it better to time control it (say 15 minutes every hour but only when the
HW is ON ) or just have it operate every time the HW is calling for demand?

--
Adam


Heliotrope Smith

unread,
Apr 13, 2012, 3:16:33 PM4/13/12
to
"ARWadsworth" <adamwa...@blueyonder.co.uk> wrote in
news:jm9tnc$8gp$1...@dont-email.me:

> Is it better to time control it (say 15 minutes every hour but only
> when the HW is ON ) or just have it operate every time the HW is
> calling for demand?
>

Timed for when there is 'likely' to be a demand for hot water.

No good it not circulating on an off time when tap turned on.

--- Posted via news://freenews.netfront.net/ - Complaints to ne...@netfront.net ---

John Rumm

unread,
Apr 13, 2012, 3:28:20 PM4/13/12
to
On 13/04/2012 20:10, ARWadsworth wrote:

> Is it better to time control it (say 15 minutes every hour but only when the
> HW is ON ) or just have it operate every time the HW is calling for demand?

The former (with well insulated pipes) I would think. HW demand might
only be once in the morning before anyone is up to use a tap!


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

harry

unread,
Apr 14, 2012, 2:42:12 AM4/14/12
to
On Apr 13, 8:10 pm, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
wrote:
This is the ideal way to double your hot water bill. They are
normally only needed on very large buildings (or very poorly designed
small DHW systems).

If you are going to run it only intermittantly it's pointless
insulating pipework, the heat will escape anyway however good the
insulation.

You need a corrosion free circ.pump. You will have a job finding one
small enough too, most are sized for large installations.

You stand a good chance of encouraging legionella in the standing
lukewarm water in the circ pipes.

So, all in all a very bad idea.

You would be far better thinking along the lines of reducing pipe runs/
layout of DHW store and users.

ARWadsworth

unread,
Apr 14, 2012, 3:09:01 AM4/14/12
to
Only the wiring concerns me:-) It's a new build. Is it big? Well the master
bedroom covers the same floor area as my house.

Deadlegs are the concern.


--
Adam


ARWadsworth

unread,
Apr 14, 2012, 3:20:00 AM4/14/12
to
Heliotrope Smith wrote:
> "ARWadsworth" <adamwa...@blueyonder.co.uk> wrote in
> news:jm9tnc$8gp$1...@dont-email.me:
>
> > Is it better to time control it (say 15 minutes every hour but only
> > when the HW is ON ) or just have it operate every time the HW is
> > calling for demand?
> >
>
> Timed for when there is 'likely' to be a demand for hot water.
>
> No good it not circulating on an off time when tap turned on.

Tricky. I have no idea when the owners will use the HW.

Had I known about this on the first fix I would have installed PIRs in the
ensuites to activate the pump. The kitchen and utility are close enough to
the boiler/cylinder and so are not on the return.

I might still be able to do the PIRs (the place is not fully boarded) if you
think it would be a better idea. How long does it take the pump to get the
HW to the taps? It's a 15mm return.

--
Adam


YAPH

unread,
Apr 14, 2012, 6:50:13 AM4/14/12
to
On Fri, 13 Apr 2012 23:42:12 -0700, harry wrote:

> On Apr 13, 8:10 pm, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
> wrote:
>> Is it better to time control it (say 15 minutes every hour but only
>> when the HW is ON ) or just have it operate every time the HW is
>> calling for demand?

> This is the ideal way to double your hot water bill. They are normally
> only needed on very large buildings (or very poorly designed small DHW
> systems).
>
> If you are going to run it only intermittantly it's pointless insulating
> pipework, the heat will escape anyway however good the insulation.

*Some* heat will escape anyway but insulation *reduces* the amount of
heat lost. So if you don't insulate it you will lose more heat than if
you do. You'd have to be using very little hot water and/or have a lot of
uninsulated pipework for that to double the amount of energy (and
therefore money) you use for water heating, though.

> You stand a good chance of encouraging legionella in the standing
> lukewarm water in the circ pipes.

One reason for SHW circulation is to avoid the possibility of Legionella
multiplying in standing lukewarm water by circulating *hot* water.

[sigh]

--
John Stumbles -- http://yaph.co.uk

Praying is talking to yourself and pretending someone intelligent is
listening.

Nospam

unread,
Apr 14, 2012, 7:08:32 AM4/14/12
to
On 13/04/2012 20:10, ARWadsworth wrote:
> Is it better to time control it (say 15 minutes every hour but only when the
> HW is ON ) or just have it operate every time the HW is calling for demand?
>

I fitted a Grundfos UP15-14BU which has a built-in programmable timer.
We experimented with timing for the first few months so I've no idea now
what it's set to, but it works. Nearly instant hotwater is rather nice
and even the kids (rather inappropriately) said it was "cool" ;-)

We have a megaflow cylinder so I guess the downside is that it's being
churned-up rather then stratifying, but I don't know what the impact is
on DHW heating demand - subjectively it doesn't seem excessive (whatever
that means!)

John Rumm

unread,
Apr 14, 2012, 7:10:47 AM4/14/12
to
About as long as running off the dead leg of water with the tap...

NT

unread,
Apr 14, 2012, 8:35:35 AM4/14/12
to
On Apr 13, 8:10 pm, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
wrote:
> Is it better to time control it (say 15 minutes every hour but only when the
> HW is ON ) or just have it operate every time the HW is calling for demand?

The ideal control is a thermostat on the coldest bit of the pipe, ie
next to the furthest tap. Then the pump just runs when needed to stay
hot. Add a PIR near the taps and the system only keeps itself hot when
someone enters a tap room.

Obviously insulation is necessary. The pipework isn't just hot when in
use, its hot 24/7.


NT

John Rumm

unread,
Apr 14, 2012, 9:40:35 AM4/14/12
to
On 14/04/2012 08:20, ARWadsworth wrote:
I expect you need a combination of a 24hr timer, and a pipe stat. Set
the timer to enable secondary circulation during typical waking hours.
Then leave the pump to cycle on a pipe stat set in the return leg of
pipework beyond the furthest draw off point.

ARWadsworth

unread,
Apr 14, 2012, 10:29:52 AM4/14/12
to
John Rumm wrote:
> On 14/04/2012 08:20, ARWadsworth wrote:
> > Heliotrope Smith wrote:
> > > "ARWadsworth"<adamwa...@blueyonder.co.uk> wrote in
> > > news:jm9tnc$8gp$1...@dont-email.me:
> > >
> > > > Is it better to time control it (say 15 minutes every hour but
> > > > only when the HW is ON ) or just have it operate every time the
> > > > HW is calling for demand?
> > > >
> > >
> > > Timed for when there is 'likely' to be a demand for hot water.
> > >
> > > No good it not circulating on an off time when tap turned on.
> >
> > Tricky. I have no idea when the owners will use the HW.
> >
> > Had I known about this on the first fix I would have installed PIRs
> > in the ensuites to activate the pump. The kitchen and utility are
> > close enough to the boiler/cylinder and so are not on the return.
> >
> > I might still be able to do the PIRs (the place is not fully
> > boarded) if you think it would be a better idea. How long does it
> > take the pump to get the HW to the taps? It's a 15mm return.
>
> I expect you need a combination of a 24hr timer, and a pipe stat. Set
> the timer to enable secondary circulation during typical waking hours.
> Then leave the pump to cycle on a pipe stat set in the return leg of
> pipework beyond the furthest draw off point.

Nice idea.

And another question. All the pipework is plastic apart from this secondary
return. Any reason it was done in copper?
--
Adam


Nospam

unread,
Apr 14, 2012, 12:09:13 PM4/14/12
to
Ref my other reply: use a (Grundfos) pump with a built-in timer. I wired
mine via a pipe stat fixed on the return - the stat may call for heat
but the pump will only run if the timer "aaproves". With timer and stat
accessible the users can change them to suit their needs.

John Rumm

unread,
Apr 14, 2012, 1:19:59 PM4/14/12
to
On 14/04/2012 15:29, ARWadsworth wrote:
No, but then again a pipe stat will work better on copper...

ARWadsworth

unread,
Apr 15, 2012, 2:21:54 AM4/15/12
to
The plumbing side of things is out of my hands!

And believe me. The owners of the house are unable to do much themselves
other than eat money so I need something idiot proof.

--
Adam


harry

unread,
Apr 15, 2012, 2:57:04 AM4/15/12
to
On Apr 14, 8:09 am, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
wrote:
> Adam- Hide quoted text -
>
> - Show quoted text -

The way forward is "point of use hot water generation" in mid sized
buildings.
http://en.wikipedia.org/wiki/Water_heating#Point-of-use_.28POU.29_vs._Centralized_hot_water

harry

unread,
Apr 15, 2012, 3:06:21 AM4/15/12
to
On Apr 14, 8:20 am, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
wrote:
> Heliotrope Smith wrote:
> > "ARWadsworth" <adamwadswo...@blueyonder.co.uk> wrote in
No-one can say, depends on the pipe size/length, size of pump etc.

harry

unread,
Apr 15, 2012, 3:03:28 AM4/15/12
to
Well if they're only turning the circ pump on intermiitantly when
water is required the water will cool and legionella is what they'll
get. Whether they catch it of not is another matter.
An ideal system for breeding it.
What they should have done is installed point of use heaters.
(Electric or Gas)

There's a lot of plumbers/heating engineers don't know the first thing
about the design of large/medium DHW systems.

harry

unread,
Apr 15, 2012, 3:14:34 AM4/15/12
to
On Apr 14, 3:29 pm, "ARWadsworth" <adamwadswo...@blueyonder.co.uk>
wrote:
> John Rumm wrote:
> > On 14/04/2012 08:20, ARWadsworth wrote:
> > > Heliotrope Smith wrote:
> > > > "ARWadsworth"<adamwadswo...@blueyonder.co.uk>  wrote in
The purpose of plastic is to save money.
Absolutely no point in have the return done in copper.
Copper does inhibit some algal growths and other bugs but the effect
is slight.
Plastic on the other hand positively encourages it. At least some
sorts do.

The problem with hot water is that the chlorine has been driven out so
the bugs grow freely.

Tim

unread,
Apr 15, 2012, 3:21:27 PM4/15/12
to
harry <harol...@aol.com> wrote:

> Well if they're only turning the circ pump on intermiitantly when
> water is required the water will cool and legionella is what they'll
> get. Whether they catch it of not is another matter.
> An ideal system for breeding it.

You mean just like a conventional system when you turn off the hot tap
leaving a long "dead leg" of water cooling?

Can't see why it would be much worse with a circulator.

Tim

NT

unread,
Apr 15, 2012, 3:40:03 PM4/15/12
to
On Apr 15, 8:21 pm, Tim <timdownie2...@yahoo.co.uk> wrote:
harry must be dribble


NT

John Rumm

unread,
Apr 15, 2012, 4:58:13 PM4/15/12
to
It would actually be better, especially if you blended it down to final
mix temperature at the point of use - since you would be periodically
raising all the pipework to full temperature.

harry

unread,
Apr 16, 2012, 1:54:43 AM4/16/12
to
On Apr 15, 8:21 pm, Tim <timdownie2...@yahoo.co.uk> wrote:
It will be worse because there is more water the was once heated in
the system and more places fort he bugs to lodge. If the pump is not
started for any reason, the water will remain in deadlegs for long
periods too,gradually getting worse.

harry

unread,
Apr 16, 2012, 2:06:04 AM4/16/12
to
On Apr 15, 9:58 pm, John Rumm <see.my.signat...@nowhere.null> wrote:
> On 15/04/2012 20:21, Tim wrote:
>
> > harry<haroldhr...@aol.com>  wrote:
>
> >> Well if they're only turning the circ pump on intermiitantly when
> >> water is required the water will cool and legionella is what they'll
> >> get.  Whether they catch it of not is another matter.
> >> An ideal system for breeding it.
>
> > You mean just like a conventional system when you turn off the hot tap
> > leaving a long "dead leg" of water cooling?
>
> > Can't see why it would be much worse with a circulator.
>
> It would actually be better, especially if you blended it down to final
> mix temperature at the point of use - since you would be periodically
> raising all the pipework to full temperature.


You need a temperature of 80-90degC to kill off legionella. The
normal 60-70 is not enough.
The best you can hope to do is to flush it away in a domestic system.

But what happens with a circulator is that you pump it back into the
tank.
So instead of just having it confined in a "dead leg", you have it in
the whole system.

So water in the DHW loop cools, you may get legionella breeding , pump
starts, sends the polluted waterback to the tank and pollutes the
whole system.
Wonderful eh?

Legionella breeds best at around human body temperature,

charles

unread,
Apr 16, 2012, 2:18:08 AM4/16/12
to
In article
<4b21e135-0362-4a50...@n5g2000vbf.googlegroups.com>,
and, having had it, I wouldn't wish it on anyone else.

--
From KT24

Using a RISC OS computer running v5.18

Onetap

unread,
Apr 16, 2012, 4:36:25 AM4/16/12
to
On Sunday, April 15, 2012 8:03:28 AM UTC+1, harry wrote:

>
> There's a lot of plumbers/heating engineers don't know the first thing
> about the design of large/medium DHW systems.

On the contrary. I've read your posts on this thread; without exception they are all utter bullshit, nonsense you've made up or read on the internet and misunderstood.

The DHWS secondary circulation pump should run 24/7 or is operated by a timer to run during the anticipated hours of occupation.

DHWS secondary circulation systems are necessary in commercial buildings because there is a legal duty to comply with the Code of Practice in the HSE document, L8, 'Legionnaires' disease. The control of legionella bacteria in water systems'. This has a requirement for the water from hot taps to reach 50 degC within 60 seconds. This can only be achieved in a large building with central stored DHW with a secondary circulating system OR electric trace heating on the DHW pipework.

Some offices use point of use electric heaters for washrooms and tea facilities with small demands. It would be daft to install such heaters for domestic bathrooms and showers if there was conventional central heating installed.

In domestic buildings, DHWS secondary circulation is usually solely for convenience.

The additional heat loss should be negligible IF the pipework is properly insulated. The secondary circulation system is simply there to compensate for the heat loss from the pipework; the flow rates required are tiny (compared to CH). PIR occupancy sensors can't be justified for commercial buildings.

The water and pipework is pasteurised by high temperatures. 50 to 60 degC is ISTR in the range of temperatures which will kill legionella, but it takes some time, a minute or two I think; there's a table in L8. The pipework can be recolonized since there's legionella in the environment and in mains water, but it takes some time at suitable tepid temperatures for the organism to proliferate to dangerous levels. There's no reason why previously heated water would be more prone to bacteria.

The 'anti-bacterial properties' of copper is a myth that grew from a research paper about exposed copper alloy fittings in hospitals. The paper is on the CDA website I believe. Copper does not contribute to the legionella control regimes, which are almost exclusively by temperature control.

Onetap

unread,
Apr 16, 2012, 4:56:21 AM4/16/12
to
On Monday, April 16, 2012 7:18:08 AM UTC+1, charles wrote:


> and, having had it, I wouldn't wish it on anyone else.
>
Did you? Do you know how you got it, did they trace the source?
Did they have a reason to suspect legionella, had there been an outbreak? Or did they identify it by routine swab tests?

I'm just interested in how the hospitals and HSE work.

Onetap

unread,
Apr 16, 2012, 4:52:08 AM4/16/12
to
On Monday, April 16, 2012 7:06:04 AM UTC+1, harry wrote:
>
> You need a temperature of 80-90degC to kill off legionella. The
> normal 60-70 is not enough.

Bollocks; 3 minutes at 60 degC. 60 degC isn't a problem for most competently designed and managed systems.

There were problems with old cylinders because the coil was some way up the cylinder and the bottom
of the cylinder never got hot. The legionella colonized the limescale deposits in the bottom of the
cylinder and recolonized the distribution system when it could.

Newer cylinders have the coil lower down and are better insulated. There's also stuff like pasteurisation
cycles and shunt pumps for bigger stuff, but you won't know about that, Harry.

Fredxx

unread,
Apr 16, 2012, 6:21:00 AM4/16/12
to
On 13/04/2012 20:10, ARWadsworth wrote:
> Is it better to time control it (say 15 minutes every hour but only
> when the
> HW is ON ) or just have it operate every time the HW is calling for
> demand?
>
For sinks and the like, what's wrong with an undersink water heater.
Probably less heat loss that lagged pipes and running a pump near 24/7.

harry

unread,
Apr 16, 2012, 12:15:43 PM4/16/12
to
Well I used to design and run large DHW systems in hospitals.

harry

unread,
Apr 16, 2012, 12:19:14 PM4/16/12
to
Yes I do having worked in hospitals for thirty years.

harry

unread,
Apr 16, 2012, 12:20:35 PM4/16/12
to
The source of many outbreaks is never discovered. Legonella is endemic.

harry

unread,
Apr 16, 2012, 12:17:53 PM4/16/12
to
That is one sort of point of use heater. Much more sensible in
domestic situation.

harry

unread,
Apr 16, 2012, 12:28:07 PM4/16/12
to

John Rumm

unread,
Apr 16, 2012, 12:47:05 PM4/16/12
to
On 16/04/2012 07:06, harry wrote:
> On Apr 15, 9:58 pm, John Rumm<see.my.signat...@nowhere.null> wrote:
>> On 15/04/2012 20:21, Tim wrote:
>>
>>> harry<haroldhr...@aol.com> wrote:
>>
>>>> Well if they're only turning the circ pump on intermiitantly when
>>>> water is required the water will cool and legionella is what they'll
>>>> get. Whether they catch it of not is another matter.
>>>> An ideal system for breeding it.
>>
>>> You mean just like a conventional system when you turn off the hot tap
>>> leaving a long "dead leg" of water cooling?
>>
>>> Can't see why it would be much worse with a circulator.
>>
>> It would actually be better, especially if you blended it down to final
>> mix temperature at the point of use - since you would be periodically
>> raising all the pipework to full temperature.
>
>
> You need a temperature of 80-90degC to kill off legionella. The
> normal 60-70 is not enough.

Its plenty.

Note also many boilers with capabilities for split temperature operation
also allow for routing anti legionella cycles to ensure the system is
heated to a higher temperature at least once a week.

> The best you can hope to do is to flush it away in a domestic system.

Since it is easy to kill, why?

> But what happens with a circulator is that you pump it back into the
> tank.
> So instead of just having it confined in a "dead leg", you have it in
> the whole system.

If its there, its in the whole system anyway. A circulation loop
eliminates any deadlegs on infrequently used hot taps, and thus reduces
the possibility of breeding happening there.

> So water in the DHW loop cools, you may get legionella breeding , pump
> starts, sends the polluted waterback to the tank and pollutes the
> whole system.

Mixes with the hot water and dies...

> Wonderful eh?

Actually, yes.

> Legionella breeds best at around human body temperature,

Is your body routinely at the 60 - 70 one would run a cylinder at?

John Rumm

unread,
Apr 16, 2012, 12:48:15 PM4/16/12
to
Higher install cost, and you are shifting your hot water heating
requirements from cheap centrally heated water, to that done with peak
rate electricity in most cases.

Onetap

unread,
Apr 16, 2012, 3:14:38 PM4/16/12
to
On Monday, April 16, 2012 5:19:14 PM UTC+1, harry wrote:

> Yes I do having worked in hospitals for thirty years.

They still remember you in Domestic Services.

Onetap

unread,
Apr 16, 2012, 3:19:45 PM4/16/12
to
On Monday, April 16, 2012 5:20:35 PM UTC+1, harry wrote:

> The source of many outbreaks is never discovered. Legonella is endemic.

Get away!

That's why I asked if they'd discovered the source. Some outbreaks are traced, mainly in hospitals. Some aren't. I was saying windscreen washer bottles were a hazard 7 or 10 years before the HSE realised that.

Jim K

unread,
Apr 16, 2012, 3:39:06 PM4/16/12
to
I'll buy you a pint if you tell us what they used to call him?

Jim K

harry

unread,
Apr 17, 2012, 2:02:38 AM4/17/12
to
I was the hospital engineer in several hospitals. In charge of all
maintenance.

One place had 12" pipework for the DHW at source.

I als worked in the design office.

So I think I know something about very large DHW systems.

So what is your experience? Working for Barret homes? Installing one
tap here and there?

harry

unread,
Apr 17, 2012, 2:03:40 AM4/17/12
to
They called me Hiawatha at one place behind my back.

Onetap

unread,
Apr 17, 2012, 4:20:06 AM4/17/12
to
On Monday, April 16, 2012 8:39:06 PM UTC+1, Jim K wrote:
> > On Monday, April 16, 2012 5:19:14 PM UTC+1, harry wrote:
> >
> >> Yes I do having worked in hospitals for thirty years.
> >
> > They still remember you in Domestic Services.
>
> I'll buy you a pint if you tell us what they used to call him?
>

Benny

Onetap

unread,
Apr 17, 2012, 4:26:39 AM4/17/12
to
On Tuesday, April 17, 2012 7:02:38 AM UTC+1, harry wrote:

> I was the hospital engineer in several hospitals. In charge of all
> maintenance.
>
> One place had 12" pipework for the DHW at source.
>
> I als worked in the design office.
>
> So I think I know something about very large DHW systems.
>
> So what is your experience? Working for Barret homes? Installing one
> tap here and there?

If that were true, you'd have some inkling of the contents of L8 and the prevention of legionella; you do not. You lie.

I worked for a RHA nearly 30 years ago, doing design. AS you have seen I know something about legionella and DHWS. The tag was a nickname in previous job and had nowt to do with taps.

Onetap

unread,
Apr 17, 2012, 4:33:31 AM4/17/12
to
On Monday, April 16, 2012 5:28:07 PM UTC+1, harry wrote:

>
> http://en.wikipedia.org/wiki/Legionella#Source_control

Which confirms what I'd said. 50 to 60 degC kills it, 2 minutes at 60 degC kills 90%.

Problems arise in DHWS systems with poor circulation ( never reach 60 degC), poor insulation
(the pipe wall is a lot cooler allowing bugs to survive) and in older calorifiers,
as mentioned previously. Also in CW systems with poor insulation where the water temperature
can exceed 20 degC and allow legionella to multiply.

gri...@gmail.com

unread,
Apr 17, 2012, 7:33:51 AM4/17/12
to
On Tue, 17 Apr 2012 01:26:39 -0700 (PDT), Onetap <one...@talk21.com>
wrote:

> The tag was a nickname in previous job and had nowt to do with taps.

One would assume you lend fags once, or are skilled at administering a
fatal shot in the head, or worked in a slaughterhouse.

Fredxx

unread,
Apr 17, 2012, 2:11:54 PM4/17/12
to
On 16/04/2012 17:48, John Rumm wrote:
> On 16/04/2012 11:21, Fredxx wrote:
>> On 13/04/2012 20:10, ARWadsworth wrote:
>>> Is it better to time control it (say 15 minutes every hour but only
>>> when the
>>> HW is ON ) or just have it operate every time the HW is calling for
>>> demand?
>>>
>> For sinks and the like, what's wrong with an undersink water heater.
>> Probably less heat loss that lagged pipes and running a pump near 24/7.
>
> Higher install cost, and you are shifting your hot water heating
> requirements from cheap centrally heated water, to that done with peak
> rate electricity in most cases.
>

Agreed, but it depends on what pipe length and corresponding heatloss,
together with a pump running for most of the day, compared to actual
water usage.

YAPH

unread,
Apr 17, 2012, 3:33:42 PM4/17/12
to
On Sun, 15 Apr 2012 23:06:04 -0700, harry wrote:

> You need a temperature of 80-90degC to kill off legionella. The normal
> 60-70 is not enough.

According to sources cited in the Wikipedia article http://
en.wikipedia.org/wiki/Legionella tends to die above 50 - 55C, and
absolutely dies at 70C. But maybe harry knows something the World Health
Organisation, Chartered Institute of Plumbing ad Heating Engineering and
Health and Safety Executive don't?

--
John Stumbles -- http://yaph.co.uk

"Write a wise saying and your name will live forever." -- anonymous

John Rumm

unread,
Apr 17, 2012, 3:56:16 PM4/17/12
to
Indeed - as with all these things, the devil is in the detail, and there
is no substitute for actually doing the numbers to see what really makes
sense.
Reply all
Reply to author
Forward
0 new messages