I need help in calculating the size of a possible heatbank store.

There are two main drivers behind this - one is that the house is
single storey so there's minimal pressure in the DHW system, and the
second is that we are rural and don't have gas; the CH boiler is a
10yr old oil burner (15kw) - perfectly effective but the radiators
heat up rather slowly so I assume that it doesn't produce it's heat
that rapidly! There's a log burner for winter assistance to the CH/
DHW system via a Dunsley Neutraliser. The roof is long and facing
south so would be suitable for solar panels if I wanted to go further.

So my thinking is to have a store that will supply the DHW at mains
pressure, and will act as a buffer for the CH system until the boiler
gets up to temperature.

What information do I need for the tank size ?

Thanks
Rob

robgraham
wibbled on Wednesday 02 December 2009 13:10

I need help in calculating the size of a possible heatbank store.

There are two main drivers behind this - one is that the house is
single storey so there's minimal pressure in the DHW system, and the
second is that we are rural and don't have gas; the CH boiler is a
10yr old oil burner (15kw) - perfectly effective but the radiators
heat up rather slowly so I assume that it doesn't produce it's heat
that rapidly! There's a log burner for winter assistance to the CH/
DHW system via a Dunsley Neutraliser. The roof is long and facing
south so would be suitable for solar panels if I wanted to go further.

So my thinking is to have a store that will supply the DHW at mains
pressure, and will act as a buffer for the CH system until the boiler
gets up to temperature.

What information do I need for the tank size ?

Thanks
Rob

There's 2 ways of looking at this:

1) Specific heat capacity of water is around 4.2kJ/kgK (ie you need
4.2kJoules to heat 1 kg (1 litre) of water 1 degree C

From that you can easily calculate the heating time of x litres and the
depletion time if you assume some rates for hot water supply[1]

Apologies if any of this insults your braincells - not intended...

[1] Incoming water = X celcius
Heat to Y celcius
Flow is Z litres / minute (more useful than l/sec)

Power in kW (kJ/sec) is

(Y-X) x Z / 60 x 4.2 in kW

Now 4 l/min is a fairly feeble flow (I have a bath tap limited to that so my
9.5kW heater can cope). If we reckon that a tap consumes 8l/min and produces
at 50C and your incoming water is 5C (worst case ground average temp - wild
guess). A shower might consume 10-15l/min from the hot depending on whether
it's normal or a power shower (guess again). Your bath tap would probably
like to run at 10l/min but could go either way.

Let's take 1 bath, and 1 shower as a minumum as they tend to run for say 10
minutes - so lets assume you might get away with 25l/min bottom end.

Most people's mains supply might be capable of producing a max flow of 30-50
l/min. This fixes the top end, though you'd be unlikely to call on HW at
50l/min for any length of time.

Bottom end power required for hot water is therefore 25*(50-5)/60*4.2 = 79kW
Top end insane case is 50*(50-5)/60*4.2 = 157kW

Most of these systems tend to run with plate heat exchangers rated at
100-160kW (the former being a reasonable choice for a medium house) anyway
which gives you your (reasonably generous) bottom end plus some spare. You
could draw more but the temperature would simply tail off - would still
produce usefully hot water at higher flow rates though.

OK - so via 2 independent means, we can assume that you will draw about 80kW
max for maybe 10 minutes with long rest periods (how many teenage daughters
do you have?!!)

If we assume you run your store at 75C and return at 15C (bit of a guess,
but Plate Exchangers are very efficient) from the HW plate, that's a dT of
60C

You are able to insert 15kW into the system whilst it's running, so our draw
from the stored energy is around 80-15 = 65kW

So (assuming 1l water is about 1kg)

V * 4.2 * 60 = energy stored by V litres in kJ

Energy drained by plate exchanger = 85kW for 10 minutes = 85*10*60 kJ =
51,000 kJ

So V is 51000/60/4.2 = about 200l tank size.

Your 15kW boiler could charge that from depleted (15C) in about an hour if
nothing was taking heat out.

(Someone tell me if my maths is up the creek!

If I understand you correctly, you are able to input some heat from your log
burner (heatbanks are excellent to couple with multiple heat sources).

Hopefully, there's enough there for you to play with some numbers. 200l is a
fairly mid sized bank. Typical cylinder width, but much taller. I reckon you
could go either way quite happily. It depends on whether you think my
guesstimates for your tap/shower usage is sensible.

Have a look at www.heatweb.com (DPS) - they have some nifty online tools for
doing heatbank guesstimation...

HTH

Tim

--
Tim Watts

This space intentionally left blank...

On 2 Dec, 14:47, Tim W wrote:
robgraham
* wibbled on Wednesday 02 December 2009 13:10



I need help in calculating the size of a possible heatbank store.

There are two main drivers behind this - one is that the house is
single storey so there's minimal pressure in the DHW system, and the
second is that we are rural and don't have gas; the CH boiler is a
10yr old oil burner (15kw) *- perfectly effective but the radiators
heat up rather slowly so I assume that it doesn't produce it's heat
that rapidly! *There's a log burner for winter assistance to the CH/
DHW system via a Dunsley Neutraliser. *The roof is long and facing
south so would be suitable for solar panels if I wanted to go further.

So my thinking is to have a store that will supply the DHW at mains
pressure, and will act as a buffer for the CH system until the boiler
gets up to temperature.

What information do I need for the tank size ?

Thanks
Rob

There's 2 ways of looking at this:

1) Specific heat capacity of water is around 4.2kJ/kgK (ie you need
4.2kJoules to heat 1 kg (1 litre) of water 1 degree C

From that you can easily calculate the heating time of x litres and the
depletion time if you assume some rates for hot water supply[1]

Apologies if any of this insults your braincells - not intended...

[1] Incoming water = X celcius
* * Heat to Y celcius
* * Flow is Z litres / minute (more useful than l/sec)

* * Power in kW (kJ/sec) is

* * (Y-X) x Z / 60 x 4.2 in kW

Now 4 l/min is a fairly feeble flow (I have a bath tap limited to that so my
9.5kW heater can cope). If we reckon that a tap consumes 8l/min and produces
at 50C and your incoming water is 5C (worst case ground average temp - wild
guess). A shower might consume 10-15l/min from the hot depending on whether
it's normal or a power shower (guess again). Your bath tap would probably
like to run at 10l/min but could go either way.

Let's take 1 bath, and 1 shower as a minumum as they tend to run for say 10
minutes - so lets assume you might get away with 25l/min bottom end.

Most people's mains supply might be capable of producing a max flow of 30-50
l/min. This fixes the top end, though you'd be unlikely to call on HW at
50l/min for any length of time.

Bottom end power required for hot water is therefore 25*(50-5)/60*4.2 = 79kW
Top end insane case is 50*(50-5)/60*4.2 = 157kW

Most of these systems tend to run with plate heat exchangers rated at
100-160kW (the former being a reasonable choice for a medium house) anyway
which gives you your (reasonably generous) bottom end plus some spare. You
could draw more but the temperature would simply tail off - would still
produce usefully hot water at higher flow rates though.

OK - so via 2 independent means, we can assume that you will draw about 80kW
max for maybe 10 minutes with long rest periods (how many teenage daughters
do you have?!!)

If we assume you run your store at 75C and return at 15C (bit of a guess,
but Plate Exchangers are very efficient) from the HW plate, that's a dT of
60C

You are able to insert 15kW into the system whilst it's running, so our draw
from the stored energy is around 80-15 = 65kW

So (assuming 1l water is about 1kg)

V * 4.2 * 60 = energy stored by V litres in kJ

Energy drained by plate exchanger = 85kW for 10 minutes = 85*10*60 kJ =
51,000 kJ

So V is 51000/60/4.2 = about 200l tank size.

Your 15kW boiler could charge that from depleted (15C) in about an hour if
nothing was taking heat out.

(Someone tell me if my maths is up the creek!

If I understand you correctly, you are able to input some heat from your log
burner (heatbanks are excellent to couple with multiple heat sources).

Hopefully, there's enough there for you to play with some numbers. 200l is a
fairly mid sized bank. Typical cylinder width, but much taller. I reckon you
could go either way quite happily. It depends on whether you think my
guesstimates for your tap/shower usage is sensible.

Have a look atwww.heatweb.com(DPS) - they have some nifty online tools for
doing heatbank guesstimation...

HTH

Tim

--
Tim Watts

This space intentionally left blank...

Hi Tim

That's remarkable - many thank for the time you put into putting that
together. And you end up with the near enough right answer at the end
too - right in that it isn't 20 litres or 20,000 as so many of my
calculations end up !!

There's no daughter's - all driven out thank goodness - just a
reasonably responsible son, and a wife of course. So water demand
isn't too heavy.

I'll now sit and absorb your sums; I've had a bit of a poke around on
the Navitron site and it does seem that 200L is the bottom end but
maybe that is all we will need. We're near Edinburgh so sun is a
luxury (BG) - a small store seemingly will require a dump if and when
I go down the solar input, and we have our one day of summer.

Very many thanks

Rob

robgraham
wibbled on Wednesday 02 December 2009 15:04


You are able to insert 15kW into the system whilst it's running, so our
draw from the stored energy is around 80-15 = 65kW

So (assuming 1l water is about 1kg)

V * 4.2 * 60 = energy stored by V litres in kJ

Energy drained by plate exchanger = 85kW for 10 minutes = 85*10*60 kJ =
51,000 kJ

So V is 51000/60/4.2 = about 200l tank size.

In fact there's an error the

I said let's assume depletion rate of 65kW (must be a typo)

so that would give a net energy depletion (if the boiler is running) of

65*10*60 kJ = 39,000 kJ

So V = 39000/60/4.2 = about 150l tank.

Sorry - was in a rush to do the school run...

Cheers

Tim

robgraham
wibbled on Wednesday 02 December 2009 15:04

Hi Tim

That's remarkable - many thank for the time you put into putting that
together. And you end up with the near enough right answer at the end
too - right in that it isn't 20 litres or 20,000 as so many of my
calculations end up !!

There's no daughter's - all driven out thank goodness - just a
reasonably responsible son, and a wife of course. So water demand
isn't too heavy.

I'll now sit and absorb your sums; I've had a bit of a poke around on
the Navitron site and it does seem that 200L is the bottom end but
maybe that is all we will need. We're near Edinburgh so sun is a
luxury (BG) - a small store seemingly will require a dump if and when
I go down the solar input, and we have our one day of summer.

Very many thanks

Rob

This would be worth a look:

http://wiki.diyfaq.org.uk/index.php?title=DIY_Heat_Bank

It's a DIY system by one of our resident GasSafe men, John which has some
rather useful theoretical and practical issues addressed.


--
Tim Watts

This space intentionally left blank...

On Wed, 02 Dec 2009 14:47:05 +0000, Tim W wrote:
(Someone tell me if my maths is up the creek!

*cheer* - nice to see something with some actual calculations in it for
a change :-)

Your feed water temp might be a bit pessimistic - even out here in
the wilds of Minnesota with Winter air temps of -30C I still see feed
water coming in at around 10C (and it seems to stay pretty constant
throughout the year). That's with supply at around 7' below ground level
though; maybe UK mains is a little higher up and so your 5C value is right?

cheers

Jules

Jules
wibbled on Wednesday 02 December 2009 15:16

On Wed, 02 Dec 2009 14:47:05 +0000, Tim W wrote:
(Someone tell me if my maths is up the creek!

*cheer* - nice to see something with some actual calculations in it for
a change :-)

Your feed water temp might be a bit pessimistic -

Yes - it's the finest physics combined with the ultimate in guesstimates, as
so often these things are.

even out here in
the wilds of Minnesota with Winter air temps of -30C I still see feed
water coming in at around 10C (and it seems to stay pretty constant
throughout the year). That's with supply at around 7' below ground level
though; maybe UK mains is a little higher up and so your 5C value is
right?

Difficult to say. My mains supply is about 2' down so I would expect it to
run fairly cold when the ground is frozen. I think your 10C is a better
guess for deep ground temperature though.

--
Tim Watts

This space intentionally left blank...

On 2 Dec, 17:35, Tim W wrote:
Jules
* wibbled on Wednesday 02 December 2009 15:16

On Wed, 02 Dec 2009 14:47:05 +0000, Tim W wrote:
(Someone tell me if my maths is up the creek!

*cheer* *- nice to see something with some actual calculations in it for
a change :-)

Your feed water temp might be a bit pessimistic -

Yes - it's the finest physics combined with the ultimate in guesstimates, as
so often these things are.

even out here in
the wilds of Minnesota with Winter air temps of -30C I still see feed
water coming in at around 10C (and it seems to stay pretty constant
throughout the year). That's with supply at around 7' below ground level
though; maybe UK mains is a little higher up and so your 5C value is
right?

Difficult to say. My mains supply is about 2' down so I would expect it to
run fairly cold when the ground is frozen. I think your 10C is a better
guess for deep ground temperature though.

--
Tim Watts

This space intentionally left blank...

Ah well, how about bang in the middle; I've just run a bowl of water
which is measuring 7.6C .

Rob