DIYbanter - Combi boilers, electric showers etc. revisited

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Combi boilers, electric showers etc. revisited

This has been somewhat covered in a couple of recent threads but they are
long so I'm trying to pick up on a few points.

(1) The maximum power output of a UK electric shower is around 10 kW. The
output of a small(ish) combi boiler is around 28 kW. So the combi should
be able to apply 2.8 times the heat to incoming cold water and so (I
assume) heat 2.8 times the amount of incoming water to an acceptable
temperature compared to the electric shower.

(2) Electricity costs around 3 times gas (per unit of energy) which kind
of balances out. You can either have about 3 times the hot water in your
shower for the same price or save money each time you shower at lower flow
rates.

I think most people would go for the more satisfying shower which does
reduce/eliminate the cost saving from using a combi boiler.

{Wanders off into speculation}

Hmmm....what is the maximum (ignoring wiring restrictions for the moment)
that you could get out of a UK domestic 2 phase supply?

Simplistically, using (Volts * Amps = Watts) and rounding outrageously 4
amps = 1 kW.

So again rounding outrageously a 13 Amp circuit could support a 3 kW fire,
for example.

If your mains incomer is 60 amps (a lot go up to 100 amps) then you might
be able to draw (60/4) = 15 kW is nothing else is powered up, or (100/4) =
25 kW for a 100 amp supply.

I realise there are all sorts of minor (hah!) constraints but if you had a
100 amp incomer then is there any reason that you couldn't (with some very
thick cabling) take 60 amps for a shower giving you 15 kW of toasty
goodness?

I do note that taking all the output from a 100 amp incomer still gives
you less heat input than a combi boiler.

3 phase electric shower, anyone?

Cheers

Dave R

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Combi boilers, electric showers etc. revisited

On Sunday, 28 April 2019 13:36:27 UTC+1, David WE Roberts (Google) wrote:
(1) The maximum power output of a UK electric shower is around 10 kW. The
output of a small(ish) combi boiler is around 28 kW. So the combi should
be able to apply 2.8 times the heat to incoming cold water and so (I
assume) heat 2.8 times the amount of incoming water to an acceptable
temperature compared to the electric shower.

Yes.

(2) Electricity costs around 3 times gas (per unit of energy) which kind
of balances out. You can either have about 3 times the hot water in your
shower for the same price or save money each time you shower at lower flow
rates.

Yes.

I think most people would go for the more satisfying shower which does
reduce/eliminate the cost saving from using a combi boiler.

Possibly, but the saving is only eliminated if you are using 3x the heat. You might in winter, but for the rest of the year only use 2x the heat. That would be a better shower than electric, and still a bit cheaper.

I realise there are all sorts of minor (hah!) constraints but if you had a
100 amp incomer then is there any reason that you couldn't (with some very
thick cabling) take 60 amps for a shower giving you 15 kW of toasty
goodness?

(a) they don't make 15 kW showers [1]
(b) you'd have to get permission from (or at least notify) from your DNO if you got one

I do note that taking all the output from a 100 amp incomer still gives
you less heat input than a combi boiler.
3 phase electric shower, anyone?

3-phase common in European countries, but max demand is usually more limited than here.

[1] But you can get 3-phase instantaneous water heaters eg

https://www.aquahot.co.uk/stiebel-el...er-heater-27kw

https://www.stiebel-eltron.co.uk/en/...r-heating.html

"45% of new build properties in the UK are city centre flats and apartments.. For most of these electricity is the preferred fuel. And for many years the default solution for hot water for each new appartment has been an unvented cylinder heated by 2 x 3kW electric immersion heaters. By its very nature this solution takes up a lot of space.
... 3 phase electric supply, which has not traditionally been run to individual domestic units in the UK.
But market conditions in the UK are now changing. Space is more than ever at a premium. Instantaneous water heaters on a 3 phase supply give the kind of powerful, limitless hot water supply that young city dwellers look for.
New build apartments have 400V provided to each floor in the riser. Currently this is then split down into a phase per apartment. By taking the 400V to each apartment the instantaneous solution becomes possible."

(Would need 3-phase metering though.)

Owain

Combi boilers, electric showers etc. revisited

On 28/04/2019 13:36, David wrote:

This has been somewhat covered in a couple of recent threads but they are
long so I'm trying to pick up on a few points.

(1) The maximum power output of a UK electric shower is around 10 kW. The

There are some at around 11kW

output of a small(ish) combi boiler is around 28 kW. So the combi should

24kW is a common size for a small one.

be able to apply 2.8 times the heat to incoming cold water and so (I
assume) heat 2.8 times the amount of incoming water to an acceptable
temperature compared to the electric shower.

Pretty much - it a linear relationship. See the table he

http://wiki.diyfaq.org.uk/index.php/...ting_the_Power

(2) Electricity costs around 3 times gas (per unit of energy) which kind
of balances out. You can either have about 3 times the hot water in your
shower for the same price or save money each time you shower at lower flow
rates.

I think most people would go for the more satisfying shower which does
reduce/eliminate the cost saving from using a combi boiler.

Probably... Still there are plenty of people happy with the performance
of their electric showers. They also have some advantages like being
less affected by people running hot water elsewhere in the house.

{Wanders off into speculation}

Hmmm....what is the maximum (ignoring wiring restrictions for the moment)
that you could get out of a UK domestic 2 phase supply?

Do you actually mean 2 phase (since that is not a common option). Single
phase or three phase would be a more common choice.

For single phase with a 100A incomer, about 24kW total.

Simplistically, using (Volts * Amps = Watts) and rounding outrageously 4
amps = 1 kW.

So again rounding outrageously a 13 Amp circuit could support a 3 kW fire,
for example.

Yup.

If your mains incomer is 60 amps (a lot go up to 100 amps) then you might
be able to draw (60/4) = 15 kW is nothing else is powered up, or (100/4) =
25 kW for a 100 amp supply.

Yup.

I realise there are all sorts of minor (hah!) constraints but if you had a
100 amp incomer then is there any reason that you couldn't (with some very
thick cabling) take 60 amps for a shower giving you 15 kW of toasty
goodness?

You could...

Perhaps pairing up a couple of lower powered showers in parallel,
feeding a common shower head.

I do note that taking all the output from a 100 amp incomer still gives
you less heat input than a combi boiler.

Keep in mind you can vary the demand dramatically by variation of she
spray pattern, and jet size, as these will have a large effect on the
actual flow rate required.

3 phase electric shower, anyone?

Here you go:

https://www.aquahot.co.uk/zip-dbx24-...er-heater-24kw

--
Cheers,

John.

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Combi boilers, electric showers etc. revisited

On 28/04/2019 16:08, John Rumm wrote:

Here you go:

https://www.aquahot.co.uk/zip-dbx24-...er-heater-24kw

The IP25 rating is a bit odd

The 2 meaning that you can stick a small child's finger in it and the 5
meaning that it is protected against water jets from all angles.

--
Adam

Combi boilers, electric showers etc. revisited

On 28/04/2019 18:07, ARW wrote:
On 28/04/2019 16:08, John Rumm wrote:

Here you go:

https://www.aquahot.co.uk/zip-dbx24-...er-heater-24kw

The IP25 rating is a bit odd

The 2 meaning that you can stick a small child's finger in it and the 5
meaning that it is protected against water jets from all angles.

Yup, I had not spotted that - it does sound a bit suspect!

--
Cheers,

John.

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

Combi boilers, electric showers etc. revisited

On 29/04/2019 10:39, John Rumm wrote:
On 28/04/2019 18:07, ARW wrote:
On 28/04/2019 16:08, John Rumm wrote:

Here you go:

https://www.aquahot.co.uk/zip-dbx24-...er-heater-24kw

The IP25 rating is a bit odd

The 2 meaning that you can stick a small child's finger in it and the
5 meaning that it is protected against water jets from all angles.

Yup, I had not spotted that - it does sound a bit suspect!

I think that's IP25 for the outer case (giving assurance that can cope
with showers but that "small stuff" could get inside). The electrical
connection is in a separate enclosure inside with sleeve for the cable.

https://d3mxxf350gpf35.cloudfront.net/zipwater.uk/zipwater.uk.manuals/manuals.051218/Hot+Water/Instantaneous/DBX+DEX+instructions+9120-34373+v4.00+Nov+2018.pdf

(Yes, I am sadly displacing.)

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