Boiler Pressure Keeps Dropping?
If your boiler pressure keeps dropping, water is leaving a sealed heating system somewhere, or the system cannot cope with expansion as it heats. Topping it up gets the heating back on, but a pressure that falls again within days or weeks has a cause that needs finding. This guide explains what normal pressure looks like, how to top up safely, and how to track down the usual culprits.
The short answer
A sealed system should hold its pressure. Cold, most boilers want about 1 to 1.5 bar on the gauge; it rises a little as the system heats up and falls back as it cools. If it keeps dropping, the usual causes are a small leak (often a radiator valve or a joint under a floor), an expansion vessel that has lost its air charge, or a pressure relief valve that is letting water by.
What normal pressure looks like
On a sealed system, the gauge on the front of the boiler shows the water pressure in the heating circuit. A typical pattern:
| Reading | What it means | What to do |
|---|---|---|
| Below about 1 bar, cold | Too low; many boilers lock out with a low-pressure code | Top up to the level in your manual, then watch it |
| About 1 to 1.5 bar, cold | Normal for most homes | Nothing |
| Rises a little when hot | Normal: water expands as it heats | Nothing |
| Climbs towards 3 bar when hot | Too high; usually the expansion vessel or a filling loop left open | See our guide to boiler pressure that is too high |
The exact figures are in your boiler's manual, and our guide to what your boiler pressure should be goes into the normal range in more detail.
Topping up safely
Most combi and system boilers have a filling loop: a flexible hose or a built-in key with two valves, linking the mains water to the heating circuit. With the boiler off and the system cold, open the valves slowly, watch the gauge rise to the figure your manual gives, then close both valves fully. If it is an external flexible loop, disconnect it afterwards; it is meant to be temporary. Then reset the boiler if it had locked out. If you are not sure where the filling loop is or how yours works, stop and ask, because over-filling pushes the pressure up to the point where the relief valve discharges.
Cause 1: a leak somewhere on the system
Most pressure loss is a small, slow leak, and on a sealed system even a drip a day shows on the gauge. Check every radiator valve and its gland nut with a dry tissue, look under the boiler for drips or staining, check the pipes in airing cupboards and under sinks, and look for marks on ceilings below upstairs radiators. Leaks under floors are harder: a damp patch on a floor or ceiling, or a section of carpet that stays damp, is the clue. A leak inside the boiler itself, from a seal or the heat exchanger, needs a Gas Safe engineer to open the case.
Cause 2: the expansion vessel has lost its charge
The expansion vessel is a small tank, usually inside the boiler, with a rubber diaphragm and a cushion of air on one side. As the water heats and expands, the air compresses and takes up the extra volume. If the air charge leaks away over the years, the expanding water has nowhere to go: pressure shoots up when the heating is on, the relief valve lets some water out, and then the pressure is low again when the system cools. That rise-and-fall pattern is the giveaway. Re-charging the vessel, or replacing it if the diaphragm has failed, is a routine repair.
Cause 3: the pressure relief valve
The pressure relief valve is a safety device that opens to release water if the pressure gets too high, usually through a copper pipe that runs outside. If it has opened a few times, debris can stop it seating properly, and it lets water by even at normal pressure. Look at the end of that pipe outside: drips or staining there point to the relief valve or to an expansion vessel problem pushing it open. Either way it needs an engineer.
Why constant topping up is a bad idea
Every top-up brings fresh mains water into the heating circuit. That dilutes the inhibitor that protects the radiators and boiler from corrosion, and fresh oxygenated water speeds up the formation of black magnetite sludge. A system that has been topped up every week for a winter often ends up with cold radiators and a sludged heat exchanger as well as the original leak. Find the cause. If the pressure has dropped more than once in a month, book a boiler repair visit, and if the water you bleed out of the radiators is black, a powerflush may be needed once the leak is fixed.
Common questions
Why does my boiler pressure drop overnight?
Usually a small leak somewhere on the system, or an expansion vessel that has lost its air charge so the relief valve discharges when the heating runs and the pressure falls again as it cools. Check radiator valves, the area under the boiler and the end of the relief valve pipe outside.
Is it safe to keep topping up my boiler pressure?
Topping up once is fine. Doing it repeatedly without fixing the cause brings in fresh water that dilutes the inhibitor and speeds up sludge, so a pressure that keeps dropping should be looked at by an engineer.
Can low boiler pressure damage the boiler?
Most modern boilers protect themselves by locking out when pressure is too low, so the heating stops rather than the boiler running dry. The bigger risk is the underlying leak, and the corrosion that repeated top-ups cause.
What pressure should my boiler be when the heating is on?
A little higher than when cold. Most systems sit at about 1 to 1.5 bar cold and rise somewhat when hot. If it heads towards 3 bar when hot, that points to an expansion vessel fault or an over-filled system.
Updated 15 October 2026
