To cut out summertime natural gas use my daughter wants to have DHW
heated by electricity.

Her combi boiler is in the loft and there is room for a 100ltr cylinder
in an airing cupboard or the loft.

Is it possible to have them work together or must the water be switched
between winter and summer running?

On 19/04/2024 09:23, ajh wrote:
> To cut out summertime natural gas use my daughter wants to have  DHW
> heated by electricity.
>
Extraordinary. End up paying more and emitting more CO2.

What's to like?

> Her combi boiler is in the loft and there is room for a 100ltr cylinder
> in an airing cupboard or the loft.
>
> Is it possible to have them work together or must the water be switched
> between winter and summer running?

If the combi is actually heating a hot water TANK, then no reason not to
simply switch on the leccy full time.

Combi will kick in when its trimer is on and water is cold,

--
Gun Control: The law that ensures that only criminals have guns.

On 19/04/2024 10:23, The Natural Philosopher wrote:
> End up paying more and emitting more CO2.

No additional CO2 emitted

On 19/04/2024 09:23, ajh wrote:

> To cut out summertime natural gas use my daughter wants to have  DHW
> heated by electricity.

ok.. might not be everyone's first choice :-)

> Her combi boiler is in the loft and there is room for a 100ltr cylinder
> in an airing cupboard or the loft.
>
> Is it possible to have them work together or must the water be switched
> between winter and summer running?

It can work, but you will need an external valve, a cylinder stat, a
dual channel timer, and some wiring changes.

Probably easiest / most efficient configuration would be a W plan
system. This will give hot water priority operation, and allow the full
output of the boiler to be directed to the cylinder. Unvented cylinders
tend to have large indirect heating coils, ans so can be very fast
recovery.

(this assumes that the combi does not have weather compensation - since
that would complicate it a bit)

The DHW maximum temp would be limited by the flow temp set on the
boilers CH temperature setting, *not* its DHW temp setting. The set temp
would be controlled by the cylinder stat.

--
Cheers,

John.

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

On 19/04/2024 12:09, ajh wrote:
> On 19/04/2024 10:23, The Natural Philosopher wrote:
>> End up paying more and emitting more CO2.
>
> No additional CO2 emitted

Additional to what?

Do you think that electricity is clean and doesn't emit any CO2?

On 19/04/2024 09:23, ajh wrote:
> To cut out summertime natural gas use my daughter wants to have  DHW
> heated by electricity.
>
> Her combi boiler is in the loft and there is room for a 100ltr cylinder
> in an airing cupboard or the loft.
>
> Is it possible to have them work together or must the water be switched
> between winter and summer running?

Day time electricity is extortionate, plus the heat losses from the
tank. I suggest you do your sums, which may only add up if you use night
time rates and then of course pay more during the day!

At the moment gas is king.

Fredxx <fredxx@spam.invalid> wrote:
> On 19/04/2024 12:09, ajh wrote:
> > On 19/04/2024 10:23, The Natural Philosopher wrote:
> >> End up paying more and emitting more CO2.
> >
> > No additional CO2 emitted
>
> Additional to what?
>
> Do you think that electricity is clean and doesn't emit any CO2?

Right now, the carbon intensity is 41 gCO2/kWh here (east of England). The
current spread goes from 0 g (north Scotland) to 94 g (south west England).

Natural gas including distribution[1] has 202 gCO2/kWh when burnt.

So yes right now the electricity is cleaner than burning gas, and so
switching to electricity is carbon negative.

Obviously the gCO2 figure varies with time, but in a lot of the country it's
less than 202 most of the time. Northern Scotland is pretty much zero
carbon all the time nowadays.

Theo

[1] https://www.forestresearch.gov.uk/tools-and-resources/fthr/biomass-energy-resources/reference-biomass/facts-figures/carbon-emissions-of-different-fuels/

John Rumm <see.my.signature@nowhere.null> wrote:
> On 19/04/2024 09:23, ajh wrote:
>
> > To cut out summertime natural gas use my daughter wants to have  DHW
> > heated by electricity.
>
> ok.. might not be everyone's first choice :-)

Could make sense if you have solar, so you can divert generation into the
cylinder immersion for 'free' hot water.

> Probably easiest / most efficient configuration would be a W plan
> system. This will give hot water priority operation, and allow the full
> output of the boiler to be directed to the cylinder. Unvented cylinders
> tend to have large indirect heating coils, ans so can be very fast
> recovery.

By 'large' do you mean 'long' or 'wide'?

Thinking that if you're going to install a cylinder you might want it to be
heat pump ready, which means having a long coil to extract maximum heat from
a low-temp flow input. Not sure how compatible that would be with a
low-flow high-temp output from a combi, or would the end result be similar?

I'd likely want more than a 100 litre tank though: one tankful would be just
enough for a bath, give or take. If you had a tank of hot water and emptied
it, wouldn't you then have a tank of cold water with the combi frantically
trying to reheat it - not ideal for that to happen mid bath, although it
would probably be hot again after 10 mins or whatever.

How would the boiler know to fire up when the tank got cold, would it need a
thermostat in the tank?

> The DHW maximum temp would be limited by the flow temp set on the
> boilers CH temperature setting, *not* its DHW temp setting. The set temp
> would be controlled by the cylinder stat.

Wouldn't that mean condensing takes a hit, if your flow temp now has to be
60C or whatever, rather than a lower more efficient flow?

Theo

In article <uvtqpj$30som$1@dont-email.me>,
Fredxx <fredxx@spam.invalid> wrote:
> On 19/04/2024 12:09, ajh wrote:
> > On 19/04/2024 10:23, The Natural Philosopher wrote:
> >> End up paying more and emitting more CO2.
> >
> > No additional CO2 emitted

> Additional to what?

> Do you think that electricity is clean and doesn't emit any CO2?

when it comes froma wind farm or solar panels -it doesn't - at the time of
use.

--
from KT24 in Surrey, England - sent from my RISC OS 4té²
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

On 19/04/2024 14:15, Fredxx wrote:
> On 19/04/2024 12:09, ajh wrote:
>> On 19/04/2024 10:23, The Natural Philosopher wrote:
>>> End up paying more and emitting more CO2.
>>
>> No additional CO2 emitted
>
> Additional to what?
>

The embedded energy that was used to get the PV panels on her roof,
inverter etc..
>
>

On 19/04/2024 15:53, Theo wrote:
> John Rumm <see.my.signature@nowhere.null> wrote:
>> On 19/04/2024 09:23, ajh wrote:
>>
>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>> heated by electricity.
>>
>> ok.. might not be everyone's first choice :-)
>
> Could make sense if you have solar, so you can divert generation into the
> cylinder immersion for 'free' hot water.

Yes this is the case, nearly every day from now until October there will
be surplus over her household use and probably her car charging.

John's helpful solution presupposes she will want to heat the tank with
gas at some stage. This is unlikely and the energy is better stored as
gas rather than hot water. In winter the combi boiler will provide DHW
and CH without the need for the hot water cylinder.

It would need to be a very well insulated tank in case it needed topping
up with cheap rate electricity to coincide with car charging, which is
sometimes needed after a long journey. Apart from heat loss and over
capacity the gas at 6p/kWh for instant hot water or heating a cylinder
at 7.5p/kWh should be about the same.I think that would be a maximum of
8kWh/day if the tank of 120ltr was used up each day.

I wonder what the heat loss would be over 24 hours.

So the system would be electrically heated cylinder OR the combi.

The extra cost of a heat pump/cylinder combination doesn't seem worth it
compared with an immersion element, though a clever one that heated from
the top down like I have would be better.
>
>
> I'd likely want more than a 100 litre tank though:

Three person household, it looks like 40ltr per person is about right,
so maybe a bigger for resilience is worthwhile.

ajh <news@loampitsfarm.co.uk> wrote:
> On 19/04/2024 15:53, Theo wrote:
> > Could make sense if you have solar, so you can divert generation into the
> > cylinder immersion for 'free' hot water.
> John's helpful solution presupposes she will want to heat the tank with
> gas at some stage. This is unlikely and the energy is better stored as
> gas rather than hot water. In winter the combi boiler will provide DHW
> and CH without the need for the hot water cylinder.

So would you want a setup where you can draw water from the cylinder or from
the combi, but not use both together. ie some kind of switch to change from
one to the other? Maybe you could do that with a 3 port valve and a tank
stat: when the cylinder is hot, use that. When it's not, use the combi.
Solar puts energy into the cylinder but will carry on heating it without
drawing while it's lukewarm.

The downside of that is aren't using any of that solar heat until the water
is 'hot enough', ie you're not using it as a preheat for the boiler on days
the solar is weak. Maybe the amount of energy wasted there is small enough
to not matter.

> It would need to be a very well insulated tank in case it needed topping
> up with cheap rate electricity to coincide with car charging, which is
> sometimes needed after a long journey. Apart from heat loss and over
> capacity the gas at 6p/kWh for instant hot water or heating a cylinder
> at 7.5p/kWh should be about the same.I think that would be a maximum of
> 8kWh/day if the tank of 120ltr was used up each day.
>
> I wonder what the heat loss would be over 24 hours.
>
> So the system would be electrically heated cylinder OR the combi.
>
> The extra cost of a heat pump/cylinder combination doesn't seem worth it
> compared with an immersion element, though a clever one that heated from
> the top down like I have would be better.

I wouldn't suggest a heat pump at this point, just that when the boiler
needs replacing and a heat pump makes sense, it would be sad if you had to
replace the cylinder as well. Spend a little now on a better
cylinder and leave you options open in future.

> > I'd likely want more than a 100 litre tank though:
>
> Three person household, it looks like 40ltr per person is about right,
> so maybe a bigger for resilience is worthwhile.

Where's your 40 litre figure coming from?

This:
https://mcdonaldwaterstorage.com/hot-water-cylinder-sizes/

says 75-80 litres to run a bath. Presumably that's mixed down with cold,
but if you allow a lower tank temperature say 45C (heat pump output or weak
solar generation) then you need more hot water. Demand is more if multiple
draws are happening at once.

The problem with averages like 40 litres per person per day, but you might
have a whole bath every few days, not a third of a bath per day. So you
need a cylinder enough for peak demand.

I suppose if your answer to peak demand is just to run the combi then that
works, as long as you always have the combi.

Theo

On 19/04/2024 15:53, Theo wrote:
> John Rumm <see.my.signature@nowhere.null> wrote:
>> On 19/04/2024 09:23, ajh wrote:
>>
>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>> heated by electricity.
>>
>> ok.. might not be everyone's first choice :-)
>
> Could make sense if you have solar, so you can divert generation into the
> cylinder immersion for 'free' hot water.
>
>> Probably easiest / most efficient configuration would be a W plan
>> system. This will give hot water priority operation, and allow the full
>> output of the boiler to be directed to the cylinder. Unvented cylinders
>> tend to have large indirect heating coils, ans so can be very fast
>> recovery.
>
> By 'large' do you mean 'long' or 'wide'?

Long typically - many turns of pipe. A traditional cylinder had an
indirect heating coil that could shift around 5kW tops. Hence why Y plan
and gravity circulation was popular, you could trickle heat into the
cylinder at the same time as running the rads. Doing the cylinder in
isolation would have meant much short cycling.

A modern fast recovery cylinder may be able to absorb 20kW for a good
part of the reheat.

> Thinking that if you're going to install a cylinder you might want it to be
> heat pump ready, which means having a long coil to extract maximum heat from
> a low-temp flow input. Not sure how compatible that would be with a
> low-flow high-temp output from a combi, or would the end result be similar?

You can have cylinders with multiple coils designed for things like
solar thermal. So they can take advantage of lower temp heat sources.

> I'd likely want more than a 100 litre tank though: one tankful would be just
> enough for a bath, give or take. If you had a tank of hot water and emptied
> it, wouldn't you then have a tank of cold water with the combi frantically
> trying to reheat it - not ideal for that to happen mid bath, although it
> would probably be hot again after 10 mins or whatever.

If you set your control system to bring in the boiler as the temp starts
dropping, then you should be able to get more out of the cylinder than
volume alone will allow.

It can be worth playing with the numbers as described here:

https://wiki.diyfaq.org.uk/index.php/Sizing_a_hot_water_cylinder

you can see what volume of "mix temperature" water you can get out of
cylinders based on the variables.

> How would the boiler know to fire up when the tank got cold, would it need a
> thermostat in the tank?

Yes. See the W plan wiring and plumbing scheme that I linked to. The hot
water would be controlled by both timer and cylinder stat. When both
indicate a call for heat - that is send to the boiler, and the diversion
valve directs the boiler output to the indirect coil in the cylinder.
When that is satisfied (or the time slow expires on the timer). The
boiler will revert to control by the timer and room stat (or possibly
prog stat acting as both)

>> The DHW maximum temp would be limited by the flow temp set on the
>> boilers CH temperature setting, *not* its DHW temp setting. The set temp
>> would be controlled by the cylinder stat.
>
> Wouldn't that mean condensing takes a hit, if your flow temp now has to be
> 60C or whatever, rather than a lower more efficient flow?
If you set your flow temp at say 70 deg, then allowing for a 20 degree
drop across the rads, that would keep the return temp around 50. That
should still get reasonable condensing efficiency on the CH. When doing
a DHW reheat, the efficiency would fall as the cylinder temp gets closer
to its set point temp - assuming you have it at 60 degrees.

--
Cheers,

John.

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

On 19/04/2024 17:59, ajh wrote:
> On 19/04/2024 15:53, Theo wrote:
>> John Rumm <see.my.signature@nowhere.null> wrote:
>>> On 19/04/2024 09:23, ajh wrote:
>>>
>>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>>> heated by electricity.
>>>
>>> ok.. might not be everyone's first choice :-)
>>
>> Could make sense if you have solar, so you can divert generation into the
>> cylinder immersion for 'free' hot water.
>
>
> Yes this is the case, nearly every day from now until October there will
> be surplus over her household use  and probably her car charging.
>
> John's helpful solution presupposes she will want to heat the tank with
> gas at some stage.

Sorry, I misread the intent.

If electric only, then you would need a "direct" rather than indirect
(unless you want the option of heat pump or solar thermal inputs)

> This is unlikely and the energy is better stored as
> gas rather than hot water. In winter the combi boiler will provide DHW
> and CH without the need for the hot water cylinder.

If you only want electric heating on the cylinder, then indeed you could
instead have manual valves to switch the DHW feed to the property from
the output of the cylinder to the DHW output of the combi. You might be
able to achieve some automation with motor actuated values (but not the
CH system type)

(picking a larger cylinder might make sense since the reheat will be
much slower on immersion(s))

> It would need to be a very well insulated tank in case it needed topping
> up with cheap rate electricity to coincide with car charging, which is
> sometimes needed after a long journey. Apart from heat loss and over
> capacity the gas at 6p/kWh for instant hot water or heating a cylinder
> at 7.5p/kWh should be about the same.I think that would be a maximum of
> 8kWh/day if the tank of 120ltr was used up each day.
>
> I wonder what the heat loss would be over 24 hours.

The cylinder makers, normally quote this figure in their specs. I think
the loss from my 210L cylinder is around 60W. Their insulation is
normally pretty good - most loss will be from the parts that pierce the
outer case.

(it might not always be considered a "loss" if you want some warmth for
an airing cupboard / drying room etc)

>
> So the system would be electrically heated cylinder OR the combi.
>
> The extra cost of a heat pump/cylinder combination doesn't seem worth it
> compared with an immersion element, though a clever one that heated from
> the top down like I have would be better.
>>
>>
>> I'd likely want more than a 100 litre tank though:
>
> Three person household, it looks like 40ltr per person is about right,
> so maybe a bigger for resilience is worthwhile.

If you want two showers in sequence, then you may want more. A couple of
immersions will only be able to reheat at 6kW - the output you would
expect from a very poor electric shower!

--
Cheers,

John.

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

On 19/04/2024 18:25, Theo wrote:

>
> So would you want a setup where you can draw water from the cylinder or from
> the combi, but not use both together. ie some kind of switch to change from
> one to the other? Maybe you could do that with a 3 port valve and a tank
> stat: when the cylinder is hot, use that. When it's not, use the combi.
> Solar puts energy into the cylinder but will carry on heating it without
> drawing while it's lukewarm.

Yes but if the tank could be heated top down then the available water
should all be at the high temperature even if the whole cylinder is not
heated.
>
> The downside of that is aren't using any of that solar heat until the water
> is 'hot enough', ie you're not using it as a preheat for the boiler on days
> the solar is weak. Maybe the amount of energy wasted there is small enough
> to not matter.

Can a combi be run in series with its water feed preheated by a hot
water tank? What if the combi only supplies 50C water and the tank were
at 70C?
>
> I wouldn't suggest a heat pump at this point, just that when the boiler
> needs replacing and a heat pump makes sense, it would be sad if you had to
> replace the cylinder as well. Spend a little now on a better
> cylinder and leave you options open in future.

An air source heat pump doesn't fit well at the moment. It would be
different if the house had underfloor heating and the screed had enough
thermal inertia, then it may be possible to store enough heat in the
floor during a cheap rate but it would clash with car charging some
winter days. Even putting 7kW into a heat pump with a COP of 3 would be
limited to 4 hours and the house may be cosy at 5:00 but less so at
20:00. She did use over 30kWh of gas on a few days this last winter.

> Where's your 40 litre figure coming from?

looked it up on a .gov.uk site.

ajh <news@loampitsfarm.co.uk> wrote:
> On 19/04/2024 15:53, Theo wrote:
>> John Rumm <see.my.signature@nowhere.null> wrote:
>>> On 19/04/2024 09:23, ajh wrote:
>>>
>>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>>> heated by electricity.
>>>
>>> ok.. might not be everyone's first choice :-)
>>
>> Could make sense if you have solar, so you can divert generation into the
>> cylinder immersion for 'free' hot water.
>
>
> Yes this is the case, nearly every day from now until October there will
> be surplus over her household use and probably her car charging.

Good if you’re on an old FIT scheme where you get paid to generate rather
than just export.

Nowadays if you’re not on a FIT scheme and if you have an EV you can often
get paid more to export than import costs during the night.

I get paid 15 per kWhr to export at any time but only pay 7.5p per kWhr to
heat my hot water at night so using “spare” solar to heat my water during
the day doesn’t make any economic sense.

This also applies to EV charging from solar. If you can charge cheaper
from the grid at night for significantly less than you get paid for
exporting to the grid solar charging becomes redundant.

Tim

--
Please don't feed the trolls

On 19/04/2024 19:42, John Rumm wrote:

> If you only want electric heating on the cylinder, then indeed you could
> instead have manual valves to switch the DHW feed to the property from
> the output of the cylinder to the DHW output of the combi. You might be
> able to achieve some automation with motor actuated values (but not the
> CH system type)

As it would only be changed for the summer this should not be a problem.
Does a combi hot water outlet have a non return valve? Would the change
over involve shutting off the feed to the combi and diverting it to the
new hot water cylinder?

> The cylinder makers, normally quote this figure in their specs. I think
> the loss from my 210L cylinder is around 60W. Their insulation is
> normally pretty good - most loss will be from the parts that pierce the
> outer case.

That's reasonable. My cylinder seems to lose most heat from the top
mounted immersion, which cannot be insulated for safety reasons, so a
bottom mounted immersion feeding hot water directly to the top and
remaining stratified would be ideal, I think one of the built in air to
water heat pump-cylinders does this. My Willis does the same but is not
integrated.

> If you want two showers in sequence, then you may want more. A couple of
> immersions will only be able to reheat at 6kW - the output you would
> expect from a very poor electric shower!

This depends on the length of shower, I guess a shower is at ~40C and
the water could be stored at 70C. I easily get a shower out of the top
30 litres of my tank.
>

On 19/04/2024 20:25, Tim+ wrote:
>
> Good if you’re on an old FIT scheme where you get paid to generate rather
> than just export.

Whole set up was commissioned March 23 so no grants or subsidies.

> Nowadays if you’re not on a FIT scheme and if you have an EV you can often
> get paid more to export than import costs during the night.

Yes, this does cause a quandary but I do not expect the high export rate
and the cheap 6+hours of cheap rate to last, currently the cheapest
rates and high export are not available to her because of car/charger
compatibility problems.
>
> I get paid 15 per kWhr to export at any time but only pay 7.5p per kWhr to
> heat my hot water at night so using “spare” solar to heat my water during
> the day doesn’t make any economic sense.
> This also applies to EV charging from solar. If you can charge cheaper
> from the grid at night for significantly less than you get paid for
> exporting to the grid solar charging becomes redundant.

I know but...

It can also mean that one does not need PV panels to make the most of
it, just a car with power to home capability.

ajh <news@loampitsfarm.co.uk> wrote:
> On 19/04/2024 20:25, Tim+ wrote:
>>
>> Good if you’re on an old FIT scheme where you get paid to generate rather
>> than just export.
>
> Whole set up was commissioned March 23 so no grants or subsidies.
>
>> Nowadays if you’re not on a FIT scheme and if you have an EV you can often
>> get paid more to export than import costs during the night.
>
> Yes, this does cause a quandary but I do not expect the high export rate
> and the cheap 6+hours of cheap rate to last, currently the cheapest
> rates and high export are not available to her because of car/charger
> compatibility problems.

She should look at Eon Next Drive. Only need a working smart meter giving
half hourly data. Works with any car. Rates are currently better than
Octopus’s.

Tim

--
Please don't feed the trolls

On 19/04/2024 15:53, Theo wrote:
> John Rumm <see.my.signature@nowhere.null> wrote:
>> On 19/04/2024 09:23, ajh wrote:
>>
>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>> heated by electricity.
>>
>> ok.. might not be everyone's first choice :-)
>
> Could make sense if you have solar, so you can divert generation into the
> cylinder immersion for 'free' hot water.

<snip>

Not a suggestion, I know little of this, but would it be possible to
charge a battery and then use an instant hot water heater? I realise
that would have to probably be upwards of 20kW. Maybe even pre-heat the
water using a battery/inverter cooling system.

--
Cheers
Clive

On 19/04/2024 23:13, Clive Arthur wrote:

>
> Not a suggestion, I know little of this, but would it be possible to
> charge a battery and then use an instant hot water heater?  I realise
> that would have to probably be upwards of 20kW.  Maybe even pre-heat the
> water using a battery/inverter cooling system.

The scale of heat required is of the wrong order for a battery.

In my case I have a median use of 8kWh of electricity and I have about
6.5kWh of battery, usable 5.7kWh at a round trip efficiency of about
80%. That's okay for lighting and TV of an evening and cooking a meal
for two.

Storing 8kWh as heat in a tank is a lot cheaper than increasing battery
size or installing a heat pump.

On 19/04/2024 20:29, ajh wrote:
> On 19/04/2024 19:42, John Rumm wrote:
>
>> If you only want electric heating on the cylinder, then indeed you
>> could instead have manual valves to switch the DHW feed to the
>> property from the output of the cylinder to the DHW output of the
>> combi. You might be able to achieve some automation with motor
>> actuated values (but not the CH system type)
>
> As it would only be changed for the summer this should not be a problem.
> Does a combi hot water outlet have a non return valve?

Depends on the combi I expect -easy enough to add one if required.

> Would the change
> over involve shutting off the feed to the combi and diverting it to the
> new hot water cylinder?

I would just shut off the output of it instead rather than leave the
plate heat exchanger empty / at low pressure.

>> The cylinder makers, normally quote this figure in their specs. I
>> think the loss from my 210L cylinder is around 60W. Their insulation
>> is normally pretty good - most loss will be from the parts that pierce
>> the outer case.
>
> That's reasonable. My cylinder seems to lose most heat from the top
> mounted immersion, which cannot be insulated for safety reasons, so a
> bottom mounted immersion feeding hot water directly to the top and
> remaining stratified would be ideal, I think one of the built in air to
> water heat pump-cylinders does this. My Willis does the same but is not
> integrated.
>
>> If you want two showers in sequence, then you may want more. A couple
>> of immersions will only be able to reheat at 6kW - the output you
>> would expect from a very poor electric shower!
>
> This depends on the length of shower, I guess a shower is at ~40C and
> the water could be stored at 70C. I easily get a shower out of the top
> 30 litres of my tank.
>>
>

--
Cheers,

John.

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

On 4/19/2024 6:13 PM, Clive Arthur wrote:
> On 19/04/2024 15:53, Theo wrote:
>> John Rumm <see.my.signature@nowhere.null> wrote:
>>> On 19/04/2024 09:23, ajh wrote:
>>>
>>>> To cut out summertime natural gas use my daughter wants to have  DHW
>>>> heated by electricity.
>>>
>>> ok.. might not be everyone's first choice :-)
>>
>> Could make sense if you have solar, so you can divert generation into the
>> cylinder immersion for 'free' hot water.
>
> <snip>
>
> Not a suggestion, I know little of this, but would it be possible to charge a battery and then use an instant hot water heater?  I realise that would have to probably be upwards of 20kW.  Maybe even pre-heat the water using a battery/inverter cooling system.
>

It's an amusing exercise. It's like planning an installation
for the "yard" at Dow Chemical.

Powerwall 2 Powerwall 3

Energy Capacity 13.5 kWh* 13.5 kWh

On-Grid Power 5 kW continuous 11.5 kW continuous

Backup Power 7 kW peak 11.5 kW continuous
106A LRA motor start 185 LRA motor start
Seamless backup transition Seamless backup transition

Up to 10 units Up to 4 units

It would depend on how long you expect to draw this 20kW. When assembling
this thing, I've tried to arrange the pack discharge time to 3 hours for the packs,
to limit stress on the batteries. Since the load is non-inductive,
the inverter might not blow up. Even though an inverter has a
Locked Rotor Amps rating, inverters do not really "like" running
inductive loads, or handling the back-EMF from such loads.
You would not try to actually *use* the 185 LRA, maybe a single
furnace air handler (2HP motor) would be as far as I would go. The Tesla
inverter might be orders better than a similarly rated Chineseum inverter.
When there is no way of knowing what you're getting, it adds to the "excitement"
at the 20kW level.

(+)----(+)----(+)----+
| | | |
PW3 PW3 PW3 Inverter ---> 11.5kW continuous
| | | | 40kWh
(-)----(-)----(-)----+ 3 hour run time (charge batt 80% not 100%)

(+)----(+)----(+)----+
| | | |
PW3 PW3 PW3 Inverter ---> 11.5kW continuous
| | | | 40kWh
(-)----(-)----(-)----+ 3 hour run time (charge batt 80% not 100%)

£7,800 * 6 + two inverters = £48,000 + £10,000 in approx round numbers.

And you're splitting the electrical load into two pieces.
As far as I know, the PW inverter applies to the whole bank (of 3 in example),
rather than each pack "load sharing" and increasing output radically.

Your solar array in this case, could store 80kWh * 0.8 = 64kWh (get 5000 charge cycles max)

And now, you're capturing a lot of your solar output. You could even export (some of it) :-)

You will likely get a much better quote locally.
You could tighten up the project as you saw fit.

Paul

Clive Arthur <clive@nowaytoday.co.uk> wrote:
> Not a suggestion, I know little of this, but would it be possible to
> charge a battery and then use an instant hot water heater? I realise
> that would have to probably be upwards of 20kW. Maybe even pre-heat the
> water using a battery/inverter cooling system.

If you do that you're going to be limited by the output of your inverter. A
typical domestic inverter is say 3-6kW, so you'd need several. A domestic
battery may not be designed to produce enough current (20kW = 416A @ 48V) if
it's tailored for a 6kW load, so you'd then need several packs. As Paul
suggests, a trio of 7kW peak Powerwalls would do it, if you don't mind them
being £5k+installation each.

More efficient would be to heat the instant hot water from the DC battery
voltage directly. Most EVs are much more than 20kW so an EV pack plus a big
water-cooled resistor would work, but that's all custom engineering. The
safety system on what's effectively a DIY DC electric shower would be
interesting.

Theo

ajh <news@loampitsfarm.co.uk> wrote:
> On 19/04/2024 18:25, Theo wrote:
>
> >
> > So would you want a setup where you can draw water from the cylinder or from
> > the combi, but not use both together. ie some kind of switch to change from
> > one to the other? Maybe you could do that with a 3 port valve and a tank
> > stat: when the cylinder is hot, use that. When it's not, use the combi.
> > Solar puts energy into the cylinder but will carry on heating it without
> > drawing while it's lukewarm.
>
> Yes but if the tank could be heated top down then the available water
> should all be at the high temperature even if the whole cylinder is not
> heated.

If you had a switch, you'd have to work out where in the cylinder to put the
sensor - maybe putting it at the top of the cylinder would be enough so that
you'd use any stored hot water, and then switch to combi when it got cold?
Or maybe it would oscillate when water was drawn and heated at the same
time?

> > The downside of that is aren't using any of that solar heat until the water
> > is 'hot enough', ie you're not using it as a preheat for the boiler on days
> > the solar is weak. Maybe the amount of energy wasted there is small enough
> > to not matter.
>
> Can a combi be run in series with its water feed preheated by a hot
> water tank? What if the combi only supplies 50C water and the tank were
> at 70C?

I think there's a limit to how much a combi can modulate down, ie if takes
20kW for 10C water to make 60C water, will it go down to 4kW when starting
from 50C water?

I'd guess the safety interlock would refuse to heat if the input temperature
was out of range. You might end up with the input water being 'too hot'
while the output is 'too cold' - eg if the input was 35C that could be too
high for the boiler to heat but not enough for bathing.

> An air source heat pump doesn't fit well at the moment. It would be
> different if the house had underfloor heating and the screed had enough
> thermal inertia, then it may be possible to store enough heat in the
> floor during a cheap rate but it would clash with car charging some
> winter days. Even putting 7kW into a heat pump with a COP of 3 would be
> limited to 4 hours and the house may be cosy at 5:00 but less so at
> 20:00. She did use over 30kWh of gas on a few days this last winter.

I don't see why you'd try to only run it during the middle of the night -
that's going to be a terrible way to heat a house. At normal rate
electricity after the COP it's roughly in line with the same price of gas,
so it would be more or less neutral in terms of running costs. Of course
you can optimise that with fancier tariffs (eg Octopus Tracker which
gives you a daily market price ~15-20p rather than the standard 25p, or time
of use tariffs) but even the standard price cap tariffs are about break
even. It depends a bit how much you drive, whether the car usage or the
heating usage is greater.

> > Where's your 40 litre figure coming from?
>
> looked it up on a .gov.uk site.

I'd do the calculations for your specific needs - how many baths / showers /
sinks / etc you have. John's wiki page is a good one:
https://wiki.diyfaq.org.uk/index.php/Sizing_a_hot_water_cylinder

Theo