Boiler Losing Pressure: Causes, Safe Checks and When to Call

A boiler that keeps losing pressure is one of the most common heating problems, and one of the most fixable. On most homes in South London the heating runs on a sealed system: the water in the radiators and pipework is held at a set pressure, shown on a gauge on the front or underneath the boiler. If that pressure falls too far, the boiler stops working to protect itself.

This guide explains what the pressure should be, why it drops, the checks that are safe to do yourself and the point where an engineer is needed.

What should boiler pressure be?

Check the gauge when the heating has been off for a while and the system is cold. On most boilers:

Your boiler's manual gives the exact figures for your model. Older open-vented systems with a tank in the loft do not have a pressure gauge in the same way; this guide is about sealed systems.

Why boilers lose pressure

You have just bled the radiators

Bleeding a radiator lets air out, and some water with it. A drop in pressure afterwards is expected, and topping up once is normal. If the pressure stays where you set it, there is nothing more to do.

A leak somewhere on the system

The most common cause of pressure that keeps falling is a small leak. It can be at a radiator valve, a joint, a towel rail or pipework under the floor, and it can be slow enough that the water evaporates before you notice it. Leaks inside the boiler itself are possible too, but finding those means removing the casing, which is a job for a Gas Safe registered engineer.

The pressure relief valve is letting water out

The pressure relief valve protects the system by releasing water if the pressure gets too high. It drains through a small pipe, usually copper, that runs through an outside wall near the boiler. If that valve opens regularly, or has started to let by, the system loses water through it.

The expansion vessel has lost its charge

Inside or beside the boiler is an expansion vessel: a tank with an air cushion that absorbs the extra volume when water heats up. Over time the air charge leaks away. Without it, the pressure climbs steeply when the heating comes on, the relief valve opens to release water, and the pressure falls once the system cools. The tell-tale pattern is high pressure when hot, low pressure when cold, and a boiler that needs topping up again and again.

An automatic air vent or valve is weeping

Small vents and valves can let out tiny amounts of water each time they operate. On their own these losses are slow, but they add up.

Checks you can safely do yourself

None of these involve opening the boiler. The casing should only ever be removed by a Gas Safe registered engineer.

1. Note the cold reading. Check the gauge first thing in the morning before the heating comes on, so you are comparing like with like.
2. Look for damp or drips. Check under each radiator and its valves, along any pipework you can see, around towel rails and on the floor under the boiler. Stains, damp patches or flaking paint can show a slow leak.
3. Check the relief valve pipe outside. Find the small pipe through the outside wall near the boiler. A drip from its end, or a wet patch on the wall below it, suggests the relief valve is opening.
4. Watch the gauge with the heating on. If the pressure climbs towards 3 bar when the system is hot, the expansion vessel is the likely cause.
5. Top up, following the manual. Use the filling loop or filling key described in your boiler's user manual, with the system cold. Open it slowly, watch the gauge and close it at the pressure the manual gives. Make sure the loop is fully closed afterwards.
6. Keep a note of how often it drops. "Every two days" and "once since last winter" point to very different problems, and the note helps an engineer find the cause faster.

When to call an engineer

It is time to book an engineer if:

• the pressure keeps dropping, so you are topping up more than once after bleeding the radiators
• you can see water leaking from the boiler, a radiator or pipework
• the relief valve pipe outside is dripping
• the pressure climbs close to 3 bar when the heating is on
• the boiler keeps locking out or showing a pressure fault code even after topping up
• you are not sure how to use the filling loop on your boiler

If water is leaking near sockets, lights or the boiler's electrics, turn the water off at the stopcock and keep away from the electrics. If you smell gas at any point, leave the property and call the National Gas Emergency Service on 0800 111 999 before anything else.

What an engineer will do

A breakdown visit starts with finding the cause, not guessing at it. Depending on what the checks show, that usually means:

• checking and recharging the expansion vessel, or replacing it if it has failed
• testing the pressure relief valve and replacing it if it is letting by
• tracing a leak on the radiators, valves or pipework
• checking the automatic air vent and other valves inside the boiler
• confirming the system holds its pressure once the repair is done

The repair should be explained and quoted before any work starts, so you can decide with the price in front of you.

Getting it fixed in South London

InstaGasWorks repairs boilers that lose pressure across South West London, Croydon and Sutton. The boiler repair page has the current breakdown visit rates for every day and time and explains how repairs are quoted, and a visit can be booked online.

If the boiler also needs its annual service, the expansion vessel charge is checked as part of every boiler service, which is often how a slow pressure problem is caught before it stops the heating.