Heat bank for under a kitchen worktop

[Doctor Drivel]

Correct an idiot, so please eff off as you are a total idiot.

[Doctor Drivel]

You really don't know do you?

[Andy Hall]

This is confused nonsense. The heat exchanger does not care about the source of its heat.

ROTFL.

[Brian Sharrock]

I hestitae to intervene in such a 'big-end' ~ little-end' dispute between two obvious Laputan experts ... however let me interject some observations based on direct experience of a heat bank;-

The heat bank has several loops which all interact within the tank itself; one 'loop' is the 'heating loop where the working fluid is sent off to the boiler to garner heat. This loop has a pipe at the base of the tank (input to the boiler) and a pipe discharging the top of the tank. After a while the tank will be filled with water ,essentially, all at the desired teemperature for the tank. The DHW is obtained by taking the working fluid and passing it throygh a heat-exchanger wherein it surrenders heat to the mains water and produces DHW. The 'cooled' working fluid re-enters the tank at the bottom. Thus if one considers the working fluid to be a column of laminae, each layer / lamina to be a body of fliuid each of a discrete temperature they will self-organise with the hottest at the top and the coldest at the bottom (stratification].. As working fluid ciirculates through the 'boiler -loop' it re-enter the tank at the boiler-demand temperature and, thanks to stratification', floats at the top of the cylinder. It is this hottest layer that goes into the heat-echanger to surrender it's heat into the DHW. In pratical _experience_ once the boiler 'fires' it's heat is availlable to provide DHW _almost_ instantly! [for variable valeus of _almost_ and _instantly_]

A heat bank is not magic; there's owt for nowt; _but_ the 'useful hot water from the boiler goes pretty immediately to the taps' _via_ the plate heat-exchanger and for an almost imperceptable value of immediate. And; whilst; "can't get a two litres from a one litre pot and certainly can't defy physics."; one can _exploit_ the physics of stratification and hot 'stuff' rises while cold 'stuff' falls to ensure that the energy flow is maximised from boller->tank->heat-echanger->DHW->taps! It's not 'physics'; it's clever engineering / plumbing.

[Doctor Drivel]

You need to understand how they work.

[Doctor Drivel]

Yep, that is what I said. After designing implementing countless stores and heat banks I would know this sort of thing.

Even Stumbles noted this in his DIYed heat bank on 22.3.2007.....

With my own setup on the few occasions that we've run the bank cold (which is inevitably when you've got into a half-full bath and the hot's run cold) it takes just a few minutes to get a reasonable stream of hot water out again. With the non-rapid-recovery conventional cylinder we had before you could, if you were patient, get a trickle of hot water out after a quarter of an hour or so.

Apart from the equivalence of a directly-coupled arrangement to an infinitely rapid-recovery coil I think there's also the factor that even the fastest coil is heating the top half or so of the cylinder by convection whereas in the direct heat bank the hot flow from the boiler is probably getting sucked directly out of the top of the cylinder by the DHW pump. can't defy physics."; one can _exploit_ the physics of stratification and

Yep. Combining the output of the boiler and energy in the cylinder a smaller cylinder may be used. It also acts as a wonderful buffer for CH too. The Germans are way into thermal storage and leave us standing on some designs.

[Andy Hall]

A good explanation, Brian.

My original point was not about how quickly the heatbank can produce hot water starting from all cold condition, but rather what happens in terms of capacity to continue to produce the initially high flow rate of hot water at the taps.

It is obvious enough, as you say, that if the cylinder is cold and the DHW is turned on that heat from the boiler will be transfered fairly effectively to the heat exchanger and will produce some hot water. Assuming that little or none goes to heating the store at this time, the rate of production at the taps will depend totally on the boiler and will be similar to the behaviour of a combi of equivalent heat output.

However, there are only cases, AFAICS, where you would have this situation:

- Coming back from holiday or something, turning on the boiler and wanting DHW quickly. This is comparatively rare.

- Heatbank has run out of stored heat and DHW Iis running in "instant " mode.

I suppose that one could operate the store by letting it cool overnight and coming on in the morning (e.g. like an immersion heater) - it would seem to be an odd thing to do.

The case that I was refering to was what happens when the store is hot and one begins to draw DHW. In that scenario, the issue of whether the heat contributed by the boiler is delivered to the heat exchanger plus a smaller amount from the store or whether it's all from the store and being replenished by the boiler is somewhat irrelevant. It will still be the same time and the same amount of energy before the store runs out.

My point was about how long is that going to be. Clearly a heatbank is useful technology, that was not in dispute, but people who have been used to stored hot water expect constant flow and temperature and are frequently disappointed if they swap for an inadequate combi.

Therefore issue 1 is how long at the intended rate of use will it be before the hot water production rate drops, and issue 2 is when it has, will the user be happy with the result.

Issue 1 can be dealt with by having an adequately sized store and adequately sized boiler

Issue 2 can be dealt with by having an adequately sized boiler.

Obviously one can choose appropriately.

The danger area is replacing a typically sized conventional HW cylinder where the user has been used to a plentiful supply of HW at constant flow and temperature with volume that always exceeds his requirements with a small store and retaining a possibly inadequate boiler.

[Andy Hall]

The principle of stratification is not in dispute; nor is what happens when the cylinder is cold or runs cold.

When it is hot, and energy is being witdrawn at a higher rate than the boiler is producing it, it's completely irrelevant as to whether the heat from the boiler is going into the store or to the heat exchanger; the net effect is the same.

Have you looked up what the unit of energy is yet?

[John Stumbles]

I think McDonald engineering to a smallish squattish cylinder with a rapid recovery coil which might do the trick if the boiler has a sealed primary.

The ideal of course would be a rectangular[1] tank which would fit under the w/t with a bit of space in front for the PHE, pump etc.

[1] what's the name for 3-dimensional rectangle? i.e. square corners, unequal sides?

[John Stumbles]

[Fx: CHEERING from amassed regulars of uk.d-i-y]

But you forgot to say the combis' gas feeds should be run separately back to the meter (or better still separately all the way back to the North Sea or Russia)

[John Rumm]

That would be kJ/h^2 dribble, try again.

Interesting concept though.... perhaps he has a combi that can out accelerate his prius.

[Brian Sharrock]

You choose;- Cuboid ... or ... Right Prism. .... or ... rectangular parallelepiped .... or rectangular prism

[John Rumm]

Its just not the same without Humph and the lovely Samantha though....

[Dave Plowman (News)]

Box?

[Dave Plowman (News)]

And separate water supplies. Extra parking for the two vans at service time. Get different makes so they don't break down together.

[Dave Plowman (News)]

The mind boggles.

[John Stumbles]

were trying to impress the punters.

[Bob Mannix]

no, I choose "box" :o)

[Doctor Drivel]

You ensure only hot water enters the top of the cylinder.

[Doctor Drivel]

Please eff off.