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

Unfortunately no replies yet. Did I post in the wrong forum? Might I have more success in posting in another forum?

 

[MC_Bob]

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

Makes no sense to me. Have you any signs of leaks and why do you assume it's the store only? You'd certainly see if it was that.

 

[Trevor Rix]

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

Thanks. There are now five pressure gauges fitted to the system, so that we can hopefully determine which section is losing pressure. (1) boiler (2) expansion vessel (3) buffer tank (4) leg one underground to house (5) leg two underground to a separate building.

A leak is suspected on the buffer tank because the buffer tank is now isolated from the rest of the system, and is the only gauge that was recently dropping - until the water was drained then pressurised with air yesterday, at which point the pressure stopped dropping.

After several months investigation by the installer there are still no signs of any water or air leaks anywhere.

Do you have any suggestions for further tests?

 

[MC_Bob]

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

Is the system being run when water is being lost?

 

[Trevor Rix]

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

There has never been any visible evidence of water being lost, other than the need to top up the pressure in the system to over 0.6 bars so that the boiler would work again. There is a pressure switch between the boiler and the expansion vessel that cuts out the boiler when it detects a pressure of less than 0.60 bars which was happening every 48 hours or so. The system is topped up with mains water with 4 bars behind it but even then takes many seconds to a few minutes to top up to around 2.50 bars.

All of these tests have been conducted both with the system running and hot, and the system not running and cold.

The whole saga is very strange, but the really weird thing is that on two separate parts of the system - (3) buffer tank (4) leg one underground to house - those gauges lost pressure quickly when filled with water, but not when pressurised with air or when refilled with water after air. It's almost as if the air has a sealing effect.

 

[snowhead]

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

Has the system been run at operating temperature with just Boiler, E.V and Buffer?
i.e with the 2 underground sections isolated?

I wouldn't get distracted by the air water loss issue.
It's loosing pressure with water, so concentrate on that for leak detection.

You're topping up to 2.5 bar Cold?

 

[Trevor Rix]

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

The next step is indeed as you suggest to run the system at operating temperature for a couple of days with the two underground legs isolated. The buffer tank has been stripped of insulation so any water leak should be visible.

Yes, topping up to 2.50 bars cold. If the pressure in the buffer tank ever approaches 3.00 bars it will blow off the relief valve.

 

[voivod]

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

Is there a leak at pressure release point?

 

[Trevor Rix]

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

No visible leaks anywhere including at the buffer tank release point.

 

[voivod]

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

have you checked tundishes?

 

[Trevor Rix]

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

What are tundishes please?

 

[voivod]

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

after the pressure release valves...or at discharge from them...are you sure nothing coming out?

 

[Trevor Rix]

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

Yes, checked, no leaks. The visible open to air item through which water gushes when the pressure relief blows off or is opened manually.

 

[Dotty]

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?

 

[Last Plumber]

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?

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

 

[snowhead]

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

Is this buffer tank, fed from a tank or is it unvented ?

The way I read it is the buffer, boiler and underground pipework are all on a common circuit and unvented, (expansion vessel mentioned).
And they're cold filling to 2.5 bar with a PRV set at 3 and wondering where the water is going.
The "Installer" has been looking for the leak for several months.

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

.

 

[Stoney Ground]

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

 

[Last Plumber]

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

Also to Trevor rid.

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

 

[Trevor Rix]

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

---

 

[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.