Why Does My Boiler Pressure Keep Dropping?
By Adrian Plumbing & Heating Ltd
In short
Repeated pressure loss usually means water is escaping somewhere in the sealed heating system or a boiler component needs attention. Bleeding a radiator can also temporarily reduce pressure. The good news is most causes are identifiable and fixable by a qualified engineer.
On this page
What boiler pressure means
Your boiler has a pressure gauge — usually on the front panel or displayed on the digital screen. This shows the pressure of the water inside your sealed central heating system. The correct range varies by model, but many systems sit around 1–1.5 bar when cold. Your boiler manual is the authoritative source for the correct range for your specific model.
When the system is cold and the pressure drops below the recommended range — or frequently falls to zero — it usually indicates that water is being lost from the sealed system somewhere. Topping up the pressure temporarily fixes the symptom but does not address the underlying cause.
Safe checks you can make
Before calling an engineer, there are a few safe, visible checks you can carry out yourself:
- Radiator valves: Feel around the valve connections for dampness or drips, including under the valve caps.
- Pipe joints: Look for signs of moisture at visible pipe joints, particularly under floors if accessible.
- Damp marks: Check ceilings, walls and skirting boards for damp patches, stains or mould that could indicate a hidden leak.
- Discharge pipe outside: Look at the copper pipe that exits the wall near your boiler (the pressure relief valve discharge pipe). If it is dripping or has left a green stain, the pressure relief valve may be passing.
- Recent radiator bleeding: Bleeding a radiator releases air and can temporarily lower the system pressure. If you have recently bled radiators, a small pressure drop is expected.
Common causes of pressure loss
The most frequent reasons for a boiler losing pressure are:
- A small leak in the heating system — even a tiny weep at a pipe joint, radiator valve or under the floor can gradually reduce pressure. These can be hard to spot without lifting floorboards.
- The pressure relief valve (PRV) — if this valve is passing, water is being discharged outside through the copper pipe. This can happen if the valve is worn or if system pressure has been set too high.
- The expansion vessel — this component absorbs the expansion of water as it heats up. If it loses its air charge or the diaphragm fails, system pressure can rise too high when heating and then drop when cooled, causing the PRV to discharge.
- An internal boiler fault — a failing component inside the boiler, such as a heat exchanger seal, can cause a slow internal leak.
Repressurising your boiler safely
If your pressure has dropped and you need to repressurise, only follow the exact instructions in your boiler manufacturer's manual. The process typically involves using the filling loop to add water until the gauge reads the recommended pressure, then closing the valves.
Important safety notes:
- Never remove the boiler casing — internal components are behind sealed panels for safety reasons.
- Repeated topping-up is not a repair. If you find yourself repressurising regularly, there is an underlying issue that needs professional attention.
- Do not over-pressurise. Setting the pressure too high can cause the PRV to discharge, which itself causes pressure loss.
- If you are not confident, request a callback and Adrian can talk you through it or arrange a visit.
When to call an engineer
Call an engineer if:
- → Pressure keeps dropping after you have topped it up more than once or twice.
- → You can see water dripping from the external discharge pipe.
- → You notice damp patches, stains or water marks near radiators, pipework or the boiler.
- → The pressure rises very high when the heating is on and then drops when it cools.
- → The boiler displays a fault code related to pressure.
Book a boiler repair or heating system repair with Adrian.
Gas emergency — what to do
⚠ If you smell gas or suspect a gas leak
- Turn off the gas supply at the meter if it is safe to do so.
- Open doors and windows to ventilate the property.
- Do not use switches, light matches or create any flame or spark.
- Call the National Gas Emergency Service on 0800 111 999 immediately.
Disclaimer: This article provides general guidance only. Always follow your appliance manufacturer's instructions and use an appropriately qualified professional for gas, electrical and plumbing work. Gas work must be carried out by a Gas Safe registered engineer.