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.

 

[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.

 

[Last Plumber]

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

What capacity does the expansion vessel for the 650 buffer have ?

It will say on the side on a label. If no label, can you post measurements. Circumference or diameter along with the height.

Also, what size of pipe connects it to the buffer ?

 

[Last Plumber]

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

Also, what pressure is that expansion vessel charged to ?

 

[Trevor Rix]

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.

In recent testing it was only the buffer tank that was losing pressure. As the buffer tank is isloated from the rest of the system during this phase of testing, any defects within the boiler would be irrelevant.

To recap, there are five sections in the sealed part of the system. (1) boiler (2) expansion vessel (3) buffer tank (4) leg one underground to house (5) leg two underground to a separate building. All five sections has its own pressure gauge. By isolating different sections we should in theory be able to determine which section is losing pressure.

 

[Trevor Rix]

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

What capacity does the expansion vessel for the 650 buffer have ?

It will say on the side on a label. If no label, can you post measurements. Circumference or diameter along with the height.

Also, what size of pipe connects it to the buffer ?

Also, what pressure is that expansion vessel charged to ?

The expansion vessel is 300 litres, connected directly to the back of the boiler. See photograph for pipe sizes. The expansion vessel is charged to 1.5 bars.

 

[Last Plumber]

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

Isolate the buffer including plugging all points of entry and test to 7 or 8 bar pressure whilst full of water.

 

[voivod]

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

What fitting on top of boiler ?
And the pressure release at expansion hasn't blown at all ?

 

[Trevor Rix]

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

Isolate the buffer including plugging all points of entry and test to 7 or 8 bar pressure whilst full of water.

As the pressure has not dropped in the buffer tank since 20 Sep 2016 1012, the installers today decided to bring the boiler, expansion vessel, buffer tank and leg one into circuit, and start the boiler to see what happens. I will be monitoring the pressures over the weekend.

 

[Trevor Rix]

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

What fitting on top of boiler ?
And the pressure release at expansion hasn't blown at all ?

Did you mean "boiler" or buffer tank?

At the very top of the buffer tank the pressure gauge (see old photo) has been replaced with an automatic bleed.

The pressure release at the back of the boiler has not blown since testing during installation in 2014.

 

[Stoney Ground]

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

Would it not be advisable to remove AAVs whilst pressure testing.

 

[Trevor Rix]

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

What are AAV's?

The system is being run at normal operating pressure over the weekend, so no excess pressure is being applied.