Boiler losing pressure

What the pressure gauge should show

Vaillant's fault code guidance gives "the normal range of 1 to 1.5 bar" for system water pressure (Vaillant, read 29 October 2026). Ideal's user guidance for its Logic System boilers says the same, "between 1 and 1.5 bar", and tells you to re-pressurise to 1 bar using the filling loop, close the tap and press restart (Ideal Heating, read 29 October 2026). Below about 1 bar most boilers shut down and show a low-pressure code. Vaillant's is F.22, and on some models F.75; Ideal's is F1.

Losing pressure with no visible leak

The usual order of likelihood, from what the makers' own guidance lists as causes:

• Air, then a slow top-up habit. Baxi says it is "normal for air to get into your system and cause pressure drops from time to time", and that a drop that keeps happening points to a leak (Baxi, read 29 October 2026). One top-up a year is normal. One a week is a fault.
• A leak you can't see. Under floors, behind a bath panel, at a radiator valve tail that only weeps when hot. Look for damp patches, stains under radiators and a drop that's worse when the heating is on.
• The pressure relief valve. It discharges through a small copper pipe outside. If that pipe drips, the valve is passing water and the system loses pressure every time it heats up.
• The expansion vessel. Vaillant lists an "internal expansion vessel faulty" among the causes of F.22. When the vessel loses its charge, pressure climbs high when hot, the relief valve lets water out, and the gauge reads low again when cold. Pressure rising to 2.5 or 3 bar when hot is the giveaway.
• The pressure sensor itself. Also on Vaillant's list for F.22 and F.75. The boiler thinks it's low when it isn't.
• A leak inside the boiler. A weeping heat exchanger or a seal. You may see water or scale marks inside the casing. That's an engineer-only job.

Losing pressure overnight, or only when the heating is on

Overnight, heating off: a leak on the system that doesn't depend on heat, or a relief valve that's passing. Check the outside discharge pipe for drips in the morning.

Only when the heating is on: look at the gauge when the system is hot. If it climbs well above 2 bar and then drops back, the expansion vessel has most likely lost its charge and the relief valve is dumping water. If it never climbs much but still falls, a joint is opening up with heat somewhere on the pipework.

After a service or repair: a system that was drained and refilled holds air for a few days. Bleed the radiators, top up once, and see if it settles.

What's safe to do yourself

1. Read the gauge cold.Below 1 bar: top up. Between 1 and 1.5: the pressure is fine, so the fault is elsewhere.
2. Top up through the filling loop.Open the valve slowly until the needle reaches about 1 bar, then close it fully. Vaillant says this can be done without an engineer if you're competent; its guidance and Ideal's both walk through it.
3. Reset the boiler once.Ideal's Logic boilers need the restart button pressed after a top-up.
4. Bleed any cold radiators.Air in the system reads as low pressure. Top up again after bleeding.
5. Check the outside discharge pipe.A drip means the relief valve is passing. Note it for the engineer.
6. Write down how fast it drops.A bar a week, or a bar a day. It tells the engineer where to look.

Don't leave the filling loop open, over-pressurise above 1.5 bar cold, open the boiler casing, or keep topping up a system that drops daily. Vaillant's guidance is blunt on the second part: if the pressure shows more than 1 bar and the boiler still faults, "this is a case for a Vaillant engineer. Don't try to solve it yourself as opening the boiler casing can be dangerous."

When to call an engineer, and what it costs

Call when the pressure drops more than once a month, when the gauge climbs high when hot, when the discharge pipe drips, or when you find water anywhere near the boiler. Dublin engineers who publish a call-out or repair price charge €80 to €160 to come out, middle figure €115, with parts extra (10 published prices, 22 October 2026). An expansion vessel or relief valve is a routine part. A heat exchanger on an old boiler is where repair or replace becomes the question. See boiler repair in Dublin for every call-out price.

Questions people ask