Any plumbers here? Howling water system and expansion tank killer.

[sploo]

We've got a mains pressure hot water system that was installed just before we moved into our current property (circa 2017). The whole house was completely renovated so has new plumbing throughout.

Unfortunately the hot water system eats expansion tanks for breakfast (we're on our third already) and is now making deafening howling sounds when water is drawn.

Pipe A in the image below is the cold water feed up from the ground floor.

Pipe B (coming from pipe A) is (I assume) supplying cold water to the bathrooms on this (first) floor.

Pipe C is the hot water coming down from the top of the cylinder (not that clear with the angle in the photo).

Pipe D (coming from pipe A) is the cold water feed to the bottom of the tank, and also goes round the back of the tank to the expansion vessel in the right of the photo.

Finally, pipe E is the overflow with tundish, going out through the external wall.

When we draw either hot or cold water upstairs, or when we draw hot water downstairs, the system howls like a banshee. I'm assuming this will be an issue with pipe A as that's the only one in which water would flow in all scenarios (upstairs cold water from A to B, or refilling the cylinder from A to D as hot water is drawn). If I reduce the water flow in pipe A using the valve lever at the bottom left of the picture I can reduce, but not eliminate, the problem. Anyone got any suggestions as to what's wrong (and how I'd fix it)?

Also, the "tank killer" seems to destroy expansion tanks for fun, and almost whenever we have the hot water or heating on there will be a steady (over)flow of water down pipe E and out onto our drive. A new expansion tank fixes it for a while, but not for long. Is there a way to prevent it from happening (once we have - yet another - new expansion tank)?

 

[Sideways]

Just an observation. Howling can be caused by the pressure in the expansion tank being wrong. It messes with the valve that provides the regulated hw supply / balanced pressure cold feed (grey plastic top). Get the pressure right and the valve quietens down.

The grey topped valve is a vital component. It regulates / limits the pressure of the cold water feed from mains into the tank to (I think) 3.5 bar max. It also provides a secondary outlet of cold water regulated to the same pressure. If you have a shower or mixer taps you take the cold water supply from this valve and the the hot water from the tank. The two will be at the same pressure so they work well.

The pressure regulating valve is installed high up because it is a spring loaded component and will wear out. Being installed high up means that you don't need to drain the HW tank down far, if at all, in order to replace it.

Tell us about bleed valves: Air will naturally collect in the high spot of any collection of pipework. I see a ton of pipes all "crowned" by the pressure regulating valve and the red topped automatic bypass valve. Air is bound to collect up there. How do you get it out and let the valves work properly ?

 

[sploo]

Just an observation. Howling can be caused by the pressure in the expansion tank being wrong. It messes with the valve that provides the reglated / balanced pressure cold feed (grey plastic top). Get the pressure right and the valve quietens down.

Does that mean "knackered expansion tank likely causing noise: solution to noise is to replace expansion tank", or "adjust the grey valve at the top of pipe B to see if that fixes the noise"?

 

[Spectric]

Has the expansion vessel been properly sized for the capacity of the hot water cylinder ? I have a 120 litre OSO cylinder that has a pair of 5.3 litre expansion vessels sitting on the top so for 120 litres I have 10.5 litres of expansion capacity. The expansion vessel is normally fitted to the hot water outlet side of the tank, pipe C and not the cold feed side pipe D.

and almost whenever we have the hot water or heating on there will be a steady (over)flow of water down pipe E and out onto our drive.

This is a safety issue, but because the way the two discharges are interconnected you do not know if it is the cylinder pressure or the system pressure but I would suggest it is the cylinder pressure. When the tank is heated and the water expands if fills the expansion vessel and if under sized then the excess hot water has no where to go and so the pressure release valve opens to release the excess, if it did not then the tank could rupture. Where is the expansion vessel for the boiler heating loop ?

I will say that I do not like the safety pipework to the tundish and I think it is not in compliance with the regulations concerning an unvented hot water tank, the system over pressure valve at the top of your picture should not connect into the discharge from the cylinder over temperature / excess pressure valve in pipe E. From memory the max length of pipe between valve and Tundish is 600mm which looks close but to many bends and the pipe from the tundish should be larger which it again looks like it is but vertical with no elbows / bends for a min of 300mm, followed by a constant fall which it appears to be horizontal.

It looks like there is a pressure regulator at the end of pipe A which is setting the pressure for both the cylinder cold supply and the cold feed to the property, this is so both hot and cold are at the same pressure which is a requirement for some thermostatic shower valves and mixer taps.

 

[Sideways]

Does that mean "knackered expansion tank likely causing noise: solution to noise is to replace expansion tank", or "adjust the grey valve at the top of pipe B to see if that fixes the noise"?

Could be either of those but I encountered the problem when the precharge pressure in the expansion tank was a little low. Adding 0.2 bar of pressure fixed it, so off that one experience I'd say it was fairly sensitive to precharge pressure in the expansion tank.
This was with all new components so I wasn't second guessing if they were worn out. My assumption was everything was new and OK, at factory preset, but I was expected to tune it according to the manuals.

And an 18L expansion vessel was supplied in the package with a 180L tank, so 1 litre expansion per 10L hot water. Sizing is specified by the regs. Read G3 of the Building Regs to confirm that your plumber did it right.

I think that the connection of the expansion vessel to the regulated cold feed to the tank complies with the regs and looks OK.

There's a good straight drop of 22mm copper pipe from the tundish that looks compliant. The horizontal looking section of that pipe that goes out to the right must have a steady fall (of at least 1 in 20?) which adds up if the pipe has a distance to go to exit the property. It's worth checking there's enough drop on that pipe.

 

[Bingy man]

The one area of my many years as a gas engineer that I didn’t pursue was unvented/mains fed systems and thermal store so i won’t comment on what your problem (s) could be . @Spectric seems to of covered most of these above . But and this is fact - your system should be inspected yearly along with your boiler as if it’s not installed correctly it has the potential to seriously hurt - by way of scalding hot water . My advice is to get a professional engineer qualified in unvented/ mains hot water systems and have if fully tested. Lots of rip off merchants out there so b.gas would be my go to . If they don’t fix it they can’t charge so ask for an engineer to visit ( chargeable) then a quote to correct any issues. Hopefully it will be something simple but you don’t won’t know until a trusted engineer inspects it . Once sorted out it on a service contract so that very expensive system is covered for future repairs inc the actual cylinder £1,500 + to buy ..

 

[Spectric]

When the tank is cold, ie say 18° C then if the pressure within is regulated to 2 bar then the pressure for the expansion vessel should be 2 bar which means equilibrium and the vessel will be empty of water. Now as the cylinder is heated and the water expands it will move the diaphragm to start filling the vessel. Providing when the cylinder has reached it's setpoint temperature the expansion vessel still has capacity then the pressure will remain at 2 bar, if it has maxed out then the pressure will increase and the relief valve will open to discharge enough liquid to drop the pressure.

 

[Daniel2]

As above, your problem is almost certainly one of excess supply pressure.
It may be that the regulating valve you have up there is not sufficient for
the job if your supply pressure is too high.
At least over here, where sealed systems are the norm, a PRV is commonly
installed just after the water meter. A secondary one is then installed at the
boiler, or immersion heater.

 

[Jester129]

Get the installer back and get him to sort it out, and ask him to resolve the expansion tank issue out. Why have you not done this already?

 

[Sideways]

What @Bingy man says.
Sealed systems require an annual check.

He mentioned pressure relief valves. There will be three at least in your system.
One is in the boiler. Overpressure in the CH system and hot water heating coil causes hot water to be discharged out of a copper pipe, most likely through the wall. Leakage here is most commonly caused by a problem with the other expansion vessel that is (probably) inside your boiler. If that doesn't have the correct precharge pressure, it causes overpressure and the boiler pressure relief valve will vent. Once vented, they often tend to drip because dirt gets into the valve.

The second is the red knob directly above the tundish on your tank. That Temperature and Pressure Relief Valve lets go if the pressure in the hot water tank gets too high (7 bar) or the temp too high (90C). This is a fail safe valve in case the boiler heating controls fail and the boiler keeps heating the tank after it should have shut off. 7 bar and 90C are both very high so this valve should never be leaking in normal use. If it is, the valve is dirty or faulty, or you have a serious problem.

The third safety relief valve is up with the grey topped pressure regulating valve in the top left of your photo. I can't see if this is an "all in one" type "multiblock cold water control valve" or two pysically separate valves but either way, the grey topped one regulates your incoming mains water pressure down to 3 bar (standard factory setting) and the red topped one is a pressure relief valve set to 6 bar. This discharges into the top of the tundish just above your letter E and being set at a lower pressure should vent before the T&P relief valve fitted to the tank.

For either of the second and third PRV (Pressure Relief Valves) to vent is most commonly a result of the expansion vessel running out of expansion space. If the air pressure in your expansion vessel isn't set high enough to balance out the cold pressure in the hot water, then this very likely. During a service, the engineer should take the pressure off the hot water system and check the precharge pressure in the expansion vessel, then pump it up if necessary. You can't check this without taking the pressure off the hot water.
Provided your mains water pressure is above 3 bar, the multiblock control valve will regulate tank pressure down to 3 bar. The precharge pressure in the expansion vessel must match that. That is why expansion vessels come precharged from the factory with inert, dry, nitrogen at 3 bar. But air valves leak and this needs to be checked during installation and the annual inspection.

One last point.
Your system is an S plan type. It has separate electric valves on the hot water and central heating.
When your system has been running and the valves shut off because both the house and the water are up to temperature, the boiler will shut off and pump should continue to run for a little while to dissipate the heat in the boiler. If it doesn't the pressure in the boiler will spike because the boiler is very hot and the water that is carrying that away has suddently stopped. During pump overrun, and also in case of a potential fault with a valve, there needs to be a way for water to keep flowing through the boiler even though both valves are shut. One way is to have a pipe "short circuiting" the tank coil, with a fixed valve in it that is set part open to allow a minimum flow at all times. Another way is to have this short circuit with a spring loaded automatic bypass valve (ABV, yet another valve with a red top, but this time squareish and with an adjustment scale). The ABV works at low pressures so while the electric valves are open, all the water goes through them and only when they both close can the pump push water through the ABV instead.
I don't see an ABV in your installation. It would be worth identifying the bypass arrangement and making sure it is correctly set, hasn't been closed by accident.

This is all stuff that a G3 service engineer will know inside out but you may not. I hope it helps but @Bingy man 's advice to get a service engineer on the job is the way to go. The problem is most likely to be pressures, could be a defective valve or yet another bad expansion vessel, might be something wrong with the design that is aggravating this.

 

[Henniep]

Plenty or experience and sage advice in the responses. This may sound silly, but you indicate that this is a recent installation and noise started only after. If a stop cock has been installed with the flow counter to the direction indicated by the arrow on the stop cock, the seal inside the stop cock will 'chatter' or howl, as you put it.

 

[sploo]

Many great responses (thanks to all). To try to answer at least some of them...

The main cylinder label indicates 200 litres, expansion tank is 18L, with a 3 bar pre charge pressure. So, a little below a 10:1 ratio in litres with the main tank.

I'm reasonably certain the PRV at the top of pipes A and B is this https://www.unventedcomponentseurop...V-533002-CST-Monobloc-Manifold-Data-Sheet.pdf and the layout of our system does appear to match the drawing on the second page of that datasheet; albeit the expansion vessel is off pipe D, rather than using the optional port on the rear of the PRV.

I'm fairly certain the issue with water flowing down the tundish is cylinder pressure rather than the system pressure, as it only happens when the boiler is heating (hot water or central heating), and if I turn the 7 bar/90C red pressure valve at the top of pipe E I can get a stream of water down the tundish; which then stops the "leaking" for a while (I assume that's reducing the pressure in the tank for a short time; hence that valve doesn't then leak for a few minutes).

I believe the 7 bar/90C valve was replaced the last time we had a new expansion tank, as the plumber told me that a dead expansion tank will often damage that valve.

The original installer/plumber pre-dates us moving in. We bought the house from someone who'd had all the work done. I did get a list of the installers from the owner, but funnily enough I was never able to get a response from that plumber.

It's a "recent" installation; in the sense of mid-2016. We've always had problems with it "killing" expansion vessels and leaking through the tundish. The howling in the pipework is a recent problem (last month or so).

Could it be as simple as the PRV at the top of pipes A/B not reducing the pressure sufficiently, and that's over pressurising the cylinder?

There's a thermostat on the right side of the photo (attached to the cylinder) and it's set to ~50C. It's electrically connected to the larger white box at the bottom right of the photo; which goes to the grey boxes marked H/W and HEATING. Running hot water from a tap in the property measures at a max of 56C. As such, I'm assuming it's not a case of a faulty thermostat resulting in too high a temperature inside the cylinder.

 

[Spectric]

One way is to have a pipe "short circuiting" the tank coil, with a fixed valve in it that is set part open to allow a minimum flow at all times.

This used to be the way using a lockshield valve but are now taboo due to efficiency concerns. Like everything it has caused more complexity especially when you use a smart circulator pump. The normal way is to use something like an automatic bypass valve like this

which works on pressure, all zone valves closed and boiler on overun the pressure rises and this valve opens to provide a loop so the hot water in the boiler can flow and lose temperature otherwise you can end up with ketling. If the pump is a smart pump you have a problem in that when the zone valves close the pump backs of and the pressure does not rise so these valves cannot be used. The other option I have used with a smart pump is a three way valve as used in a Y plan system which in the unpowered state provides a bypass and only when there is a demand will it fully open to divert flow to the zone valves.

 

[Sideways]

@sploo
Your description is helpful, thanks.
The one test I would suggest that doesn't meddle with the system is this.
Turn off the boiler.
Turn off the cold water supply to the tank before the monobloc valve (top left), and run an upstairs hot water tap until all the pressure inside the hot water tank is gone.
Unscrew the black cap on the top of the expansion vessel. You'll find a schrader valve like on a car tire. Use a trustworthy tyre pressure gauge to check the precharge pressure in your expansion vessel.
It should be 3 bar as it came from the factory. If it is a little higher, 3.1 or 3.2, don't let the excess out. Better slightly high than low, and type pressure gauges have a tolerance so just a little high will guard against any pressure gauge error in the wrong direction.

If it is low, this will be part of your problem and needs to be put right. You can pump it up yourself but as said before, get a service engineer out is really the better way as they can check for more issues and they're insured.

After checking the expansion precharge pressure, you can turn on the cold supply to the tank again, and run a hot tap for a few seconds to see it's all back to normal.

Now, check the expansion vessel air pressure one more time.
If the monoblock valve is regulating pressure to 3 bar as it should, the pressures between the expansion vessel and tank wil be balanced, no water will be forced into the vessel and the air pressure should remain at 3 bar.
If the monoblock is regulating the tank water pressure to a higher value than 3 bar, then that higher pressure will push water into the expansion vessel until the pressures equalise. So this gives you an easy way to measure if the monoblock valve is over pressurising.
If the monoblock is regulating at less than 3 bar, all you'll see is the same 3 bar precharge pressure in the expansion vessel, so you won't know, but it's also less of a problem.

Put the black cap back on the expansion and let it be. Now you can turn the boiler back on.

 

[sploo]

@sploo
Your description is helpful, thanks.
The one test I would suggest that doesn't meddle with the system is this.
Turn off the boiler.
Turn off the cold water supply to the tank before the monobloc valve (top left), and run an upstairs hot water tap until all the pressure inside the hot water tank is gone.
Unscrew the black cap on the top of the expansion vessel. You'll find a schrader valve like on a car tire. Use a trustworthy tyre pressure gauge to check the precharge pressure in your expansion vessel.
It should be 3 bar as it came from the factory. If it is a little higher, 3.1 or 3.2, don't let the excess out. Better slightly high than low, and type pressure gauges have a tolerance so just a little high will guard against any pressure gauge error in the wrong direction.

If it is low, this will be part of your problem and needs to be put right. You can pump it up yourself but as said before, get a service engineer out is really the better way as they can check for more issues and they're insured.

After checking the expansion precharge pressure, you can turn on the cold supply to the tank again, and run a hot tap for a few seconds to see it's all back to normal.

Now, check the expansion vessel air pressure one more time.
If the monoblock valve is regulating pressure to 3 bar as it should, the pressures between the expansion vessel and tank wil be balanced, no water will be forced into the vessel and the air pressure should remain at 3 bar.
If the monoblock is regulating the tank water pressure to a higher value than 3 bar, then that higher pressure will push water into the expansion vessel until the pressures equalise. So this gives you an easy way to measure if the monoblock valve is over pressurising.
If the monoblock is regulating at less than 3 bar, all you'll see is the same 3 bar precharge pressure in the expansion vessel, so you won't know, but it's also less of a problem.

Put the black cap back on the expansion and let it be. Now you can turn the boiler back on.

Very clear instructions - many thanks.

The expansion tank pressure reads a nice round 0 bar. So I pumped it up to 3.1 bar. At about 1 bar the main cylinder did an impressive "fart", but otherwise nothing untoward happened.

I turned the cold water flow to the tank back on, ran some hot water, and checked the expansion tank again - still 3.1 bar.

I then ran a fair amount of either hot or cold water from the upstairs taps. Interestingly it's greatly reduced the howling in pipe A; essentially nothing when pulling hot water (and refilling the tank), and very little when pulling cold water.

Expansion tank is still at 3.1 bar, so I've switched on the hot water (which usually causes leaking via the tundish) and I'll keep an eye on what happens.

 

[sploo]

Expansion vessel at 3.4 bar after ~20 minutes of the boiler running for hot water. No signs of leaks via the tundish... yet.

 

[sploo]

Another 10+ minutes passed; boiler's just ramping down (in temperature) so I assume the tank is nearly at temperature. Expansion tank reads 3.0 bar, but there has been a little bit of dripping via the tundish (though less than usual).

I guess it's a case of keeping an eye on the tank pressure over the next few days.

 

[Sideways]

Pressure should increase as the hot tank heats without any water drawn off.
That is to be expected.
As you draw water off, it will fall back to 3 bar because the incoming pressure is limited to that.
It will then rise a bit as cold water is heated up.
It should stay well under the 6 bar at which the first pressure relief valve lets go and releases water to the tundish.
The most extreme test is a tankfull of cold at 3 bar, heated to max, with nothing drawn off.
Whatever the pressure in the expansion vessel at that point is as high as it should ever get in normal operation.
I suspect you have found and fixed your problem.
Put a note on the calendar and do this check again in a few months, and then regularly every year

 

[Sideways]

If you have drips at the tundish but otherwise the system has settled down, it is likely that the pressure relief valve(s) have suffered and or dirt has settled in the seals. You have nothing to lose by operating them manually where possible as this may flush any dirt, but mains cold water in should be very clean so you may need to pay out and have the monoblock valve serviced or replaced again, and the T&P valve on the tank if the service engineer says so.
Thanks for letting me know it at least partly worked out.

 

[sploo]

Thanks. I suppose it makes sense (based on previous comments from plumbers) that the 7 bar/90C valve can be damaged (hence it's weeping).

Expansion tank pressure stable at just over 3 bar (makes me wonder what happened to the pressure in the first place).

Unfortunately the howling from pipe A came back, but it seems to be quite random as to when it does or does not occur.