Plumbing problem...

[John Brown]

I took the valve core out - zero water, but if I poke a matchstick up the valve outer, I can feel a springy resistance, which I guess is the diaphragm. I put in a new valve core, opened a radiator bleed valve, and tried again with the pumping. Nothing.
Is it possible that I need to ensure the motorised valve that enables flow to the rads is open? Am I trying to pump up the hot water tank? Or, if neither valve is open(S configuration), just trying to pump up a few feet of piping?

 

[Spectric]

I think you need to dump the pressure, then ensure that the expansion vessel is pressurised to 1 Bar and then re-pressurise the system but make sure the expansion vessel holds the air pressure first. It might sound like a good idea to fit an isolation valve in the pipe to the primary expansion vessel but this is NOT allowed because you will be providing a means to isolate the vessel from the system.

 

[Pedronicus]

I think you need to dump the pressure, then ensure that the expansion vessel is pressurised to 1 Bar and then re-pressurise the system but make sure the expansion vessel holds the air pressure first.

Exactly that. You should only top up the air side of the expansion vessel when there is no pressure on the water side.

If you can feel what you think is the membrane than the air pressure has gone. When was the expansion vessel air side last checked/topped up? My guess would be never since installation. Very few service bods (and certainly not the 'flying blue van' mob) actually check this pre-charge pressure.

 

[John Brown]

No pressure entails a total drain down, as the expansion vessel is on the ground floor, and there are 8 or 9 rads upstairs. As I said, I want to change some trvs as well, so I think we'll limp along until the weather changes.

 

[John Brown]

I'm still thinking about this - I think the problem might be that the diaphragm is pressing on the back end of the valve. I may try and extension piece with its own valve, and remove the core from the vessel..

 

[Fitzroy]

Sounds same as when my expansion vessel failed. The diaphragm in the vessel ends up forced against the vessel walls. My theory is once the diaphragm fails the air in the vessel leaks onto the water side, most boilers have a auto air release, or you bleed the air out of on of the radiators which is cold. When you then pressure the system you force the diaphragm back into the vessel. The remaining air leaks back to the water side. This process is repeated until the diaphragm is firmly forced onto the vessel wall where it becomes stuck.

A tall house is 30ft/10m this is only 1bar or 15psi of static head. This would not stop you inflating the diaphragm with a regular bike pump.

 

[Fitzroy]

This thread is interesting, because we have a mains pressure hot water system that kills expansion vessels like they're going out of fashion. We're on our third in seven years, and that one's failed too

An undersized expansion vessel for your system volume will put more stress on the diaphragm, could be part of your issue.

We installed a number of old school radiators and had to install secondary expansion vessel due to the extra volume.

 

[John Brown]

Sounds same as when my expansion vessel failed. The diaphragm in the vessel ends up forced against the vessel walls. My theory is once the diaphragm fails the air in the vessel leaks onto the water side, most boilers have a auto air release, or you bleed the air out of on of the radiators which is cold. When you then pressure the system you force the diaphragm back into the vessel. The remaining air leaks back to the water side. This process is repeated until the diaphragm is firmly forced onto the vessel wall where it becomes stuck.

A tall house is 30ft/10m this is only 1bar or 15psi of static head. This would not stop you inflating the diaphragm with a regular bike pump.

Yes, but the pressure of the water concentrated onto the pin at the back of the valve..?

 

[Fitzroy]

Yes, but the pressure of the water concentrated onto the pin at the back of the valve..?

When I took mine out the system I could only force air into the vessel with a small bike suspension pump at 100psi+ the diaphragm was firmly adhered to the vessel wall in all locations. It was the adhesion causing the issue not the pressure in the system.

 

[John Brown]

When I took mine out the system I could only force air into the vessel with a small bike suspension pump at 100psi+ the diaphragm was firmly adhered to the vessel wall in all locations. It was the adhesion causing the issue not the pressure in the system.

Interesting, thanks. I hope to borrow a Makita cordless inflater from a friend in the village, not sure if that'll help, but certainly easier to try than trying to use a floor standing hand pump against the wall.

 

[Spectric]

We installed a number of old school radiators and had to install secondary expansion vessel due to the extra volume.

This is common when people want the traditional cast iron radiators, they also work well with condesning boilers as they can easily deliver a good heat output with the lower water temperatures.

If you get a good pump like

https://www.cityplumbing.co.uk/p/regin-pressure-vessel-hand-pump-regk30/p/557235

then it has a long hose and you clip the adaptor onto the valve, then you have both hands to pump and a foot to support the pump.

The problem is that you need to release the pressure in the system but as you said

No pressure entails a total drain down, as the expansion vessel is on the ground floor, and there are 8 or 9 rads upstairs. As I said, I want to change some trvs as well, so I think we'll limp along until the weather changes.

so when you do then rather than mess around with the old one just fit a new one and then check it annually, this should be part of the annual service. Also use a decent corrosion inhibitor as this can save a lot of issues from happening.

 

[Pedronicus]

No pressure entails a total drain down, as the expansion vessel is on the ground floor, and there are 8 or 9 rads upstairs. As I said, I want to change some trvs as well, so I think we'll limp along until the weather changes.

You do not need a total drain down. Just release the pressure via a rad vent until the boiler pressure gauge reads zero. Then pump air into the expansion vessel. The boiler gauge will increase so you let this out from the rad vent and add more air to the vessel. Carry on with this procedure until the vessel pressure (read from the pump gauge) is up to the required charge pressure. Then add water into the system via the filling loop until the boiler gauge reaches 1 bar (or just above the join between the red and green segments on the gauge if that is the case). All this is done with the boiler off and system cold. If the gauge goes up to 2 bar or above when the system is running then that would indicate that the vessel is too small. If water discharges from the pressure relief valve then that will probably have to be replaced as well.

 

[John Brown]

You do not need a total drain down. Just release the pressure via a rad vent until the boiler pressure gauge reads zero. Then pump air into the expansion vessel. The boiler gauge will increase so you let this out from the rad vent and add more air to the vessel. Carry on with this procedure until the vessel pressure (read from the pump gauge) is up to the required charge pressure. Then add water into the system via the filling loop until the boiler gauge reaches 1 bar (or just above the join between the red and green segments on the gauge if that is the case). All this is done with the boiler off and system cold. If the gauge goes up to 2 bar or above when the system is running then that would indicate that the vessel is too small. If water discharges from the pressure relief valve then that will probably have to be replaced as well.

You may not have read the entire thread. I can't get any air into the expansion vessel at all. I have opened, and in fact totally removed a bleed valve from an upstairs radiator, but to no avail. I think Fitzroy has the answer - the "diaphragm"(looks more like a bladder in the YouTube vid I watched) is firmly adhered to the inside of the outer container. No water has come out, even though I've removed the Schrader valve core.
I'm with whoever it was that said the expansion vessel pressure had never been checked. Certainly hasn't been in the 4.5 years we've been here.

Consequently, I shall probably not even try to repressurize it, as there's a good chance the rubber could split or tear even if I could get it to separate from the housing.
As I said, a job for warmer weather. The thing I must do, however, is change the HW motorised valve head. I changed the CH one last year, almost entirety by feel, but thankfully the HW one is a little easier to get to. Just wish I'd made a note of the wire colours last year, as although it's exactly the same Honeywell part, the colour codes are slightly different.

 

[Jaydee]

Is the expansion vessel on the hot water system. Because if it is you need to open a hot water tap to relieve the pressure to put air into the vessel. That’s what I had to do on my system. I isolated the vessel by way of a valve from the hot water tank, then opened a hot water tap. Then as I pumped in air the tap let out water trapped in the vessel. It was not easy as I was using a car foot pump in an airing cupboard. If this is not applicable I apologise. If it is I hope it helps.

 

[Setch]

Is the expansion vessel on the hot water system. Because if it is you need to open a hot water tap to relieve the pressure to put air into the vessel. That’s what I had to do on my system. I isolated the vessel by way of a valve from the hot water tank, then opened a hot water tap. Then as I pumped in air the tap let out water trapped in the vessel. It was not easy as I was using a car foot pump in an airing cupboard. If this is not applicable I apologise. If it is I hope it helps.

I don't think this is correct. The hot water is not pressurized, it is the coil which heats the hot water (as part of the sealed system) which needs to be depressurized.

Opening a hot tap will release water from your tank, which will be topped up by cold water from the mains feed.

 

[Jaydee]

Yes, but you isolate the vessel first from the hot tank. It should have an isolation valve before the vessel. Then as you put air in the vessel the excess water comes out of the opened hot water tap.
This is of course if the expansion vessel is indeed on the hot water side.
The reason for the expansion vessel is to even out the mains pressure running through the hot water system.
This is how it works on my system. Of course your system could be completely different.
I’m not a plumber, but this is how it was explained to me by my plumber when I needed to re pressurise my system. Which I managed to do.
If this is just confusing the issue I apologise.

 

[John Brown]

The expansion vessel in question is attached to the central heating(radiators) and the indirect heating coil in the hot water cylinder.

 

[Jaydee]

Obviously my system is different. I apologise for any confusion.

 

[John Brown]

Obviously my system is different. I apologise for any confusion.

No apologies needed, thanks for trying to help!

 

[Spectric]

Just wish I'd made a note of the wire colours last year, as although it's exactly the same Honeywell part, the colour codes are slightly different.

Where possible just cut the cable close to the junction box and then fit the new head. Now you still have the original connections and can just match the colors. Blue = Neutral Brown = Thermostat Grey = Live Orange = Boiler

Operation is power supplied via the thermostat to the valve head motor on brown wire which opens the valve, this operates the micro switch which connects the grey live to the orange boiler supply.

The reason for the expansion vessel is to even out the mains pressure running through the hot water system.

With an unvented hotwater tank the expansion vessel is there for just one purpose and the giveaway is in the name, expansion. A volume of water in a sealed vessel expands upon heating, the expansion vessel is there to accomodate this excess volume of water which if it had no where to go would result in an increase in pressure which is why there is a PRV's , to stop the tank from rupturing. The expansion vessel on the heating side is there for the same reason, to allow the water in the rads to expand upon heating without causing a large rise in pressure, this is why you need the correct size or an oversized expansion vessel.

Not along ago I had a noise when my hotwater came on, a clanging sound which I identified as being caused by a volume of water trapped between the three way bypass valve and the hotwater zone valve. This water was at 70° C when the valves closed, it cools down and contracts so reducing the pressure in this section of pipework. Now when the valve re-opens you get the clang as the pressure equalises and was fixed by fitting a small expansion vessel in this pipework, a 0.5 litre did not work and it needed a 1 litre vessel which shows how large the expansion can be in a heating system.