Leaking H/W pressure release valve

[Graham.]

I meant I *can't* see the point.

Sorry

[Graham.]

Are you sure the device on the water main isn't a softener, I can see the point of an expansion vessel there.

[Lee Nowell]

Hi All,

Well I think that's what it is. It looks similar one of these

In my situation the large black bit is positioned to the left and the other ports are Top - cold water feed to house Bottom - mains water feed from outside Right - connected to top of h/w cylinder (it is a mega flo type thing for mains pressure hot water)

The leak is coming from the big black bit but not at the end somewhere between the brass bit and the end.

I have done some googling and there seems to be a mix of views

1. This is normal and that's what happens when the pressure is high. Seems odd to me and mine continually drips. Also not sure what pressure is too high - boiler circuit is set at 1.5 bar and inbound pressure is not great
2. It is because the expansion vessels have run out of air. I appear to have 2. One in the inbound supply side and one on the circuit coming out of the boilers. Some people seem to say to just check them and if low too them up (not sure to what) with a tyre inflator (presume the little compressor thing I use to pump tyres up). Others say this is very dangerous. Also TBH not sure what the purpose of either is and therefore which one is likely to be it. If safe, I could do both of course.
3. The valve is faulty and it needs to be changed by a qualified person as could be dangerous.

I have done a lot of plumbing including designing and fitting the whole plumbing (obviously no gas work!) in our last place and adding to the current one so am pretty able but some of the "this is dangerous" is making me think twice.

Any thoughts appreciated. Also if anyone know why I have 2 expansion vessels and what they do that would fill a void in my understanding too.

Thanks in advance

Lee.

[Lee Nowell]

Definitely not a water softener. Both look largely the same and are mounted on the wall by a thread bar attached to the bit where the vessel connects to the pipework. The one on the supply side is plain grey in colour with no labels etc on it whereas the other one is red/ orange and has manufacturer labels on it (not at home at the moment so can't tell you what they say)

[Lee Nowell]

Hi All,

I went to look again at this with a torch and discovered a part number of the valve. It is actually one of these

I have the old version in the diagram and the drip is coming from the end of the fatter bit at the bottom where the plastic nut is. Is this valve to prevent some part of the system having too much pressure? It says the Outlet pressure preset is 3.5bar and the Expansion Valve preset is 6 bar. Not sure what this means? Is it worth trying to tighten the plastic nut? I am afraid it might break and bugger the whole thing up?

The expansion vessel on the boiler output circuit is a Zilmat 130-cal-pro 20013 which looks like a standard expansion vessel. I have double checked the other expansion vessel and no marking on it so don't know exactly what it is.

Thanks

Lee.

[Andy Burns]

It does two jobs, one for proper operation, one for safety of an unvented system.

That's the pressure supplied to the H/W cylinder and the cold taps (so they are equal and mixers work properly) obviously it can be less if your supply is less

That's the relief pressure that the system will 'blow off' at, and is the safety part.

Have you got any way of reading the pressure in the system? (e.g a schrader valve where you would pump up the expansion vessel)

Do you see water dripping through the tundish?

[The Natural Philosopher]

That is NOT a pressure relief valve. It is a pressure REDUCING valve. Designed to keep the whole unvented system from going over 2.1 or maybe

3 bar.

that will be one 3 bar feed.

Unreglated input

Another 3 bar feed to the sylinder

Replace the sodding thing. Mine failed in a different way and a neighbours is alos dripping like yours is.

Bollocks.

That may be becuas yourr cylinder doesnt have one. Ther are tow pressure zones - one is te DHW/CH *primary* that has an expanosion vessel in or near the boiler usually and the other is a DHW secondary in the 'pressuerised' part of te hot water that is regulated by that valve.

i.e cold wetra enters tank till pressure is 2.1 - or maybe 3 bar - depemndiong on te tank.

That water then gets hot. Pressure ises in te tank. So there is an expansion vessel. In mine its a bubble at the top of the tank. Looks like you have a separate vessel

Yes. If they have not failed. If when you try and deflate them WATER instead of air comes out, they have failed.

Pump up to usially a couple of bar - 28 pisi or thereabouts

The world ois full of ignoirant snowflakes

It will be the one vonnected to te hotr weatr tank outlet not the primary heating circuit

No bad plan to check em all.

1. The valve is faulty and it needs to be > changed by a qualified person as could be dangerous.

The valve is almost certainly faulty but that may be becauise the tank has overpressured due to a faulyy expansiuon vessel on the hot water side.

It is only dangerous if you are a typical thick British plumber or a snowflake.

That valve is there to regulate mains pressure down to a safe level for the tank. As long as it is fitted cleanly with no leaks thats it, job done. Mine got scaled up and failed. I had NO pressure at all! It was cocered in scale outside so had been leaking like yours

One for the boiler primnary circuit, one for the domestic hot water tank

how to fix this.

1/. Check all expansion tanks. This is something you should do annually anyway as failed expansion tanks cause a LOT of damage. Whilst my Ex had the house this happened and I lost three gate valves and two motorised valves as pressures went mad and blew seals everywhere.

The key sign is that water comes out where air should be. These tanks are essentiually made in tow halces with a rubber membrane between. Air is then pumped into one half to allow expansion of water in the other half. They fail because the membrane tears or leaks or whatever allowing water everywhere. The air ends up in the water an you bleed it out and then its all gone.

If at all unsure replace the sodding things. £50-£150 is the going rate.

2/. Replace the valve. £42 quid - mine was £100 - is a cheap price to pay to fix a nasty plumbing problem. Obviosuly you need to depressurise everything first by turning off te mains supply and then opening hot and cold water taps still it all stops gushing

Try and get exact replacement. Mine wasnt and I had to mess with cutting olives off pipes to get the old one totally out, and then adding pipe section to nake it all play nice again.

So factor in that you make need to do this and get some 22mm pipe couplings and pipe in case you need to essentially cut the old one out.

If you are prepared you wont get so upset when it all turns out to be corroded and scaled to shit.

[Dave Liquorice]

Water expands when heated, the expansion vessels give some where for this increase in volume to go. They are simply a chamber with a stretchy membrane across with the water one side and trapped air the other.

You have two as the boiler circuit is seperate to the domestic HW. Both being sealed both need expansion vessels. As we are looking at the HW side then it's only the expansion vessel on that we need to worry about. If it has failed then the pressure can get *very* high and this could force water out of places it shouldn't come out of, like that pressure reducing valve.

The vessel should have car tyre like valve press the little bobble in the middle air should hiss out, no wet. If wet the vessel has failed and needs replacing. Not 100% fool proof if the valve isn't at the bottom but they normally are... If dry it may of lost pressure on the air side but this isn't the normal failure mode. To repressurise I think you'd need to de-pressure the wet side and pump up the air side to a preset value dependant on that particular vessel, that figure might be on the label.

[Lee Nowell]

Thanks all. Seems like getting the actual part number has aided my googling... Looks like other have has the exact same issue and taking out the assembly and cleaning it seems to have sorted the problem for some. It is referred to as "cleaning the strainer". Will give this a go next weekend.

In response to the other questions...

1. yes mine has a car tyre type valve at the top will try and get a reading on it
2. no water coming out of the tundish

Couple of follow up questions

1. The valve has an adjuster on it. If I turn it up I assume I will get greater water pressure in the house (assuming it is limiting it)? The showers a ok but could be better - I had always assumed it was the inbound pressure being the issue.
2. I have recently had problems with an existing thermostatic shower where the hot seems to have "suddenly" over take the cold pressure and it is not getting much colder when you turn it down. Similarly, as new one I fitted a few weeks ago had the opposite problem where the cold was overtaking the hot and I had to adjust (a lot) the cold flow at the shower to make it work. Wonder if these are symptoms relating to the current issue discussed above?

thanks

Lee.

[Lee Nowell]

So just checked the expansion vessels.... The one on the boiler side had air in it although unfortunately the valve was sticky to stuck open when I tested it so a fair amount of air came out. Now sitting at 1.1 bar according to one of the car tyre pressure gauges that pop out an inner cylinder to read the pressure (not sure how accurate they are).

The expansion vessel on the cold feed side nothing comes out at all. Looks like that needs filling?

[The Natural Philosopher]

If its a thermostatic shower it should be insensitive to input pressures largely

[John Rumm]

Not sure the connections as described make sense - although that depends a bit on what you mean by the connection to the top of the cylinder.

The cold water control valve normally provides a supply of mains cold water to the cylinder (through a strainer to catch particulates), at a reduced pressure. Normally it facilitates a connection to the main DHW expansion vessel, and usually provide a pressure balanced cold output for use in the house in situations where having equal hot and cold pressures is particularly desirable - e.g. showers.

See:

Dismantling, cleaning, and (possibly) descaling will often help

Nothing to do with boiler circuit pressure - that's not connected to the incoming mains water directly.

If you turn off the cold water feed to the cylinder and go draw some hot water from a tap, you would expect it to keep providing water at a reducing pressure for a short while - basically the stored compressed air in the expansion vessel will push some more water out of the cylinder even without the incoming mains pressure. If you don't see this, then that is a good indication that the vessel is de-pressurised (you would also expect water ejected to the tundish each time the cylinder heats up).

They would be the ones who don't understand unvented hot water cylinders, and prefer the "there be dragons here" policy over self education :-)

If you turn off the cold main to the cylinder, release any stored pressure by opening a hot tap, you can then measure the pressure in the vessel. Then re-pressurise with a pump to the manufacturers recommended pressure.

It allows the water in the cylinder to expand when its heated. Without it, and an otherwise closed system the pressure rise would be very high (and then typically open a safety release valve to discharge into the tundish)

It may be faulty, or may just need cleaning. How much qualification do you need to turn a spanner?

There are safety concerns with unvented cylinders, but these are addressed by the overall system design and that of the controls. Doing a like for like swap with the appropriate part carries no risk beyond that associated with any plumbing job IMHO.

No need to worry. You have a properly designed unvented cylinder from a recognised brand, with all the expected controls and interlocks by the sounds of it.

Yes in theory you can do dangerous things, but you would really have to try, and defeat/disable multiple safety interlocks to even get close to having a serious problem.

The two perform a similar function - but for different and unrelated parts of the system. The one on the boiler allows for expansion of the water in the radiator circuit. That's a sealed system with recirculating (stagnant, dirty) water that goes through the boiler's primary heat exchanger. The one on your cylinder is on the cold water mains input to the store of DHW that you consume in the house.

Have a read of:

Then see the section on sealed systems for boilers:

Then the bit on unvented hot water systems like yours:

[harry]

You seem to have a valve with dual function. These are often two separate valves. Functions are:-

1. To reduce the water pressure (PRV/pressure reducing valve)
2. To relieve excess pressure if the PRV goes faulty (SV/safety valve.)

The PRV will have a high pressure inlet and a lower pressure outlet.

The SV will have an outlet to air to discharge any over pressure. The SV is on the low pressure side of the PRV

If water is dribbling from the SV discharge, it means the PRV is not functioning correctly (allowing overpressure)/needs adjustment/maintenance. OR The SV is faulty/needs maintenance.

You have to find out which by checking low pressure side is what he PRV says on the label. If it's higher the SV is just doing it's job. If it's correct, the SV is probably faulty.

[John Rumm]

No, the outlet of the valve must not discharge to air - this would be prohibited by building regulations. See approved document G section 3.

In an operating situation the water released could be at high pressure and close to boiling point!

The relieve valves (both main cold water valve, and the additional over temperature / pressure valve), must be piped to a safe location for final discharge. There must also be an air break (a tundish) visible for inspection.

There are rules on the lengths / diameters / falls etc permitted. See:

The overpressure can also operate if there is insufficient expansion volume available, so operation does not necessarily indicate a failure of the PRV. (and even in cases where the PRV has failed its relatively uncommon for the incoming mains water pressure to actually be high enough to cause the valve to operate, since they are often set to operate at around 6 bar).

So you also need to check the function and pressure in the expansion vessel.

[Martin Brown]

+1

This is worth a try since it would be what any half decent plumber would try first. They do gradually lose capacity over time and pumping them up will get you another 5 years or so provided it hasn't actually popped.

Unfortunately you do get people who take chunks out of their hot CH system without switching off the boiler or draining it down first.

A charity shop I help out was on the receiving end of a shower outlet that came through the ceiling because the muppet that installed it did not connect up the drain properly. It was amazing how destructive falling plaster can be!

[harry]

A tun dish; ie to air.

[Robin]

While "tun dish" is found I don't think it is by any measure to be preferred over John's spelling. The OED shows "tun-dish" and "tundish" but not "tun dish".

And a tun-dish is not open to air by definition. IIRC some continuous casting processes use a tundish with air excluded.

[John Rumm]

No, a tundish introduces an air break (to protect the mains supply from back contamination) - the water is still contained and it carried by a pipe for safe disposal from the tundish.