Vented heat bank in a block of flats

I live in a 30 year old block of flats; two flats per floor, three floors.
Each flat has its own, conventional, vented hot water cylinder.
In the roof void of the building there's a set of header tanks, all interconnected (so effectively one big header tank).
In the service duct through the building is a single 3 inch riser, supplying mains water to each flat and refilling the header tanks in the roof.
From the base of the header tanks there's a single 3 inch downcomer, feeding the base of each of the six hot water cylinders. Another 3 inch downcomer feeds domestic cold to each flat.
There appears to be a separate vent pipe (22mm copper) running from the top of each cylinder up to a separate 'overflow' tank up in the roof (access is difficult, so we're not entirely sure about this). This tank has six copper pipes hooked over the top - and it's empty. There's no ballcock to fill it, and it just has a discharge/drain pipe to a visible point outside the building.
Can I, unilaterally, convert my hot water cylinder into a heat bank? Would Building Regs forbid it?
I've been trying to figure out the risks, in this arrangement, of the contents of any cylinder commingling with the contents of another cylinder in another flat. There are no check valves, so reverse flow into the common downcomer is theoretically possible, but I can't see a mechanism that would cause it.
--
Roger Morton


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Anyone?
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Roger Morton


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Roger Morton wrote:

A couple of thoughts - I converted our conventional hot cylinder to a heat bank system a couple of years ago, as we had no head or sensible flow rate in a bungalow, and have been very happy with the result.
You will need to fill the heat bank with water and inhibitor to protect the pump. and keep it reasonably sterile. It would not be a good idea to mix this water/inhibitor mix with your neighbours hot tank cold feeds, and you wont want your expensive inhibitor diluted. You will also need some degree of expansion provision as the heat bank warms and cools.
My suggestion would be to install a small expansion tank (central heating type) anywhere convenient, so long as it is above the top of your cylinder. Feed this via a ball valve from your existing cold water feeds, stored or mains, via a stopcock. This will isolate your system from that of your neighbours should you use the stored feed.
From the expansion tank feed the bottom of your cylinder WITHOUT any means of shutoff - no stopcocks etc - this is very important. Add an air vent to the top, to release air while filling. You do not need an open vent from the top of the cylinder, as it's at close to atmospheric pressure, and has an expansion path back to the header tank. I fed mine to the top of the cylinder, and find it a pig to fill - and will change this next time I re-arrange the pipework.
Add a loop from top to bottom of the cylinder for your pumped feed to the heat exchanger as per your existing planning.
If I can provide any other info let me know.
Charles F
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That would be my thoughts too, for an entirely self-contained heatbank - though the simplest (but more expensive) solution would be a thermal store type cylinder with integrated feed & expansion tank in the top half.

Sorry - but that goes against everything I know - AIUI a second safety pipe is always required on vented systems, and should loop over the f&e tank.

Again, AIUI cylinder-filling feeds are always to the bottom, often teed off from the feed to the boiler, and positioned just before the pump.
(IANAP - I Am Not A Plumber, so don't take my safety advice as gospel - get it from someone qualified and/or manufacturers/professional literature)
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I think that the vertical safety vent, certainly necessary on conventional systems, is probably not necessary for a heat bank PROVIDED that there is a permanently open pipe to the expansion tank - which can be 22mm if you prefer.
If you look at http://www.heatweb.com/techtips/Pandora.html and continue to the filling pages, there is only one pipe to the expansion tank - in this case to the top, but I don't see that that matters particularly in safety terms. Additionally, on my MK1 heatbank I find that I'm losing some heat via the vertical fill pipe from the top of the tank, even though lagged. A fill pipe from the base of the cylinder should improve this loss.
I'd be interested to know what your reasoning is to need a fill and vent pipe in the case of a heat bank?
One thing I forgot - you will need a drain cock for the cylinder, and isolation valves either side of the pump and heat exchanger save draining the cylinder if you need to remove either item.
Charles F
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That's designed to be unvented, and I would expect there are secondary safety valves as are usually used on sealed systems - "details omitted for clarity" as they say.

Usually there's a couple of near-90 degree bends to make a 300-400mm offset in the vertically-rising vent pipe to reduce in-pipe circulation.

I've recently installed an Albion mainsflow thermal store, and the manufacturers instructions say "combined feed and vent pipes should not be used".
I know combined feed and vent pipes *are* sometimes used, but I don't know when that's ok.
Speculation here - but I would imagine it's on systems that can never overheat to the point of producing steam, under any fault conditions. e.g. a cylinder that's only heated by hot water from another separate system.
Regarding using the vent pipe as fill pipe (rather than the lowest point on the cylinder), there is the possibility under fault conditions of a plastic f&e tank getting so hot that it fails.
Again, IANAP.
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Not sure I've got the height to do that within the flat - wouldn't this expansion tank have to be above the highest point in the central heating circuit, rather than (just) the top of the cylinder? I have tall radiators, to within a foot of the ceiling....
I can see that isolation from neighbours would be a Good Thing - but is it a Required Thing?
If the vent pipes from everyone's cylinders are, as we suspect, kept separate up to the common, empty, overflow tank in the roof, then back flow around vent pipes must be inconceivable, mustn't it?
It's the shared feed to the cylinder bases (plural) that's the concern.

Dumb q - why is it important, if there's also a separate vent pipe? As long as one of the feed and vent pipes has no means of isolation, isn't that enough to ensure you can't pull a vacuum on the cylinder? In fact, with our existing, common cylinder feed, each flat has to have an isolation valve on its feed branch - otherwise you'd have to drain the header tanks for the entire block before working on any one flat's cylinder :-(
Given that, would a check valve in the feed branch help assure no reverse flow into the common downcomer?
--
Roger Morton
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I think there may be some misunderstandings over how a heat bank works - the water in the bank is there long term, needs to have inhibiter, and has to have some means of expansion to maintain levels in the heatbank cylinder. Have a look at DPS's info, and also look at Gledhill's info on their Torrent heatbanks. They both use vented expansion tanks (one with a diagraph), and both mention inhibitor.
Charles F
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I'd been assuming a direct heat bank, like the Xcel 2009, with no heating coil. But I'd forgotten the indirect boiler option they've got, with a second plate exchanger. That should let me keep the boiler and the ch radiators as a sealed system, as they are at the moment.

In fact the DPS literature shows a gate valve in the feed :-)

I think I understood all that - the flow into (and/or back out of) the base of a heat bank fed from the common downcomer would be tiny, except on initial fill and heat. Probably never enough back flow to get a check valve to close, but small amounts of inhibited water will get back eventually.
I'll pursue the fully isolated (from the neighbours) option with an indirect boiler - looks the best best for this situation.
Ta for the help
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Roger Morton
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On 2 Feb, 09:45, snipped-for-privacy@b.invalid (Roger Morton) wrote:

Why would you want to?
Heat banks were a method of providing mains/high pressure hot water before unvented hot water storage cylinders were permitted in the UK. You have to kep them at around 80 degC and they don't work well with condensing boilers, which work better with a low temperature return.
Get an unvented cylinder (lease terms & conditions allowing) or a combi.
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Onetap wrote:

Mains pressure hot water with minimum complexity and maintenance cost - or maximum reliability and ease of maintenance, perhaps the same thing :-)
I don't like the control systems an unvented cylinder would need - and getting landlord consent could be a real issue. An atmospheric pressure heat bank, particularly if I can isolate it from the common services properly, would be a much easier sell.
And I want to hang on to my simple, non-condensing boiler for as long as it carries on working. It's a fine example of the dreaded Suprima (6 years old, non-condensing), and I've not seen anything on the market these days that will fit into the same space - not even the condensing version of it. Replacing it could well involve ripping up a beautifully fitted kitchen and starting again.
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Roger Morton
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On 5 Feb, 14:56, snipped-for-privacy@b.invalid (Roger Morton) wrote:

The complexity and the explosion risks are exaggerated by the UK's resident herds of Plumbosaurus Rex and the likes of Drivel who don't understand them and can't install them, I have an unvented ticket, they're good gear.The main difficulty is in fitting a metal drain pipe to a suitable discharge point. Even the Merikins use them without blowing themselves up (well, not often).

I'm sure you could find a boiler that would fit. The logic of fitting an antiquated device to preserve an antiquated boiler escapes me.
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(Roger Morton) wrote:

The complexity and the explosion risks are exaggerated by the UK's resident herds of Plumbosaurus Rex and the likes of Drivel who don't understand them and can't install them, I have an unvented ticket, they're good gear.The main difficulty is in fitting a metal drain pipe to a suitable discharge point. Even the Merikins use them without blowing themselves up (well, not often).
I'm sure you could find a boiler that would fit. The logic of fitting an antiquated device to preserve an antiquated boiler escapes me.
Part of the attraction of heatbank systems, in a DIY group, is that they are easy to produce and maintain, and can be much less costly. You can also DIY them, unlike unvented systems (to stay legal anyway)
If you are to buy an off the shelf heatbank system from Gledhill or DPS, there is probably little to choose in cost compared to an unvented system. However, there are alternatives....
My system uses the existing cylinder, so additional costs were a plate heat exchanger from DPS (less than 200), a standard central heating pump, some basic electronics, and some rearrangement of plumbing - total cost a little over 300. In the next ten years I may need a new heat exchanger if it furs up enough, and I'd like to put in a bigger cylinder - 400 ish? I understand the system, and can easily maintain it if needed. Total cost in the next ten years - less than 1000, even with improvements.
An unvented cylinder is probably around 1000, and I can't install it myself - another 250? It also should have an annual service at 100-150 pa. Total cost over ten years - 2250 to 2750, assuming nothing fails over the period.
So I have a system that can run two showers at the same time, with water as hot as is sensible to stand under, which replaced a stored water system with such poor head that one of the basins never did get any hot water. It will cost me about 1500 less than an unvented system over the next ten years.
I realise that this is not to everyone's taste, but this is a DIY group, and heatbanks make good sense for many of us.
Charles F
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(Roger Morton) wrote:

The complexity and the explosion risks are exaggerated by the UK's resident herds of Plumbosaurus Rex and the likes of Drivel who don't understand them and can't install them, I have an unvented ticket, they're good gear.The main difficulty is in fitting a metal drain pipe to a suitable discharge point. Even the Merikins use them without blowing themselves up (well, not often).
I'm sure you could find a boiler that would fit. The logic of fitting an antiquated device to preserve an antiquated boiler escapes me.
Part of the attraction of heatbank systems, in a DIY group, is that they are easy to produce and maintain, and can be much less costly. You can also DIY them, unlike unvented systems (to stay legal anyway)
If you are to buy an off the shelf heatbank system from Gledhill or DPS, there is probably little to choose in cost compared to an unvented system. However, there are alternatives....
My system uses the existing cylinder, so additional costs were a plate heat exchanger from DPS (less than 200), a standard central heating pump, some basic electronics, and some rearrangement of plumbing - total cost a little over 300. In the next ten years I may need a new heat exchanger if it furs up enough, and I'd like to put in a bigger cylinder - 400 ish? I understand the system, and can easily maintain it if needed. Total cost in the next ten years - less than 1000, even with improvements.
An unvented cylinder is probably around 1000, and I can't install it myself - another 250? It also should have an annual service at 100-150 pa. Total cost over ten years - 2250 to 2750, assuming nothing fails over the period.
So I have a system that can run two showers at the same time, with water as hot as is sensible to stand under, which replaced a stored water system with such poor head that one of the basins never did get any hot water. It will cost me about 1500 less than an unvented system over the next ten years.
I realise that this is not to everyone's taste, but this is a DIY group, and heatbanks make good sense for many of us.
Charles F
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Could you expand on that? How did you connect the boiler and/or what did you do with the old coil? One of the problems I have with my existing cylinder is the slow recovery rate.
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Charles Fearnley wrote:

Roger, I'll get back to you tomorrow - long day at work today....
Charles F
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Onetap wrote:

That's for hot escapes from PRVs? Could be tricky in a utility cupboard right in the middle of the flat - no exterior walls for many many feet.... There's a bathroom waste passing through it, but it would have to be cooled somehow?

The old Suprima body is 490mm high - it's inside a kitchen cupboard that's 700mm high; with 25mm clearance above the top of the flue bend (inside the cupboard) at the top and about 40mm at the bottom. I've spent a lot of time looking :-)

You may very well be right - I'm still very much at the pipe-dreaming stage :-)
--
Roger Morton
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