Can an unvented system lose pressure without a leak?

[Wul]

My unvented cylinder is losing pressure and needs pressurised a couple of times/week. Pressure drops from 1.5bar to almost zero over 3 days.
The thing is, I can't find any leaks and there's no evidence of water dripping anywhere in the house. There is a stain on the tank which looks like residue from a dribble but this has been there for many years and hasn't got worse.
There is also some evidence of a leak (staining, corrosion) on a boiler flange but no actual water.
Is it possible for a very small leak (which then dries up with the heat of the boiler/tank) to cause such a loss of pressure?
Could the pressure vessel (which looks like it has a diaphragm inside) loose pressure by having a leaky diaphragm?

 

[Alex10]

Could be Hydrogen gas in radiators? Are you bleeding the rads?!

 

[Wul]

I haven't bled the rads no. Hydrogen gas? How is that created?

 

[Wul]

Just checked rads. No air or gas in them and ejected water is clean.

 

[Jackrae]

Expansion vessel could have lost its pressure, or burst bladder, which means there is no expansion space so even the slightest loss of water causes a rapid loss of pressure.

By how much does the pressure rise between a cold system and a hot one

The hydrogen comes about from oxygen being extracted from the water (H20) by corrosion of the radiator internals. This then leaves hydrogen gas collecting in the radiator tops.

 

[ollski]

Is this the central heating system pressure?, if it is what boiler have you got?

 

[Wul]

The boiler is a 36Kw Ideal Mexico, condensing, floor standing boiler. Pressure difference between hot and cold doesn't seem to amount to much. I'll need to monitor it more closely to see.
The pressures I'm quoting are showing on the gauge next to the expansion vessel adjacent to the hot water tank. No other gauges present. I'm not sure if these pressures are HW or CH but I would say HW since mains pressure shower was the reason for an unvented tank.

 

[Jackrae]

If it's on the expansion tank piping then it's the CH pressure. This will change slightly between cold and hot. With the CH system cold, note the pressure (should be between 1 and 1.5bar. Then bleed off some of the CH fluid - if you bleed off say a couple of pints the pressure should remain almost the same. If however it falls substantially then your expansion vessel air-side is either depressurised or the bladder is burst.

 

[Wul]

Sorry for lack of response, been working all hours. Jackrae, thanks for that tip about testing. I'll give it a go. Boiler makes a hellish noise when the pressure drops, suspect the water inside is boiling due to reduced pressure.

 

[Jackrae]

If pressure is too low boiler should run should be inhibited so the "hellish noise" shouldn't be caused by low pressure. Maybe pump isn't getting flow through the system, is the boiler exit temperature excessively high, which could suggest blockage. But that doesn't answer the pressure loss problem.

 

[Wul]

The pressure doesn't seem to change much between cold & hot. There are 3 zones; UFH, Upstairs Rads & Hot Water. No change in pressure when UFH running, or HW but a 0.1Bar rise when upstairs rads switched on.
I've added some pics which may be helpful.

 

[Wul]

OK, no pics. Will have another go.

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• Wul
• 1 Feb 2015
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• Wul
• 1 Feb 2015
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• Wul
• 1 Feb 2015
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• Wul
• 1 Feb 2015
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[AdrianUK]

I haven't bled the rads no. Hydrogen gas? How is that created?

The black sludge found in central heating systems is Iron Oxide (FeO2) this is not the same as 'rust' which is FeO3. The iron wants to oxidise so it breaks down the water in hydrogen & oxygen. The oxygen immediately recombines with the iron of the radiators to make iron-oxide and the hydrogen is liberated as a gas.

Hence the 'air' in radiators is actually hydrogen & is a warning that your radiators are rusting from the inside.