Calculations for heat bank size

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

Reply to
robgraham
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robgraham wibbled on Wednesday 02 December 2009 13:10

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

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(DPS) - they have some nifty online tools for doing heatbank guesstimation...

HTH

Tim

Reply to
Tim W

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

Reply to
robgraham

*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

Reply to
Jules

robgraham wibbled on Wednesday 02 December 2009 15:04

In fact there's an error there:

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

Reply to
Tim W

robgraham wibbled on Wednesday 02 December 2009 15:04

This would be worth a look:

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

Reply to
Tim W

Jules wibbled on Wednesday 02 December 2009 15:16

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

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.

Reply to
Tim W

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

Rob

Reply to
robgraham

I agree with the calcs.

Might be worth remembering that we are running the rads as well as the HW from this store - so some extra capacity might be handy on days where its both cold and a high water demand may co-exist. Hence I would be tempted to go for about 250L.

Reply to
John Rumm

John Rumm wibbled on Thursday 03 December 2009 10:25

Yes - that's the size I went for. Fairly normal width but 5' or so tall - could still fit in an airing cupboard, although there wouldn't be any room left for linen. There's quite a wide choice of form factors available if space is an issue.

The other other point is that when arranging tappings for the rads, make sure to leave the top part of the tank unconsumed by the radiator circuit - this guarantees that no matter what, that hot water will be available in some useful quantity. HW feed to the Plate Exchanger comes off the top of the cylinder and as it tends to return the coolest water, the return goes to the base of the tank.

The commercial units (eg DPS) know this already but if one were doing a homebrew system and specifying tapping or modding a standard cylinder with extra flanges or whatever you'd want to give this some consideration.

I reserved the top 1/3 on my tank for HW.

Reply to
Tim W

Many thanks again guys for your advice. The maths is being worked on and extended as I have a figure from somewhere for the typical CH flow demand. All I need now is datalogg how long it takes the oil burner to get it's output up to temperature.

Is there any reason why the tank shouldn't be in the attic, bearing in mind this is a 'cottage'. The ceiling timbers are 7" x 3" x 15', but are coupled down to the similar sized floor joists in that area. On the basis that the CW tank comes out, and that sits on the ties anyway, there will be that reduction in weight (don't ask me what size it is as getting to it at the moment isn't that easy - I've just had a knee operation!).

Currently the HW tank sits through the ceiling line to get head from the CW tank. I may have to do this again to get enough height, or can I go for a shorter fatter shape - I suspect that's less desirable for stratification. What about two tanks in parallel ?

Rob

Reply to
robgraham

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