Boiler water pressure keeps dropping: expansion vessel, leak or valve?

Short answer: topping up only helps temporarily. If the water pressure drops again within a few days or weeks, there is a cause. In three quarters of cases it is an expansion vessel that has lost its pre-charge pressure; the rest is a leak, a pressure relief valve that keeps leaking or a filling loop that is not fully closed. A vessel costs between €70 and €110. The more often you top up, the more fresh, oxygen-rich water enters the system, and that is bad for the boiler.

Just want to know how to top up? Then follow the step-by-step guide to topping up your boiler.

What is normal and what is not?

In a cold system the pressure should be between 1.5 and 2 bar. As the boiler warms up, the pressure rises to about 2.5 bar; that is normal, because hot water expands. When the system cools down, the pressure drops back to its starting value.

Losing a little pressure over a whole heating season is fine: water escapes when you bleed radiators. What is not normal: topping up every month, or a pressure that is below 1 bar again after a week. Then water or air is being lost somewhere where it should not be.

The expansion vessel: the most common cause

In the boiler (or next to it) there is a sealed vessel with a rubber diaphragm. On one side there is water, on the other side air under pressure. That air cushion absorbs the expansion of the heating water. If the pre-charge pressure disappears or the diaphragm tears, the water has nowhere to go: when heating up the pressure shoots up, the pressure relief valve discharges, and after cooling down you are below 1 bar. That pattern, high pressure when warm and low pressure when cold, is the clearest proof that the vessel has failed.

A quick check you can do yourself: press the Schrader valve on the top or bottom of the vessel, just like on a bicycle tyre. If air comes out, the diaphragm is still intact and the vessel may be recharged to the correct pre-charge pressure. If water comes out, the diaphragm is broken and the vessel must be replaced. Also tap the vessel: if it sounds equally dull all over, it is full of water.

The other causes

A real leak is less often the cause than people think, but it does happen: at fittings, at underfloor heating manifolds or under the boiler. Look for damp patches, limescale rings or rust marks; a leak that evaporates on a hot pipe does leave a white edge.

The pressure relief valve can also keep leaking after it has discharged once; there is then dirt under the valve seat. You can see this from water in the drain under the valve. And a filling loop that is not fully closed does exactly the opposite: it tops up unnoticed, so that the pressure actually rises. Both parts can be found under valves.

Does the pressure drop only when you run hot water, or does it actually rise then? Then the plate heat exchanger is leaking tap water into the heating circuit. Our guide on that topic is replacing a heat exchanger.

Fault codes per brand

Brand	Code	Message
Remeha	A.02.06	Warning: water pressure below the minimum water pressure
Remeha	E.00.40	Water pressure sensor removed or reading below range
Vaillant	F.21	Water pressure too low: leak in the system or expansion vessel faulty
Vaillant	F.22	Water pressure too high: faulty expansion vessel or overfilled
Bosch	2970	Pressure loss in the heating system too fast
AWB	21	System water pressure too low

Note the difference between F.21 and F.22 on Vaillant: a faulty expansion vessel can show up as either, depending on whether the boiler is hot or cold when the sensor takes its reading.

What does an expansion vessel cost?

Part prices including VAT from our catalogue. Which vessel you need depends on the appliance and the size of the system.

Part	Manufacturer number	Price
Flamco renovation kit 18 litres (vessel, safety valve and inlet combination)	—	€82.50
Intergas expansion vessel 6 litres, Kombi Kompakt	864067	€74.95
Remeha expansion vessel 8 litre	7738455	€99.95
Bosch expansion vessel 12 litres, 3 bar	8733714389	€108.95

All vessels can be found together under expansion vessels.

Vessel inside the boiler or next to it?

If the vessel is inside the boiler, replacing it is work for a gas engineer: the casing has to come off and that falls under the sealed section of a gas appliance. If the vessel hangs outside it on the wall or on the pipework, it is water-side work that a handy DIYer can do: drain the system, unscrew the vessel, fit a new vessel with the correct pre-charge pressure, refill and bleed.

A common solution for a boiler whose internal vessel has failed is to add an extra vessel on the wall instead of opening the boiler. That is cheaper and often just as effective, provided the vessel has the correct capacity and pre-charge pressure for the system. Do check this with an installer, because a vessel that is too small will not solve the problem.

Why frequent topping up is bad

Every time you top up, fresh mains water enters the system: with oxygen and with limescale. Oxygen attacks metal parts and eventually causes magnetite, the black sludge that ruins pumps and heat exchangers. Limescale settles on the hottest spot, and that is precisely the heat exchanger. Topping up regularly is therefore not just a nuisance, it also shortens the life of the most expensive parts. Fix the cause and you are rid of both.

Frequently asked questions

How often may I top up the boiler?

A small amount once or twice a year is normal, especially after bleeding radiators. If you have to top up monthly, there is a cause you need to track down; that is not maintenance but treating the symptoms.

How do I know whether my expansion vessel is broken?

The clearest sign is pressure that rises sharply when warm and drops below 1 bar when cold. In addition, press the vessel's Schrader valve: if water comes out instead of air, the diaphragm is broken and the vessel must be replaced.

Can I recharge the expansion vessel with air myself?

If the diaphragm is still intact, an installer can restore the pre-charge pressure to the correct value. This must be done with the system depressurised, otherwise you measure the wrong value. So it is not as simple as just attaching a bicycle pump.

What is the correct pre-charge pressure?

That depends on the height of the system and is usually stated on the vessel or in the boiler manual. For an average single-family home it is around 0.8 to 1 bar. Never fill by feel; an incorrect pre-charge pressure causes exactly the same symptoms as a broken vessel.

Can I keep using the boiler if the pressure is too low?

The boiler protects itself and switches off if the pressure is too low, often first the hot water and then the heating. Repeatedly resetting and topping up without tackling the cause can lead to dry firing or damage to the pump.