Expansion Vessel Faults

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Pressure swings high and low, then boiler shuts down

All sealed central heating systems have rises and falls in pressure as the system heats up and cools down. Extreme rises of pressure are prevented by the presence of an expansion vessel (also called the expansion chamber). If it is working correctly, the pressure changes are normally less than 0.5 bar (½ bar) up and down and are quite slow.
Big swings in central heating system pressure indicate a failed or failing expansion vessel. An undersized vessel can cause big pressure swings too but those swings would have been present since installation. If, however, you've recently extended your central heating system by several radiators, the existing expansion vessel could be too small and this would also cause larger swings in pressure.
For us, a pressure swing of over ½ a bar (between hot and cold) suggests that the vessel may have a problem. If the swing is 1 bar or more, there is almost certainly an expansion vessel fault.
High and low system pressures are not always caused by expansion vessel faults. Water leaks, connected filling loops letting by, internal leaks on a secondary heat exchanger in a combi boiler, all cause pressure changes. There's more on those causes in our article about central heating system pressure.

Expansion Vessel Types

Red, lozenge shaped 10 litre expansion vessel from within a Baxi boiler

10 litre expansion vessel from a Baxi boiler

Expansion vessels in a sealed system may be internal to the boiler or separate from it. Very occasionally there may be two in one central heating system. Some internal expansion vessels can be very difficult to replace, though improved design on newer boilers now generally makes it easier. If the diaphragm in a vessel internal to the boiler fails, provided it hasn't rusted through, the vessel may be left in place and a replacement vessel fitted externally.

Central heating system expansion vessels are usually red but could also be grey. If they are within boilers, most are flattened, lozenge shaped, to allow them to fit more easily. External to the boiler they are usually spherical and the size of a large football. As we've said, the size required depends on the size of the heating system, with larger systems needing larger expansion chambers, but vessels of 12 litres and 18 litres are most common.
White or blue vessels are usually part of the unvented domestic hot water system, not the central heating system. They are used for potable water (drinkable water).

How do expansion vessels fail?

We've explained how these vessels work in our System Pressure article.

An expansion vessel may fail by rusting through and leaking water. This is a less common cause of failure, particularly in systems which have had good quality, long-life corrosion inhibitor liquids added to the water.
Each vessel has a Schraeder filling valve fitted, very similar to a car tyre valve. These can fail, but very rarely do.
More commonly, it is the butyl diaphragm dividing the two halves of the expansion chamber which fails. Over time this degrades and perishes, becoming thinner and eventually perforating.

During manufacture, the "air" side of the expansion chamber is pumped up to about 0.75 bar (¾ bar). This is the charge pressure. Air may be used or it may be pure nitrogen (air is about 78% nitrogen anyway).
The butyl rubber diaphragm is semi-permeable to gases so, in use, molecules from the air side gradually move across to the water side. This means that the pressure in the air side gradually reduces.
The oxygen in air passes across a little more quickly than nitrogen. The oxygen in air is also more reactive than pure nitrogen and will attack and weaken the butyl diaphragm more quickly than pure nitrogen. For both reasons, nitrogen makes sense in manufacture.

As the diaphragm becomes degraded, the passage of gas molecules across it becomes easier. Even if it does not become perforated, the charge pressure gradually disappears and, when the water pressure is topped up, the diaphragm is pressed further and further into the air chamber. As the air pocket reduces, the ability of the vessel to accommodate the water expansion also reduces. The size of the change in system pressure (the pressure swing) increases.

When the water expansion volume (from cold to hot) is more than the remaining air pocket volume, the system pressure will swing rapidly up. It will pass 3 bar and will force open the safety valve. Water will then be dumped outside via the safety pipe.
When the system cools, now with less water in it, there will be a rapid downwards swing. The new, low pressure may be too low to operate the boiler. Many boilers have a pressure switch built in and will shut down, often showing a low pressure fault code.
If you top up the water pressure, the process will just repeat itself. You need to re-pressurise the expansion vessel, or replace it. If the diaphragm is perforated it cannot be re-pressurised.

Can you test an expansion vessel?

With an external vessel, you can hear a difference in the sound if you "ping" the side. Vessels which have an intact air chamber make an echoey ping when you tap the side. Vessels which have lost the expansion vessel make a much duller "plunk".

First, switch off the heating system and isolate the electrical power to it. Make sure the system pressure is above 1 bar. If you think the vessel has failed, unscrew the plastic cap from the pressurisation valve (it's like a car tyre valve). Momentarily depress the pin in the valve to release a little pressure. If water comes out of the valve, the vessel has definitely failed and must be replaced.
If no water comes out but no air either, the diaphragm may still be intact and it may be possible to re-pressurise. There is no guarantee that it will last a long time.
If you had air coming out of the "tyre" valve, you may need to re-pressurise the expansion chamber anyway.

How to re-pressurise the expansion vessel

To re-pressurise correctly you must drain water from the system until the pressure is zero. If you don't do this you can't set the expansion chamber pressure correctly. It's important to leave the water drain valve open while you re-pressurise the vessel. Connect a car tyre pump to the expansion vessel valve. Stirrup type pumps (often used for bike tyres) work well. Pump up the expansion vessel to about 1 bar (15 psi). This is putting air into the "air" side. If your pump does not have a built in gauge, you'll need to use a tyre pressure gauge to check the pressure. It may be worth using a separate gauge anyway, as gauges built into pumps are often inaccurate.

As the air chamber is pumped up, the diaphragm will move across the chamber and push water out into the heating system and out of your open drain point. If you hear lots of air bubbling into the boiler and pipework, the diaphragm has almost certainly failed. You'll know for sure if you can't get the expansion vessel air chamber up to 1 bar. Remember, to test this properly you must leave the water drain cock open while you do this.
If you can get the expansion vessel to hold air pressure at 1 bar (with the drain valve open) you may be in luck. Remove the pump connector and refit the plastic valve cap.
Close the drain point and top up the system water pressure to about 1½ bar (1.5) using the filling loop. Bleed air from any radiators or air release valves and top up again to 1½ bar.
Run the central heating system as normal.
If there's any doubt about the reliability of the expansion vessel it would make sense to change it.

Temporary workaround

If your expansion vessel cannot be re-pressurised there is a temporary "fix" which might get your system up and running again. We would not leave this as a "permanent" fix but it should work until the vessel is replaced. You're aiming to produce a temporary expansion pocket for the system.

Choose an upstairs radiator in a room which you don't need to keep really warm.
Turn the heating system off and drain down to about the level of the top of that radiator. Carefully open the bleed vent on the radiator. If water continues to come out, drain the system a bit further and try again. When water stops coming out, drain the system a little further until about another bucketful is gone from the radiator. Now close the bleed vent again.

Refill the central heating system and bleed out the air but don't bleed out the radiator you let air into. With a bit of luck, the air pocket you have formed at the top of the radiator will act as an expansion chamber. Turn on the heating system and try it out. When you replace the expansion vessel you'll need to bleed out that radiator too. This will not work effectively if one of the radiator pipe connections is at the top of the radiator but nearly all radiators have their pipe connections at the bottom.


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