Why does my boiler keep losing pressure?

One of the most common calls we get from homes around Hexham goes something like this: “The boiler keeps losing pressure. I top it up, it works for a few days, and then it drops again.” It is frustrating, and it is easy to get into the habit of topping up every week without ever finding out why. Here is what is usually going on.

What boiler pressure actually is

Most modern boilers – combis and system boilers – run on a sealed heating system. The water in your radiators and pipes is kept under a little pressure, usually around 1 to 1.5 bar when the system is cold. You can see this on the pressure gauge on the front of the boiler or on its display.

That pressure should stay fairly steady. It rises a little as the water heats up and expands, then falls back as it cools. If it keeps dropping over days or weeks, water (or air pressure) is being lost from somewhere.

The most common causes

1. A leak somewhere in the system

Even a tiny leak will slowly reduce the pressure. Look for damp patches around radiator valves, under the boiler, along visible pipework and on ceilings under bathrooms. Small leaks often show as rust marks, white crusty deposits or water stains rather than drips. In older stone houses, pipes are sometimes buried in floors or walls, which makes leaks harder to spot.

2. The pressure relief valve is letting water out

Every sealed system has a pressure relief valve – a safety device that lets water out if the pressure gets too high. It is connected to a small copper pipe that usually goes through an outside wall. If that pipe is dripping, or if there are drip marks on the wall below it, the valve may be letting water out. That can happen because the valve itself is worn, or because the pressure is going too high when the heating is on – which brings us to the next cause.

3. The expansion vessel has lost its charge

The expansion vessel is a small tank (usually inside the boiler) with a cushion of air on one side. It absorbs the extra volume when the water heats up. Over time the air charge leaks away. When that happens, the pressure jumps up when the heating is on, the relief valve lets water out, and then the pressure drops when the system cools. If your gauge swings from low to near 3 bar and back again, this is a likely suspect.

4. Bleeding radiators

Bleeding radiators releases air, and with it some pressure. That is normal, and you may need to top up afterwards. But if you are bleeding radiators often, it suggests air is getting in, or gas is forming from corrosion inside the system – both worth investigating.

5. A faulty filling loop or other component

Less often, the problem lies with other parts of the boiler or system. That is where a proper diagnosis by an engineer comes in.

What you can safely check

• Look at the pressure gauge with the heating off and cold, and note the reading.
• Check radiators, valves and visible pipes for damp, rust or white deposits.
• Look outside for drips from the small copper pipe from the boiler.
• Check under the boiler for any signs of water.
• Note how often you have to top up and by how much.

Your boiler manual explains how to re-pressurise using the filling loop. If you are not comfortable doing it, or the pressure drops again quickly, leave it and get help. Never remove the boiler casing – the parts inside must only be worked on by a Gas Safe registered engineer.

What an engineer will do

An engineer will check the system for leaks, test the pressure relief valve, check and re-charge (or replace) the expansion vessel, and look at how the pressure behaves as the system heats up and cools down. Finding the cause first means you only pay for the part or repair you actually need.

Why it is worth fixing

Constantly topping up a leaking system brings fresh, oxygen-rich water into the heating, which speeds up corrosion and sludge. Over time that can damage radiators, the pump and the boiler itself. Fixing the cause protects the whole system.

If your boiler keeps losing pressure, our boiler repairs page explains how we diagnose faults, or you can request a quote and tell us what you are seeing. For general information on gas safety and checking an engineer, see the Gas Safe Register.

Frequently asked questions