I think you are talking about using the CH part of the combi boiler to in addition heat an auxiliary cylinder of water.
Two possibilities for the valve..
1. Blending valve on the output to limit the maximum temperature to avoid scalding.
2. 3 way valve combining with the cylinder thermostat to get the combi to heat the water through the coil and stopping it once the cylinder has reached the set temperature. Probably interlocked with the CH thermostat
so that it works even when the CH is not required.
Case 2 is mandatory under Part L (fuel efficiency) of building regs
Case 1 is mandatory for hospitals, residential homes etc but there are noises about making it mandatory in homes.
I think case 2 is the most likely given your description.
StarGazer
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Sounds like quite a nice setup to me - should provide the best of both worlds.
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I'm totally confused now. I thought the CH from the combi would do the majority of the heating through the coil, with the immersion element kicking in when the thermostat said it needed to. I don't understand how/why/where this valve would go, or why I need it.
I'd of thought the combi would do all of the heating through the coil. All the time the system works the immersion heater switch should be firmly in the OFF position only to be used if the combi breaks down otherwise you'll be wasting 50% of your money on heating up the sea next to the power station.
You'd definitely need the 3 way valve Stargazer mentions in Case 2 (or two 2 way valves), and I would guess a 2 channel timer otherwise you'd have no simple way of separating the heating from the HW.
Are you fitting the system yourself? Beware if your work friend is not confident about describing his system. Unvented cylinders and their expansion vessels and pressure relief valves demand respect and I believe the council should be notified of your intention to install an unvented system and, after fitting, the installation should be approved as being safe before use.
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Using a combi and an indirect cylinder?
This is really a very bad idea, leading to very dificult to control water heating.
1/ It will only work when the system is calling for central heating. Using the elctric immersion element to back this up is very expensive,. you will need to have it turned on all the time to provide hot water with no central heating. In fact in this weather you will be using electricity all the time to heat the water in your new cylinder.
2/ You will need to design and plumb a bypass for the cylinder, using a three way valve, and a cylinder theormostat (withe all the required pipework and electrics so that when the temperature of the cylnder is reached the valve operates to isolate it
Combis and indirect cylinders are NOT designed to work together.
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TourVanMan TM < Ex RF >
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OK I'm no expert in this area but I have a couple of questions.
1) Won't Simon need to have the unvented cylinder fitted by a certified person? ISTR recent regulations mentioning this. Someone posted a photo of an unvented cylinder that had exploded in an attic room, on another forum. I can tell you that the whole room was totalled and the occupants were luck that no one was killed.
2) For what reason have you discounted a vented cylinder? Technically it is possible to link this up to a combi, Maybe some of the 'real plumbers' could enlighten me.
H
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A friend at work has a simmilar set up with a combi boiler and a hot water cylinder, I do not know the full details but understand that the hot water cylinder is pressurised so thought that it was a water bomb waiting to go bang.I assume that the hot water tank was not a standard one but one designed to take the pressure.
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rustbucket (the original)
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" thought that it was a water bomb waiting to go bang"
Water is pretty much incompressible, so even under vast pressure, it cannot explode a vessel. It may rupture it and spring a leak, but no danger.
V
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" thought that it was a water bomb waiting to go bang" Water is pretty much incompressible, so even under vast pressure, it cannot explode a vessel. It may rupture it and spring a leak, but no danger. V sorry wrong terminology,
but still under pressure so water can make a mess when it splits / rips more that a leak fom a non pressurised container.
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rustbucket (the original)
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Have to disagree with Vin, they can be rather like a ticking bomb if not fitted correctly as Hugo's description depicts.
I believe the volume of water rises by about 4.4% between 4C and 100C and for the reason that water is pretty much uncompressible the pressure within the cylinder will rocket (as the non-return valve will prevent heated water leaking back into the mains) until it explodes as the material of the cylinder will not expand that much. If the expansion vessel and pressure relief valve are installed correctly though the system is quite safe.
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Unvented cylinders are widely used throughout Europe, it only seems that the UK has an issue with them. They come supplied with an overpressure relif valve that vents water to a drain. Much simpler to install ( I have done both in the last year) and has the advantage of high pressure hot water with no combi like restrictions on flow rate. My 200l cylinder is electrictally heated only and the integrated insulation is extremely effective and with a purchase cost of about Euro230 with all fittings would make it a financially viable solution in the UK.
UK plumbers seem paranoid about them (see the Screwfix site) and the uk devices require an annual service by a certified person. Also Uk ones appear to have an expansion vessel, and I am 99% sure the french ones dont have this, relying on a constant over pressure bleed.
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pmh (was peter)
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"until it explodes as the material of the cylinder will not expand that much"
The reality of that situation is that you have a material inside the tank trying to expand into a volume 4.4% bigger than it currently occupies. That will not cause an "explosion". It may well split a seam, but will not "explode". The pressure may be huge, perhaps hundreds of bars, but the water has no need to expand by more than 4.4% to relieve it.
A vessel filled with air at 200 bar is holding in a gas that, if released, will try to occupy 200 times its current volume. That'll give you an "explosion"
I knew a guy who used to make expansion chamber for motorbikes. He made them flat, then pumped them full of water to expand them. He was pushing steel plating out of shape, so you can imagine the pressures he was using: hundreds of PSI. If a seam broke, there's be a tiny spurt of water.
I'm not saying such a leak isn't a problem, just that there won't be an "explosion".
V
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I imagine that the danger arises if there is an integrated expansion chamber. Surely this will need to be filled with gas sepated from the water by a diaphram. Hence you can have compressed gas (or steam, if the thermostat fails), which can explode thro its container wall. This would presumably account for the stories (I have never seen the pictures) of shrapnel flying around the room.
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pmh (was peter)
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While I agree with the general argument about how hydraulic failures are always of lower energy than the equivalent gas failures, there is also the failure mode of the pressure vessel to consider.
If the pressure vessel fails in a ductile manner, or via springing a containable leak which relieves the pressure (although it unlikely that a so called leak before break type fracture analysis wil have performed by the manufacturer to verify this), then it is likely to be quite safe - although messy!
If the pressure vessel fails in a predominantly brittle manner, the fracture itself will absorb negligible energy, and the fractured part may "fly off".
Number_Cruncher
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Thanks for the comments.
Having read up about the unvented cylinders before I bought one, I decided I wasn't knowledgable enough, qualified or safe to install one. The OSO cyinder has a built in expansion vessel, with a full kit of pressure release valves part of the package.
I knew that during summer, and especially in weather like this, it would be expensive to run, but there is no gas for miles around, and the little oil fired combi would not cope at all with the swimming pool SWMBO has chosen for the bathroom.
I've been trying to find an unvented qualified plumber, but no-one advertises they've got the cert. in ads, and when I called a few, the only comments have been 'waste of cash'. Only thing is that my house insurance is invalidated if the cylinder is not installed and certified by a qualified unvented plumber and something goes wrong......
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Simon, All the ones I have been involved in have been mainly industrial types. We changed around twenty normal type calorifiers (cylinder) to pressurised hot water systems in my Establishment. The Industrial type has a pressure vessel mounted on the wall close by and is sized by the capacity of the calorifier which is pretty obvious really.
But included for a couple of small buildings were smaller type which I think were OSO with an integral pressure vessel but I am talking of ten years or so ago.
They were all superb in having mains pressure water available.
As I recall the OSO type were specified to be drained annually to ensure that it was still full of air.
They were relatively easy to install and came with a diagram of pipework which was very simple to follow.
As I see it there is no danger from these cylinders whatsoever and if I were considering changing my cylinder would install one of these.
Its not rocket science and is simply a vessel with an integral air tank to take the increase in pressure on heat up. I?m way out of touch with modern regs and wasn?t aware they needed to be fitted by qualified tradesmen but would have thought any Plumber (A proper one) would find it simple to install.
However regarding your combi being used as the heating boiler I think you would need someone who knows what he?s doing to sort that one out as there are a few problems as mentioned earlier in maintaining control of the system.
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It's astounding how many plumbing/CH queries come up on this site.
An unvented system should be fitted by someone who has passed the CITB/IoP or similar training course (I have). You have to notify the local authority of any installation.
You can have a combi heating a DHWS storage cylinder, either unvented or vented.
I have done this, another contractor had installed a combi boiler in the same room as a 300 litre unvented cylinder; strange.
You'd use two 2-port motorized valves (MV) ; one valve is opened by a CH demand, the other is opened by a DHWS demand. The boiler & pump are enabled by either (or both) of the MVs' micro switches, which are wired in parallel. See the S-Plan wiring diagrams on the Honeywell website, bearing in mind that you may have to adapt the wiring shown to suit your installatiuon.
content.honeywell.com/uk/homes/Catalogue/Sundial%2...f
You will get a 2-port spring return MV (usually a Honeywell V4043) with an unvented cylinder. You MUST use it. The valve will spring return shut in the event of a power failure; this is a safety feature to prevent the stored DHW overheating in the event of a system failure. You shouldn't use a 3-port valve because they spring return OPEN to the DHWS port. This would be unsafe. If you use a 3-port valve, you must also use the 2-port MV; this would be silly.
Foreign unvented cylinders don't usually have any provision to accomodate expansion, the water is just discharged to waste through the expansion/pressure relief valve. UK installations are required by the water regulations to have an expansion vessel or an internal air bubble to accomodate the expansion. Unvented HW storage cylinders are very, very safe, if installed properly and maintained. They could burst explosively if you managed to by-pass the 3 sequential safety devices. This has never happened in the UK yet, it is very unlikely to happen by accident.
See www.waterheaterblast.com if you really want to see what could happen. This is a small water heater (12 US gallons) on which the 2 safety devices have been by-passed and the pressure relief valve has been deliberately plugged.
A traditional vented cylinder could burst in the same way, if it was incorrectly installed or if the vent were plugged (by ice, say). This has happened in the UK.
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