Pressure drops when 650 litre buffer tank is filled with water but no drop with air

[Trevor Rix]

I have a 70kW woodchip biomass boiler installation including a 650 litre buffer tank, all installed in 2014. The buffer tank loses pressure from say 2.58 bars to 0.00 bars within a 48 hour period. Yet there are no visible signs of any water leaks,

When this tank is drained of water then pressurised with air to 2.40 bars the same pressure gauge mounted at the top does not drop. And when all of the joints etc. are treated with soapy water there are no visible leaks.

There was a similar scenario with a leg of 60m of highly insulated underground pipe that lost pressure when filled with water, but did not lose pressure when pressurised with air. When the air was replaced with water there is still no pressure drop.

It is thought that the system has been losing pressure ever since installation. We have yet to refill the whole system with water/inhibitor and turn the boiler back on.

Suggestions as to what may have caused this phenomenon, and any possible long term cures, are welcome.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Don't you need to be g3 to work on these?

As alluded to in post #4 it is the original installer who is doing all of this work, not myself. All I am doing is taking regular pressure readings and relaying them to the installer, and topping up the pressure when asked.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Depends on how the 650 litre bit is fed Croppie.
I've seen them open vented and just the heating system side sealed.

Which brings me to my query.

Is this buffer tank, fed from a tank or is it unvented ?
When you say you're loosing pressure is that from the tank itself ?
Meaning the 650 litre side ?

I may be misunderstanding your problem here. For a tank volume like that loosing water like you seem to think, I would expect it to be really obvious.

Are you sure it's not the heating system that's loosing ?

Don't be offended if I've got this wrong it just sounds a bit bazar

The whole system is sealed up to the point of the boiler side of the heat exchangers located in the two properties being heated. The property sides of the two heat exchangers are vented. The property sides of the heat exchangers are isolated during testing to eilimate to the possibilty of leaks within the heat exchangers.

The buffer tank is therefore part of the sealed system. Yes, when filled with pressurised water and isolated, the gauge on the 650 litre buffer tank was losing pressure quickly. When the buffer tank was drained of water then pressurised with air there is no pressure drop.

The buffer tank is normally pressurised manually by a mains feed situated between the boiler and the expansion vessel.

I agree that there should be a highly visible water leak on the buffer but there is none.

The pressure drop(s) is on the sealed system, not on the heating systems in the properties because as explained above the property sides have been isolated.

The whole scenario is very bizarre. No one has come up with an explation yet.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

To Trevor Rix,
Has the Buffer tank got a domestic hot water coil in it?

No.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Could pressure vessel be losing it on shraeder valve

The air pressure within the expansion vessel has been tested. It has always been rock solid.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Are there two expansion vessels or one ?
What size are they ?
When you pressurised the buffer with air, how was it done ?

There is one expansion vessel, size not specified in the quotation.

The installers (not me) pressurised the buffer with air using an air compressor fitted to the buffer inlet.

 

[ShaunCorbs]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Have you/ your installer pressure tested the install at say 10 bar (water)

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Have you/ your installer pressure tested the install at say 10 bar (water)

About six weeks ago the installers pressurised the underground legs with air to 5 bars. It was then that one of the legs stopped losing pressure, and continued to hold pressure when refilled with water.

No one seems to remember what the installer's plumbers did regarding pressure testing during the initial install over two years ago. There is a suspicion that the system has been losing pressure ever since the initial install.

Thanks everyone for your questions and suggestions. Keep them coming!

 

[Best]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Just one thought that might not apply here.
Sometimes push - fit fittings that are leaking can seal themselves temporary or otherwise, when pressure is released on the pipework and then pressurised again.
So don't be surprised if loss of pressure suddenly stops when you drop the pressure to zero and then build pressure up again, air or water.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Just one thought that might not apply here.
Sometimes push - fit fittings that are leaking can seal themselves temporary or otherwise, when pressure is released on the pipework and then pressurised again.
So don't be surprised if loss of pressure suddenly stops when you drop the pressure to zero and then build pressure up again, air or water.

Thanks for the thought, but there are no push fit fittings. And there have not been any visible leaks.

 

[Last Plumber]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

And this looses right down to zero ?

no pressure, not even 0.5 of a bar ?

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

And this looses right down to zero ?

no pressure, not even 0.5 of a bar ?

That cannot be determined because when the needle on the pressure gauge rests against the minimum stop, it points to 0.05 bars on the scale.

 

[Last Plumber]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Quick recap ! for my sanity

No visible Leaks !

Water pressure drops quickly on a 650 litre Volume tank that is full ! ( it is full isn't it, I mean it's not just that it gets pressurized and the air is leaking out over the time you state it loses pressure ?)

There are no problems with those heat ex's to either heating system and both of those heating systems are open vented ?

There is no underground out of sight pipe leak !

No other coils within the vessel that are connected to anything else ?

Is the Bio mass boiler piped direct to the Buffer or indirect. ?

 

[snowhead]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

All done with Xpress pipe fittings or similar? VSH XPress system | XPress
Using steel pipe?

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

Quick recap ! for my sanity

No visible Leaks !

Water pressure drops quickly on a 650 litre Volume tank that is full ! ( it is full isn't it, I mean it's not just that it gets pressurized and the air is leaking out over the time you state it loses pressure ?)

There are no problems with those heat ex's to either heating system and both of those heating systems are open vented ?

There is no underground out of sight pipe leak !

No other coils within the vessel that are connected to anything else ?

Is the Bio mass boiler piped direct to the Buffer or indirect. ?

All correct.

When the buffer tank is filled with water it is indeed full of water to the top.

At first the primary suspect for the cause of pressure drops was indeed underground pipes the majority of which are not visible because they are buried about 3'6" deep. That suspicion was knocked on the head when *** the underground legs were drained, pressurised with air to 5 bars, air released then refilled with water. After the point of *** there were no more pressure drops on the underground legs.

No coils within any vessels/tanks.

The biomass boiler is piped direct to the buffer tank.

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

All done with Xpress pipe fittings or similar? VSH XPress system | XPress
Using steel pipe?

All pipework, apart from the underground legs, is metal with all metal fittings. The underground legs are all plastic - Upanor undergound insulated central heating pipe - terminated at all ends with metal fittings.

 

[ShaunCorbs]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

All pipework, apart from the underground legs, is metal with all metal fittings. The underground legs are all plastic - Upanor undergound insulated central heating pipe - terminated at all ends with metal fittings.

metal screwed black iron or like snows link?

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

metal screwed black iron or like snows link?

What does "snows link" mean please?

 

[ShaunCorbs]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

What does "snows link" mean please?

VSH XPress system | XPress

 

[Trevor Rix]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

VSH XPress system | XPress

No, nothing like the fittings shown on your link.

There is a mixture of copper soldered joints, and metal screw to tighten unions/couplings/valves that require spanners to fit.

 

[Last Plumber]

Re: Pressure drops when 650 litre buffer tank is filled with water but no drop with a

If it's piped direct have you checked the heat exchanger on the boiler.

There could be an internal crack that either opens under pressure or heat.

I am clutching at straws now as you can tell but it could be possible in my minds eye. My real eyes haven't seen the job.

Can we have pictures ?

Someone said earlier to pressure test with water to 10 bar. Have you done that and looked everywhere including inside the boiler combustion chamber. I know it's a long shot but it is not impossible.