I have a pressurised hot water cylinder. About a year ago it started leaking water through a valve at the top, and an obviously incompetent buffoon of a plumber replaced it (he basically admitted he didn't know what he was doing). It no longer leaks water, but the pressure of the hot water as it comes out of showers and taps has got noticeably greater - when I turned on my shower this morning the shower head sort of exploded because it couldn't cope with the throughput of hot water.
Might the cylinder be about to blow up or something? Should the valve be letting through air, and might it not be? Is there anything I can check?
Sealed metal cylinders full of hot water don't blow up in a nice way. It will be more like a huge boom than a fizz. There was a story going round about one on a boat exploding and going through the hull. Doubtless someone else will provide the URL for that - I can't find it myself.
Why let the plumber replace a part if he admitted he didn't know what he was doing? I think you are supposed to have an extra qualification to service or install these beasts.
I would switch it off until you can get someone competent to take a look at it.
If it is a Megaflow type device then it may be that the internal air gap needs regenerating - there should be instructions for carrying out this process printed on the side. The airgap is compressed as the water is heated, avoiding the initial "burst" you describe when first turning on the hot tap after the boiler has come on in the morning.
I can't quite remember the procedure from the top of my head but I think it goes:
Turn of incoming cold water supply to tank.
Open nearest hot tap and wait until water stops flowing (if the air gap is the problem, this will occur very quickly otherwise it will take about 5 mins).
Close hot tap and open air valve (marked on tank) until gurgling noise stops.
Thanks very much, I shall give it a go. Would the air valve be the thing on the top then?
Incidentally, the very high pressure is not an initial "burst" - it stays abnormally high, even during the course of a lengthy hot shower. Could that have any bearing on anything?
There should be 2 safety valves, a pressure relief valve and a combined temperature and pressure relief valve, both certified to the relevant BS. If the PRV was leaking (?)it would suggest that the expansion vessel, or the air bubble, needed recharging. The plumber may have replaced a safety valve with an incorrect part, when the 'leak' was indicating another fault that needed fixing, and which he hasn't fixed.
A "plumber" should be qualified to work on these, CITB or IoP qualification or similar. He will be able to quote a number that you could check with the relevant certification body. I've got the CITB card.
I'd suggest you find someone competent to service it. If the pressure is getting high enough the wreck the shower head, it suggests that neither relief valve is working correctly; I would anticipate that presently it would split the cylinder causing a huge flood. I would turn it off (immersion heaters and/or heating coil from the boiler) until it's fixed.
I don't know mate - I'm not entirely sure what your setup is and on mine all the valves are clearly marked from manufacture and there are instructions on the side. Also when I have seen this phenomenon the "burst" only lasts a few seconds not throughout the whole duration of the shower.
This suggests to me that you might have a problem with the pressure reducing valve on the INLET to the unit, which should limit the mains pressure and is perhaps not doing so correctly. Perhaps this is what the dodgy plumber replaced? You should be able to work out which is which, the INLET will be at the bottom of the tank, and the OUTLET at the top. Inlet valve should (I think) limit incoming pressure to 4 bar, and should be marked as such.
Sorry, not really much help. Like Mr F said, it might be dangerous in its current state, so play safe.
Turn it off IMMEDIATELY!!! Remove the fuses from the immersion heater supply and turn off your central heating at the mains.
It is EXTREMELY DANGEROUS! If it explodes, which is actually relatively likely, it has the real potential to destroy the house, killing the occupants.
Now find a plumber who is qualified to work on pressurised hot water storage systems to fix it. Tell them that some buffoon has removed all the safety gear.
If you had a Megaflo, this works on a different principle to many. They have a "air bubble" at the top to act as a pressure vessel. It tends to disappear over time, leading to slight overpressure and water through the valve. The correct procedure (which should be done as part of the regular servicing that mains pressure cylinders need to remain safe) is to drain down the cylinder and refill, to restore the bubble. Blocking up the pressure relief can lead to death.
Thanks for that info too. I have just been home to look at it (it's in the garage, behind a wooden partition wall which I have removed), and as you say it has a metallic "air-bubble"-shaped object above the main cylinder. On the air-bubble is a wide plastic cap under which is a small black knob. Can the bubble be restored by turning off the cold input, opening a hot tap until it stops, and opening this little black knob until the gurgling stops? (I know ultimately I need to get it serviced - question is finding a plumber who really knows about these things, and isn't just pretending to.)
If it has a separate chamber, then it isn't an air bubble type, but an expansion vessel type. Do you have a make/model?
Quite frankly, I wouldn't suggest you try to repair it yourself, particularly as the previous plumber may have damaged that safety systems. You need a specifically qualified plumber. There is a separate qualification to work on these things.
It's an expansion vessel, with a neoprene diaphragm, separating the air charge from the water. From your description, the black knob is a valve cap on a schraeder valve, as on car tyres. You'd need to isolate the cold water, discharge the pressure from the cylinder and use a tyre pump to pump the air charge back up to the recommended pressure. If the diaphragm has failed, you'd need a new expansion vessel.
No. That applies to the integral air-bubble type; this is different.
Phone up, ask if they're qualified, get the card number, check with the certification body. If you then employ them, tell them you'll want to see their photo (NB) ID card when they come, to ensure they haven't sent the company idiot. No card, no job. Get a receipt. Anyone who's done the course will be happy to oblige, anyone who makes excuses is not qualified.
I had dealt with unvented systems in 1987 when they were first legalized and I had later resented paying the money for the course. With 20/20 hindsight, I learnt a fair bit from it. There's quite a few safety bits to go wrong, and the previous plank may have compromised the safety devices. I'd recommend you not DIYing this.
I've always wondered about this... Now, I'm not saying this can't happen, but I'm a bit perplexed by it.
So my background for my confusion is the SCUBA diving world where we regularly strap several tanks of various gases to our backs and sides with gas pressures up to 300bar. Domestic water is probably no more than
9 bar max, and typically regulated to 2-3 bar for pressurised cylinders (as I understand it - my home pressure is some 8.5 bar and I don't have a mains pressure cylinder)
SCUBA tanks need to be hydrostatically tested every 5 years. To do the test, they fill them with water, then pump the water up to their test pressure which is maybe 25-50% more than their normal working pressure. So this could be 400 bar in some cases. When a tank goes pop, all the reports I've had from the testers I've talked to indicate that it's just a bit of a big splash. (Most of the time they don't go pop, thankfully, they fail a test by not shrinking back to a percentage of their nominal size within a set time preiod - ie. the elasticity of the tank has been lost)
So this where my confusion comes from. I'm not saying that (up to)
250 litres of scalding hot water isn't dangerous, but I'm failing to see how if it's pressurised to a mere 10 bar it can do as much damage as a SCUBA cylinder pumped up to 400 bar (with water). They are placed in a cylinder with a water jacket surrounding them, so maybe this can cushion the shock a little, but even so...
If it was 10 bar of air/steam then it would be a different matter, as these will expand, causing much more damage, but water won't expand. I'd expect the tank to split at a seam or join somewhere and just vent the excess pressure - via a possibly higher pressure jet of water than a garden hose, but I still can't work out how it's going to blow the house down... Water doesn't significantly expand (unless you boil it) and you're not going to get much of a "bottle rocket" effect at 10 bar.
Being a hot water cylinder, it has heating mechanisms. It is the boiling that causes problems. And believe me, the amount of destruction that can occur when one boils with defective safety gear is frightening.
Obviously, when heated electrically using an immersion (usually the backup system), they are particularly dangerous, as the immersion heater is capable of operation long past the water boiling point, leading to superheated steam in the cylinder.
The really serious danger issues do not occur without boiling. However, there is still considerable scope for damage without. In a closed sealed system, with no expansion capability (due to failed expansion system), the pressure increase from simply heating the water normally can be really quite huge. Explosion is less likely, but it can still be very dangerous. I believe in at least one case, a couple were killed in their beds when a cylinder split and covered them in scalding hot water water through the ceiling.
the high resolution video. I wouldn't be too worried about 250 litres of scalding hot water. More worried by bits of steel flying around at several hundred miles per hour.
What is contained in the tank is crucial. Cold water (an incompressable fluid) is often used for hydraulic testing quite safely in quite large boilers (I've seen it in 10m x 2.5m ones). compressed gas is another matter, as it can expand rapidly sending bits of metal etc. long distances. The worst is a liquid above it's boiling point, which can expand into a gas on removing the pressure, and being a liquid there's more of it available as a gas.
Think about the super heated water that can erupt after microwaving, and multiply that many times, and speed it up and you've got mor like a boiler explosion. They were more common with back boilers (solid fuel) in frosty weather and always resulted in considerable damage and frequently loss of life.
If the safety devices have been tampered with, get a suitable expert to give an opinion, and make sure they know what they are doing!
And that was only a 50 litre tank, not the 200 litre or so typical of a UK mains cylinder. When these things go, they demolish buildings and sink ships.
OK, so following up my own post - I undestand what's involved now. It's not the water, it's boiling water thats the problem - coupled with a broken safety valve, one-way valve from the mains, steam, pressure ->
kaboom. I thought there was something else at work here. (And I do voluntary work for the local steam railway, so understand only too well what high pressure steam can do :)
(And you can pump 2 litre coke bottles up to about 6 bar too which make very effective bottle rockets :)
But IIRC, the OP's problem is a constant supply of higher than normal pressure hot water - which might not indicate a boiling boiler, but a faulure of the mains regulator which, although not nice for fittings, etc. that can't take the pressure, shouldn't neccessarily be a danger to life, or the tank... But getting someone in who fully understands it all would be a very sensible thing to do!
If it's the one I think you mean it was the cold water header tank that failed, flopping over after they had ignored the cylinder boiling over repeatedly, filling it with scalding hot water.
Actually, two of the three things required for explosion have now happened.
Firstly, the expansion mechanism has failed. The system has a non-return valve (the pressure reducing valve) which means that the pressure will increase rapidly when the cold water expands in normal use.
Secondly, the pressure relief and safety systems may have been disabled. This will lead to any increase in pressure being unchecked.
Now, all it needs is an unchecked heat source, such as broken boiling boiler (not very likely) or a failed thermostat in the immersion (much more likely) to have an explosion. The sheer number of safety devices is very high in UK installations (the immersion must have 2 thermostats, for example), but the OP is rapidly approaching having defeated all of them!
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.