On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
> On 29/01/2024 19:38, Animal wrote:
> > On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
> >> On 26/01/2024 00:21, Fredxx wrote:
> >>> On 25/01/2024 20:07, Animal wrote:
> >>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
> >>>>> On 24/01/2024 19:46, AnthonyL wrote:
> >>>> important if you use a low flow shower, showers as low as 3kW are
> >>>> perfectly practical.
> >>>
> >>> I dread to think of the flow rate at 3kW.
> >> < 1.5 lpm in the winter!
> >
> > nah, more than that
> Might even be less - depends on the ground water temp.
>
> Say incoming water is only 5 deg, and your target shower temp is 40
> (what I would call a warm shower rather than hot!), that is:

40 is excessive for a shower, 30 is ok.

> (40-5) x 4200 = 147 kJ of energy required per litre.
>
> A 3kW heater will transfer 3000J x 60 sec = 180 kJ per min.
>
> 180 / 147 = 1.22 litres/min.

from 10 to 30C is 20deg rise which permits 2.1lpm

> Even with incoming water at 10 degrees, it is still less than 1.5 lpm
> >
> >> < It is the norm to have a
> >>> significant flow with a combi, otherwise you might as well go electric.
> >>>
> >>>>>> We have a hot water delivery problem to the kitchen which is at
> >>>>>> the front of the house (boiler right at the back). My wife likes
> >>>>>> to wash with running water (makes some sense) but with a
> >>>>>> combination of poor modulation (burners turn off when water too
> >>>>>> hot) and the warm up time, she ends up using the cold water and
> >>>>>> screaming frost-bite at me.
> >>>>> Perhaps an under-sink heater is called for?
> >>>>
> >>>> another option is a circulation pump & pir. When you approach the
> >>>> sink, the hot water runs to the tap briefly (& back). Result is
> >>>> instant hot most of the time.
> >>>
> >>> And a significant loss of heat every time it runs.
>
> >> Hot water circulation works ok if you have a cylinder of hot water and a
> >> well insulated pipe run - you just circulate water back to another
> >> tapping on the cylinder, so you don't waste much heat, but eliminate
> >> delay at the tap since there is hot water right next to it when you need
> >
> > You could also use 22m with microbore inside so it returns along he same pipe.
> yup you could... also lots of hassle unless installing a return pipe any
> other way is particularly difficult.
> >> it. Not sure how you would do it with a combi.
> and you still have the problem of how to get the combi to re-heat it
> since many have a maximum cold water inlet temperature that is not
> particularly high.

The hw doesn't return to the inlet, it returns to the tank, whether that's an external tank or a baby one in the combi.

On 30/01/2024 20:26, Animal wrote:
> On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
>> On 29/01/2024 19:38, Animal wrote:
>>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
>>>> On 26/01/2024 00:21, Fredxx wrote:
>>>>> On 25/01/2024 20:07, Animal wrote:
>>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
>>>>>>> On 24/01/2024 19:46, AnthonyL wrote:
>>>>>> important if you use a low flow shower, showers as low as 3kW are
>>>>>> perfectly practical.
>>>>>
>>>>> I dread to think of the flow rate at 3kW.
>>>> < 1.5 lpm in the winter!
>>>
>>> nah, more than that
>> Might even be less - depends on the ground water temp.
>>
>> Say incoming water is only 5 deg, and your target shower temp is 40
>> (what I would call a warm shower rather than hot!), that is:
>
> 40 is excessive for a shower, 30 is ok.

Most people say body temperature is best:
https://www.mirashowers.co.uk/blog/mira-recommends/what-is-the-best-shower-temperature

So nearer 40C than 30C, especially if there's any heat loss on the way
to the shower.

30C will feel very cold!

On Tuesday 30 January 2024 at 21:20:43 UTC, Fredxx wrote:
> On 30/01/2024 20:26, Animal wrote:
> > On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
> >> On 29/01/2024 19:38, Animal wrote:
> >>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
> >>>> On 26/01/2024 00:21, Fredxx wrote:
> >>>>> On 25/01/2024 20:07, Animal wrote:
> >>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
> >>>>>>> On 24/01/2024 19:46, AnthonyL wrote:
> >>>>>> important if you use a low flow shower, showers as low as 3kW are
> >>>>>> perfectly practical.
> >>>>>
> >>>>> I dread to think of the flow rate at 3kW.
> >>>> < 1.5 lpm in the winter!
> >>>
> >>> nah, more than that
> >> Might even be less - depends on the ground water temp.
> >>
> >> Say incoming water is only 5 deg, and your target shower temp is 40
> >> (what I would call a warm shower rather than hot!), that is:
> >
> > 40 is excessive for a shower, 30 is ok.
> Most people say body temperature is best:
>
> https://www.mirashowers.co.uk/blog/mira-recommends/what-is-the-best-shower-temperature
>
> So nearer 40C than 30C, especially if there's any heat loss on the way
> to the shower.
>
> 30C will feel very cold!

I dug up my test temps. 39C at the shower head, 34C where water hits plastic. This felt a bit too hot, so nearer 35 sounds about right.
You can cut it further with a drain heat exchanger. Good for showers, not for baths.
You could play other games too if you want, like a hot harry preheater. And this was a standard shower, not aerated or pulsed jet. A comfortable 1kW shower should be quite doable.

On 30/01/2024 20:26, Animal wrote:
> On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
>> On 29/01/2024 19:38, Animal wrote:
>>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
>>>> On 26/01/2024 00:21, Fredxx wrote:
>>>>> On 25/01/2024 20:07, Animal wrote:
>>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
>>>>>>> On 24/01/2024 19:46, AnthonyL wrote:
>>>>>> important if you use a low flow shower, showers as low as 3kW are
>>>>>> perfectly practical.
>>>>>
>>>>> I dread to think of the flow rate at 3kW.
>>>> < 1.5 lpm in the winter!
>>>
>>> nah, more than that
>> Might even be less - depends on the ground water temp.
>>
>> Say incoming water is only 5 deg, and your target shower temp is 40
>> (what I would call a warm shower rather than hot!), that is:
>
> 40 is excessive for a shower, 30 is ok.
>
>> (40-5) x 4200 = 147 kJ of energy required per litre.
>>
>> A 3kW heater will transfer 3000J x 60 sec = 180 kJ per min.
>>
>> 180 / 147 = 1.22 litres/min.
>
> from 10 to 30C is 20deg rise which permits 2.1lpm

and still way short of even adequate.

>>>> Hot water circulation works ok if you have a cylinder of hot water and a
>>>> well insulated pipe run - you just circulate water back to another

>>>> it. Not sure how you would do it with a combi.

>> and you still have the problem of how to get the combi to re-heat it
>> since many have a maximum cold water inlet temperature that is not
>> particularly high.
>
> The hw doesn't return to the inlet, it returns to the tank, whether that's an external tank or a baby one in the combi.

Unless it is specifically a storage combi (not that common in the UK),
there is unlikely to be any way of connecting another inlet to its tank.

With an external tank sure no problem...

--
Cheers,

John.

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

On Wednesday 31 January 2024 at 18:55:58 UTC, John Rumm wrote:
> On 30/01/2024 20:26, Animal wrote:
> > On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
> >> On 29/01/2024 19:38, Animal wrote:
> >>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
> >>>> On 26/01/2024 00:21, Fredxx wrote:
> >>>>> On 25/01/2024 20:07, Animal wrote:
> >>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
> >>>>>>> On 24/01/2024 19:46, AnthonyL wrote:

> >>>>>> important if you use a low flow shower, showers as low as 3kW are
> >>>>>> perfectly practical.
> >>>>>
> >>>>> I dread to think of the flow rate at 3kW.
> >>>> < 1.5 lpm in the winter!
> >>>
> >>> nah, more than that
> >> Might even be less - depends on the ground water temp.
> >>
> >> Say incoming water is only 5 deg, and your target shower temp is 40
> >> (what I would call a warm shower rather than hot!), that is:
> >
> > 40 is excessive for a shower, 30 is ok.
> >
> >> (40-5) x 4200 = 147 kJ of energy required per litre.
> >>
> >> A 3kW heater will transfer 3000J x 60 sec = 180 kJ per min.
> >>
> >> 180 / 147 = 1.22 litres/min.
> >
> > from 10 to 30C is 20deg rise which permits 2.1lpm
> and still way short of even adequate.

nonsense. The fashion for high flow showers is pointless.

> >>>> Hot water circulation works ok if you have a cylinder of hot water and a
> >>>> well insulated pipe run - you just circulate water back to another
> >>>> it. Not sure how you would do it with a combi.
>
> >> and you still have the problem of how to get the combi to re-heat it
> >> since many have a maximum cold water inlet temperature that is not
> >> particularly high.
> >
> > The hw doesn't return to the inlet, it returns to the tank, whether that's an external tank or a baby one in the combi.

> Unless it is specifically a storage combi (not that common in the UK),
> there is unlikely to be any way of connecting another inlet to its tank.
>
> With an external tank sure no problem...

Heh, return the HW back into the same connection you get it from, microbore inside 22mm. I shouldn't need to pont out that the ubore should be a little longer.

On 02/02/2024 22:06, Animal wrote:
> On Wednesday 31 January 2024 at 18:55:58 UTC, John Rumm wrote:
>> On 30/01/2024 20:26, Animal wrote:
>>> On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
>>>> On 29/01/2024 19:38, Animal wrote:
>>>>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
>>>>>> On 26/01/2024 00:21, Fredxx wrote:
>>>>>>> On 25/01/2024 20:07, Animal wrote:
>>>>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
>>>>>>>>> On 24/01/2024 19:46, AnthonyL wrote:
>
>>>>>>>> important if you use a low flow shower, showers as low as 3kW are
>>>>>>>> perfectly practical.
>>>>>>>
>>>>>>> I dread to think of the flow rate at 3kW.
>>>>>> < 1.5 lpm in the winter!
>>>>>
>>>>> nah, more than that
>>>> Might even be less - depends on the ground water temp.
>>>>
>>>> Say incoming water is only 5 deg, and your target shower temp is 40
>>>> (what I would call a warm shower rather than hot!), that is:
>>>
>>> 40 is excessive for a shower, 30 is ok.
>>>
>>>> (40-5) x 4200 = 147 kJ of energy required per litre.
>>>>
>>>> A 3kW heater will transfer 3000J x 60 sec = 180 kJ per min.
>>>>
>>>> 180 / 147 = 1.22 litres/min.
>>>
>>> from 10 to 30C is 20deg rise which permits 2.1lpm
>> and still way short of even adequate.
>
> nonsense. The fashion for high flow showers is pointless.

I think you need your flow rate recalibrated! High flow showers would be
15+ lpm - and possibly significantly more with larger soaker heads or
body jets etc. Most folks idea of a "normal" shower would be something
in the range 6 to 8 lpm.

Very few people buy 7kW electric showers unless supply constraints rule
out anything bigger, because they know the performance is so dire.

Screwfix have ~125 electric showers listed, the lowest power ones being
7.5kW - and they only have 2 models at that output.

>>>>>> Hot water circulation works ok if you have a cylinder of hot water and a
>>>>>> well insulated pipe run - you just circulate water back to another
>>>>>> it. Not sure how you would do it with a combi.
>>
>>>> and you still have the problem of how to get the combi to re-heat it
>>>> since many have a maximum cold water inlet temperature that is not
>>>> particularly high.
>>>
>>> The hw doesn't return to the inlet, it returns to the tank, whether that's an external tank or a baby one in the combi.
>
>> Unless it is specifically a storage combi (not that common in the UK),
>> there is unlikely to be any way of connecting another inlet to its tank.
>>
>> With an external tank sure no problem...
>
> Heh, return the HW back into the same connection you get it from, microbore inside 22mm. I shouldn't need to pont out that the ubore should be a little longer.

Perhaps I am missing something, but I can't wee what that would achieve
with a combi.

The idea of a circulation loop is to ensure that hot water is available
at the point of use with no delay. With some stored hot water you can
pump hot water from the cylinder, round the loop and back to another
tapping a bit lower down the cylinder. You are circulating hot water
past the point of use so it will be hot when required.

If you are proposing that you tee into the DHW *output* of the combi,
and pump back to that, all you will be doing is circulating cold water,
since the combi will only fire when it sees an actual flow of hot water.

--
Cheers,

John.

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

Animal wrote:
> John Rumm wrote:
>
>> Say incoming water is only 5 deg, and your target shower temp is 40
>> (what I would call a warm shower rather than hot!)
>
> 40 is excessive for a shower, 30 is ok.
Mine is 42°C as it leaves the shower head.

On 03/02/2024 11:45, Andy Burns wrote:
> Animal wrote:
>
>> John Rumm wrote:
>>
>>> Say incoming water is only 5 deg, and your target shower temp is 40
>>> (what I would call a warm shower rather than hot!)
>>
>> 40 is excessive for a shower, 30 is ok.
>
> Mine is 42°C as it leaves the shower head.

Hotter the better I say - up to physical damage anyway.

--
“A leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
“We did this ourselves.”

― Lao Tzu, Tao Te Ching

The Natural Philosopher wrote:
> Andy Burns wrote:
>
>> Mine is 42°C as it leaves the shower head.
>
> Hotter the better I say - up to physical damage anyway.
My mum always used to say I had lily-livered skin

On 03/02/2024 11:45, Andy Burns wrote:
> Animal wrote:
>
>> John Rumm wrote:
>>
>>> Say incoming water is only 5 deg, and your target shower temp is 40
>>> (what I would call a warm shower rather than hot!)
>>
>> 40 is excessive for a shower, 30 is ok.
>
> Mine is 42°C as it leaves the shower head.

In a hotel I stayed at the other day the thermostat (detent peg) was set
to 35C which I found to be cool. I raised the control to 45C which was
comfortable BUT I don't actually know the actual temperature of the
water, just what the control indicated.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On 2024-02-03 15:52, alan_m wrote:
> On 03/02/2024 11:45, Andy Burns wrote:
>> Animal wrote:
>>
>>> John Rumm wrote:
>>>
>>>> Say incoming water is only 5 deg, and your target shower temp is 40
>>>> (what I would call a warm shower rather than hot!)
>>>
>>> 40 is excessive for a shower, 30 is ok.
>>
>> Mine is 42°C as it leaves the shower head.
>
> In a hotel I stayed at the other day the thermostat (detent peg) was set
> to 35C which I found to be cool. I raised the control to 45C which was
> comfortable BUT I don't actually know the actual temperature of the
> water, just what the control indicated.
>

Best bathwater is about 38 or 39C (by thermometer).

nib

nibble wrote:

> alan_m wrote:
>
>> In a hotel I stayed at the other day the thermostat (detent peg) was
>> set to 35C which I found to be cool. I raised the control to 45C which
>> was comfortable BUT I don't actually know the actual temperature of
>> the water, just what the control indicated.
>
> Best bathwater is about 38 or 39C (by thermometer).

Shower droplets cool pretty quickly in air

On 03/02/2024 17:06, Andy Burns wrote:
> nibble wrote:
>
>> alan_m wrote:
>>
>>> In a hotel I stayed at the other day the thermostat (detent peg) was
>>> set to 35C which I found to be cool. I raised the control to 45C
>>> which was comfortable BUT I don't actually know the actual
>>> temperature of the water, just what the control indicated.
>>
>> Best bathwater is about 38 or 39C (by thermometer).
>
> Shower droplets cool pretty quickly in air
>

And the water entering a metal bath.

--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

On Friday 2 February 2024 at 23:56:18 UTC, John Rumm wrote:
> On 02/02/2024 22:06, Animal wrote:
> > On Wednesday 31 January 2024 at 18:55:58 UTC, John Rumm wrote:
> >> On 30/01/2024 20:26, Animal wrote:
> >>> On Monday 29 January 2024 at 21:44:24 UTC, John Rumm wrote:
> >>>> On 29/01/2024 19:38, Animal wrote:
> >>>>> On Monday 29 January 2024 at 02:07:47 UTC, John Rumm wrote:
> >>>>>> On 26/01/2024 00:21, Fredxx wrote:
> >>>>>>> On 25/01/2024 20:07, Animal wrote:
> >>>>>>>> On Thursday 25 January 2024 at 00:27:22 UTC, Fredxx wrote:
> >>>>>>>>> On 24/01/2024 19:46, AnthonyL wrote:
> >
> >>>>>>>> important if you use a low flow shower, showers as low as 3kW are
> >>>>>>>> perfectly practical.
> >>>>>>>
> >>>>>>> I dread to think of the flow rate at 3kW.
> >>>>>> < 1.5 lpm in the winter!
> >>>>>
> >>>>> nah, more than that
> >>>> Might even be less - depends on the ground water temp.
> >>>>
> >>>> Say incoming water is only 5 deg, and your target shower temp is 40
> >>>> (what I would call a warm shower rather than hot!), that is:
> >>>
> >>> 40 is excessive for a shower, 30 is ok.
> >>>
> >>>> (40-5) x 4200 = 147 kJ of energy required per litre.
> >>>>
> >>>> A 3kW heater will transfer 3000J x 60 sec = 180 kJ per min.
> >>>>
> >>>> 180 / 147 = 1.22 litres/min.
> >>>
> >>> from 10 to 30C is 20deg rise which permits 2.1lpm
> >> and still way short of even adequate.
> >
> > nonsense. The fashion for high flow showers is pointless.

> I think you need your flow rate recalibrated! High flow showers would be
> 15+ lpm - and possibly significantly more with larger soaker heads or
> body jets etc. Most folks idea of a "normal" shower would be something
> in the range 6 to 8 lpm.

I've lived with industrial style high flow showers, typical domestic showers and 3kW only. IME all are entirely adequate. I can also safely conclude the high flow ones had no real advantage.

> Very few people buy 7kW electric showers unless supply constraints rule
> out anything bigger, because they know the performance is so dire.
>
> Screwfix have ~125 electric showers listed, the lowest power ones being
> 7.5kW - and they only have 2 models at that output.

There are many things that become fashionable and often described as 'must have.' Many of course are not, and many are later accepted by society as a whole as not. If the obsession with green continues I expect we'll all have to get used to 1kW showers.

> >>>>>> Hot water circulation works ok if you have a cylinder of hot water and a
> >>>>>> well insulated pipe run - you just circulate water back to another
> >>>>>> it. Not sure how you would do it with a combi.
> >>
> >>>> and you still have the problem of how to get the combi to re-heat it
> >>>> since many have a maximum cold water inlet temperature that is not
> >>>> particularly high.
> >>>
> >>> The hw doesn't return to the inlet, it returns to the tank, whether that's an external tank or a baby one in the combi.
> >
> >> Unless it is specifically a storage combi (not that common in the UK),
> >> there is unlikely to be any way of connecting another inlet to its tank.
> >>
> >> With an external tank sure no problem...
> >
> > Heh, return the HW back into the same connection you get it from, microbore inside 22mm. I shouldn't need to pont out that the ubore should be a little longer.

> Perhaps I am missing something, but I can't wee what that would achieve
> with a combi.
>
> The idea of a circulation loop is to ensure that hot water is available
> at the point of use with no delay. With some stored hot water you can
> pump hot water from the cylinder, round the loop and back to another
> tapping a bit lower down the cylinder. You are circulating hot water
> past the point of use so it will be hot when required.
>
> If you are proposing that you tee into the DHW *output* of the combi,
> and pump back to that, all you will be doing is circulating cold water,
> since the combi will only fire when it sees an actual flow of hot water.

Some boilers include a tiny hot water tank which it keeps hot. Those are the type where you can add circulation this way.

On Saturday 3 February 2024 at 17:06:24 UTC, Andy Burns wrote:
> nibble wrote:
> > alan_m wrote:
> >
> >> In a hotel I stayed at the other day the thermostat (detent peg) was
> >> set to 35C which I found to be cool. I raised the control to 45C which
> >> was comfortable BUT I don't actually know the actual temperature of
> >> the water, just what the control indicated.
> >
> > Best bathwater is about 38 or 39C (by thermometer).
> Shower droplets cool pretty quickly in air

Some data I collected for comfortable temp showering:
Cold winter day in November: 34C shower head, 30 on tray surface.
Hot summer day: 36C shower head, 34C shower tray.

I like a perceived hotter shower in winter, cooler in summer, but the data shows once again the difference between factual temps & perception.

alan_m 3 Feb 2024 wrote:
> In a hotel I stayed at the other day the thermostat (detent peg) was set
> to 35C which I found to be cool. I raised the control to 45C which was
> comfortable BUT I don't actually know the actual temperature of the
> water, just what the control indicated.

It has become fashionable to mark showers in degrees C even when there is no connection between the temp markings & fact. It gives the impression of precision where no control, monitoring or precision exists. The Chinese are masters of creating false positive impressions of their products.

On 04/02/2024 21:18, Animal wrote:
> On Friday 2 February 2024 at 23:56:18 UTC, John Rumm wrote:
>> On 02/02/2024 22:06, Animal wrote:

>>> Heh, return the HW back into the same connection you get it from, microbore inside 22mm. I shouldn't need to pont out that the ubore should be a little longer.
>
>> Perhaps I am missing something, but I can't wee what that would achieve
>> with a combi.
>>
>> The idea of a circulation loop is to ensure that hot water is available
>> at the point of use with no delay. With some stored hot water you can
>> pump hot water from the cylinder, round the loop and back to another
>> tapping a bit lower down the cylinder. You are circulating hot water
>> past the point of use so it will be hot when required.
>>
>> If you are proposing that you tee into the DHW *output* of the combi,
>> and pump back to that, all you will be doing is circulating cold water,
>> since the combi will only fire when it sees an actual flow of hot water.
>
> Some boilers include a tiny hot water tank which it keeps hot. Those are the type where you can add circulation this way.

With a storage combi with a relatively large cylinder (40+ litres), you
probably could rig a circulation loop - but might need to make mods
inside the boiler if they cylinder does not already have a tapping to
allow a return connection.

For normal combis, I can see two problems. Firstly the cylinder is even
less accessible than that of a storage combi with less scope for adding
a return connection to it, and secondly, most of the combis that feature
this type of pre-heat don't do seem to do it thermostatically - they
just have a timed firing every so often to keep it warm. There are only
have a couple of litres of stored warm water - enough to partly offset
the warm up time of the boiler when someone turns a tap on.

If you are now going to start pumping that very limited amount of warm
water round the circulation loop, you are just going to dump all its
heat into the pipes of the loop, leaving your store of water tepid at
best. Since the boiler is not using closed loop control on the
temperature of that small cylinder, it will likely stay that way until
it does its next tempering cycle.

The only way I can see a circulation loop working with a normal combi
would be if you return the end of the circulation loop the the mains
cold inlet of the combi (after a double check valve in the cold mains
feed). The circulation pump can then cause a flow of mains cold through
the boiler that it can sense and fire in response to. You will be
returning increasingly hot water to the mains inlet which may be a
problem if the boiler has a maximum inlet water temperature specified.

You also have the issue that if the circulation pump is running on a
timer, it will force the combi into DHW mode all the time the pump is
running, and that will take priority over the CH - so no CH for the
duration even if the DHW has reached the max set point temp causing the
boiler to cycle the burner off to limit the temperature rise.

Perhaps a pump controlled from a single shot run on timer, that is
triggered from a PIR in the bathroom. So when you walk in, it triggers
the pump for a minute in the anticipation that hot water might be needed
soon.

--
Cheers,

John.

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

On Monday 5 February 2024 at 15:34:53 UTC, John Rumm wrote:
> On 04/02/2024 21:18, Animal wrote:
> > On Friday 2 February 2024 at 23:56:18 UTC, John Rumm wrote:
> >> On 02/02/2024 22:06, Animal wrote:
>
>
> >>> Heh, return the HW back into the same connection you get it from, microbore inside 22mm. I shouldn't need to pont out that the ubore should be a little longer.
> >
> >> Perhaps I am missing something, but I can't wee what that would achieve
> >> with a combi.
> >>
> >> The idea of a circulation loop is to ensure that hot water is available
> >> at the point of use with no delay. With some stored hot water you can
> >> pump hot water from the cylinder, round the loop and back to another
> >> tapping a bit lower down the cylinder. You are circulating hot water
> >> past the point of use so it will be hot when required.
> >>
> >> If you are proposing that you tee into the DHW *output* of the combi,
> >> and pump back to that, all you will be doing is circulating cold water,
> >> since the combi will only fire when it sees an actual flow of hot water.
> >
> > Some boilers include a tiny hot water tank which it keeps hot. Those are the type where you can add circulation this way.

> With a storage combi with a relatively large cylinder (40+ litres), you
> probably could rig a circulation loop - but might need to make mods
> inside the boiler if they cylinder does not already have a tapping to
> allow a return connection.

Using the standard output connection for return with microbore, no mod is needed inside the boiler or hot cylinder.

> For normal combis, I can see two problems. Firstly the cylinder is even
> less accessible than that of a storage combi with less scope for adding
> a return connection to it, and secondly, most of the combis that feature
> this type of pre-heat don't do seem to do it thermostatically - they
> just have a timed firing every so often to keep it warm. There are only
> have a couple of litres of stored warm water - enough to partly offset
> the warm up time of the boiler when someone turns a tap on.
>
> If you are now going to start pumping that very limited amount of warm
> water round the circulation loop, you are just going to dump all its
> heat into the pipes of the loop, leaving your store of water tepid at
> best. Since the boiler is not using closed loop control on the
> temperature of that small cylinder, it will likely stay that way until
> it does its next tempering cycle.

sure. Or until you turn the hot tap on.

> The only way I can see a circulation loop working with a normal combi
> would be if you return the end of the circulation loop the the mains
> cold inlet of the combi (after a double check valve in the cold mains
> feed). The circulation pump can then cause a flow of mains cold through
> the boiler that it can sense and fire in response to. You will be
> returning increasingly hot water to the mains inlet which may be a
> problem if the boiler has a maximum inlet water temperature specified.
>
> You also have the issue that if the circulation pump is running on a
> timer, it will force the combi into DHW mode all the time the pump is
> running, and that will take priority over the CH - so no CH for the
> duration even if the DHW has reached the max set point temp causing the
> boiler to cycle the burner off to limit the temperature rise.

yuck :)

> Perhaps a pump controlled from a single shot run on timer, that is
> triggered from a PIR in the bathroom. So when you walk in, it triggers
> the pump for a minute in the anticipation that hot water might be needed
> soon.