Boiler Losing Pressure? What It Means and How Urgent It Is
A sealed system does not consume water.
A sealed heating system does not consume water, so if you are topping up regularly the water is escaping. The three causes are a leak on the pipework or a radiator valve, a failed expansion vessel, or a passing pressure relief valve discharging outside. Topping up repeatedly hides the fault and corrodes the system.
Published 8 July 2026 · Written by the Emergency Plumb engineers
What the gauge should read
Cold, between 1 and 1.5 bar. Running, it will rise to somewhere between 1.5 and 2.5 bar as the water expands, and that is normal. Below about 0.8 bar most boilers lock out and you get no heat and no hot water. The number itself is less useful than how fast it falls.
How fast is it dropping?
Once a year is normal and probably just air being released. Once a month suggests a small leak or a tired expansion vessel. Every week, or every few days, means water is actively leaving the system and something will get damaged, that is worth a call now rather than in the spring.
The three usual causes
A leak on the pipework or a radiator tail, often under a floor and invisible. A failed expansion vessel, where the internal diaphragm has gone and the system cannot absorb expansion. Or a pressure relief valve passing, which discharges outside through a small copper pipe on the external wall.
Check the outside pipe
This is the check almost nobody makes. Find the small copper pipe coming through the wall near the boiler. If it is dripping, damp, or has a limescale trail beneath it, your pressure is going out of the building through the relief valve, and no amount of topping up will fix it.
How to repressurise safely
Boiler off and cold. Open both filling loop valves slowly and watch the gauge, it moves quickly. Close at 1.2 bar, then close both valves fully and, if it is a removable loop, disconnect it. Leaving a filling loop connected and slightly open is a mains-to-system cross-connection and it is a Water Regulations breach.
When to stop topping up and call
If you are repressurising more than once a month, adding water is treating the symptom. Every top-up introduces fresh oxygenated water, which corrodes radiators from the inside and produces the black sludge that eventually blocks the heat exchanger. The leak is cheaper to find than the damage is to undo.
The expansion vessel case, which people miss
If there is no damp patch anywhere and the pressure still falls, suspect the expansion vessel before tearing up floors. The vessel is a sealed chamber with a rubber diaphragm and a charge of air behind it, and its job is to absorb the expansion of water as the system heats. When the charge is lost, heated water has nowhere to expand into, so pressure spikes well above 2.5 bar, the pressure relief valve opens, and water discharges through a copper pipe outside. The system then cools and reads low. Nothing leaked indoors, the water left through the wall. Checking the discharge pipe outside for dripping or limescale staining takes thirty seconds and settles it.
Tracing a genuine leak on the heating circuit
Real leaks on a sealed system are usually undramatic. Radiator valve glands weep just enough to evaporate before it puddles, leaving a faint white or rusty crust on the valve body. Towel rails in bathrooms are a frequent culprit for the same reason. Pipework buried in a solid floor is the harder case, and it shows up as a warm patch underfoot rather than visible water. If nothing is visible anywhere, an engineer can isolate the boiler from the circuit overnight, if the pressure holds with the boiler isolated, the fault is on the pipework, and if it still drops, the fault is inside the appliance.
Why topping it up repeatedly makes things worse
Every top-up introduces fresh mains water, and fresh water carries dissolved oxygen. Oxygen is what drives the corrosion that produces magnetite sludge, which then blocks heat exchangers, pinholes radiators from the inside, and wrecks pumps. A system topped up weekly for a year has taken in a substantial volume of oxygenated water and diluted whatever inhibitor was in it. So the habit that seems to be managing the problem is quietly creating a second, more expensive one.
What a proper repair looks like
Find the cause, fix it, then restore the system properly: refill to 1.0-1.5 bar cold, bleed the radiators, re-check the pressure, test the system water, and dose inhibitor to the correct concentration. If the water sample comes back dirty, a power flush and a magnetic filter belong in the same visit, otherwise the sludge already circulating carries on doing damage regardless of the leak being fixed. Ask for the inhibitor level to be recorded on the invoice; it is the number nobody checks and the one that predicts the next five years.
Why does boiler pressure keep dropping?
Because water is leaving the system. A sealed heating system uses the same water over and over, so it should not need topping up more than once or twice a year. If it keeps dropping, there are three usual reasons. One: a small leak, often a radiator valve, a towel rail or a joint under the floor. Two: a failed expansion vessel, which makes the pressure jump when the heating is on and pushes water out of a safety valve. Three: that safety valve, the pressure relief valve, letting water past even when it should be shut. Only the first one leaves any water indoors, which is why so many people cannot find a leak. The answer is to find which of the three it is, not to keep topping up.
Pressure dropping on boiler every day
A drop every day is fast, and it tells you something. A slow weep from a radiator valve usually takes weeks to empty a system. Needing to top up every day means a lot of water is going somewhere. The most common reason is the expansion vessel. It fails, the pressure shoots up each time the heating runs, and the safety valve dumps water outside. A drop that only happens on days the heating is on fits that pattern well. The other possibility is a bigger leak, often hidden in a floor or inside the boiler. Either way, this is worth a call soon, not in the spring. Our guide Boiler Losing Pressure But No Leak explains the outside-pipe check that tells you which it is.
Combi boiler losing pressure
Combi boilers lose pressure for the same reasons as any sealed system, with two extras worth knowing. First, the expansion vessel is built into the boiler itself, so it is easy to forget it exists. It still needs checking at the yearly service. Second, the filling loop on a combi is often a small tap or lever underneath the boiler. If it has been left slightly open, pressure will rise, not fall, and the safety valve then lets water out, so the pressure ends up low. Check it is fully closed. If your combi also gives hot water that runs warm, scale inside the boiler may be part of the problem. The fix for pressure loss is the same: find where the water goes.
Is low boiler pressure dangerous?
Not in the way people fear. A boiler with low pressure will not explode. Modern boilers switch themselves off when the pressure falls too low, usually below about 0.5 bar, to protect themselves. So the main effect is that you lose heating and hot water. That can be a real risk for babies, older people and anyone unwell in cold weather, so treat it as urgent in winter. What is harmful over time is topping up again and again. Fresh water brings oxygen into the system, which rusts radiators from the inside and makes black sludge. If you ever smell gas or see soot around the boiler, that is a different danger: turn it off and ring 0800 111 999.