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.

 

[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.

 

[Stoney Ground]

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

What are AAVs?

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

Automatic Air Vent (Bleed) as you stated had been fitted, designed to let air out and consequently reduces pressure in the system.

 

[Trevor Rix]

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

Automatic Air Vent (Bleed) as you stated had been fitted, designed to let air out and consequently reduces pressure in the system.

Thanks for the observation. The red plastic cap at the top of the AAV is I think temporarily screwed closed.

 

[voivod]

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

Sorry buffer

 

[Gray0689]

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

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.

View attachment 27395View attachment 27396View attachment 27397View attachment 27398

Looks like an eta boiler and buffer
Where is the job

 

[Trevor Rix]

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

Looks like an eta boiler and buffer
Where is the job

Yes, it is an ETA boiler and buffer tank. The job is in West Suffolk.

 

[acecharly]

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

Not read through but if theres a vent with a ball in it would float up when full of water but not air.

 

[Trevor Rix]

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

There is no vent and no ball because it is a sealed system as far as the two heat exchangers in the two properties being heated.

 

[acecharly]

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

All I can suggest here is (if you already have not)

1. Isolate each of the 5 sections.
2. Test each section individually (you mentioned earlier testing to 5bar with air I call that a bomb) use water.
3. You should now have narrowed it down to 1 section.
4. Losing that much pressure over that volume of water will be noticeable.
5. If there is pressure loss and no evidence of water then you must have a faulty pressure gauge or that its passing back through any isolation valve you've turned off. So first change the ball valves.

In my opinion and im guessing because like the other guys here im not infront of it, id say your losing it of one of the legs that are out of view or youd see it...only Paul Daniels can make things disappear and im afraid hes no longer an option.

All the best.

 

[devoncreamy]

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

a wild stab in the dark, is the buffer heating a dhw cylinder? maybe a leak on the coil?

 

[Trevor Rix]

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

All I can suggest here is (if you already have not)

1. Isolate each of the 5 sections.
2. Test each section individually (you mentioned earlier testing to 5bar with air I call that a bomb) use water.
3. You should now have narrowed it down to 1 section.
4. Losing that much pressure over that volume of water will be noticeable.
5. If there is pressure loss and no evidence of water then you must have a faulty pressure gauge or that its passing back through any isolation valve you've turned off. So first change the ball valves.

In my opinion and im guessing because like the other guys here im not infront of it, id say your losing it of one of the legs that are out of view or youd see it...only Paul Daniels can make things disappear and im afraid hes no longer an option.

All the best.

Could you please read the whole thread.