Boiler Losing Pressure? Causes, How to Repressurise and When to Call an Engineer

You go to run a bath, nothing happens, and the little gauge on the front of the boiler is sitting below 1. A boiler losing pressure is one of the most common reasons the heating and hot water stop working, and the good news is that it is often something you can put right yourself in a few minutes.

The catch is that pressure does not disappear on its own. If your boiler keeps losing pressure, the water is going somewhere, and topping it up again and again only hides the problem. This guide explains what the gauge should read, why pressure drops, how to repressurise a boiler safely, and the point at which it is time to get an engineer in.

What should your boiler pressure be?

Combi and system boilers run on a sealed, pressurised heating circuit. The gauge on the front (either a dial or a number on the display) shows the pressure of the water in that circuit.

With the heating off and the system cold, the reading should usually sit between 1 and 1.5 bar. That is the range Viessmann gives for a boiler that is not in use, and most other manufacturers say much the same. Many dials have a green band to make it easy. Always check your own boiler’s manual, because the exact figure varies slightly between models.

When the heating is running the water expands and the needle will rise a little. That is normal. What you are looking for is a reading that stays steady from one week to the next when the system is cold.

If the pressure falls below the minimum required by your boiler, the appliance may display a warning, show a fault code or stop operating. The exact threshold and response depend on the boiler model.

Why boilers lose pressure

A sealed system should hold its pressure. When you find your boiler losing pressure, the cause is almost always one of the following.

1. You have recently bled the radiators

Bleeding lets air out, and the pressure drops as the air is replaced with water. One top up afterwards is expected and nothing to worry about. If you have just been through our guide to radiators not heating up and bled a few, check the gauge before you do anything else.

2. A leak somewhere on the heating pipework

This is the most common cause by a distance. It can be a weeping radiator valve, a joint on a towel rail, or a pinhole in a pipe under the floorboards that you will never see. Have a look under each radiator and around the valves for drips, rust marks or staining on the carpet or ceiling below. Leaks under floors and inside walls are harder, and that is where proper leak detection saves a lot of lifted floorboards.

3. The pressure relief valve is passing

Every sealed system has a pressure relief valve that opens if pressure climbs too high. It is piped to the outside of the house, usually as a short copper pipe coming out of the wall near the boiler. If that pipe is dripping, or there is a damp patch or limescale streak beneath it, the valve is letting water out when it should not.

4. The expansion vessel has lost its charge

The expansion vessel is a small tank, usually inside the boiler, that gives the water room to expand as it heats up. When it loses its air charge, pressure shoots up every time the heating comes on, the relief valve opens to release it, and the pressure then falls back too low once the system cools. If your gauge swings a long way between cold and hot, this is the likely cause.

5. A leak inside the boiler itself

Less common, but seals and internal pipework do fail. Signs include water on the boiler case, drips from underneath it, or staining on the wall below. Do not take the casing off to look. That is a job for a Gas Safe engineer.

How to repressurise a boiler and restore the pressure

Most boilers are topped up through a filling loop. On many it is a braided silver hose with a small valve or tap at each end, fitted under the boiler. Some newer boilers have a built-in filling key or lever instead, so if you cannot see a hose, check the manual for your model.

1. Turn the boiler off and let it cool. You want to read the pressure with the system cold, otherwise you will overfill it.
2. Check the loop is connected at both ends. If it is a removable hose, screw it on hand tight.
3. Open both valves slowly. You will hear water flowing. Keep your eye on the gauge the whole time.
4. Close both valves when the gauge reaches about 1 to 1.5 bar. It only takes a few seconds, so do not walk away.
5. Disconnect a removable loop and keep a cloth handy, as a little water will drip out.
6. Turn the boiler back on and reset it if it is showing a fault code. The reset method is in the manual.

Then watch the gauge over the next few days. If it holds, the drop was probably down to bleeding or a very slow loss over months. If it falls again, something needs fixing.

What if your boiler pressure is too high?

High pressure usually comes from overfilling, a filling loop that is letting water through, or the failed expansion vessel described above.

If you have simply overfilled it, Viessmann’s advice is that at 2.75 bar or above, bleeding a radiator will bring it back down. Catch the water in a cloth, because it can be hot. The pressure relief valve on most domestic systems is designed to open at around 3 bar, so a gauge sitting near that line is worth acting on.

If the pressure climbs high every time the heating runs and drops back when it cools, do not keep bleeding radiators to manage it. That points to the expansion vessel or the filling loop, and both need looking at properly.

When a boiler losing pressure needs an engineer

Call us, or any Gas Safe registered engineer, if any of these apply:

• You are topping up more than once every few weeks.
• The pressure drops back within a day or two of repressurising.
• The relief valve pipe outside is dripping.
• The gauge swings above 2.5 bar when the heating is on.
• There is water on or under the boiler.
• The boiler still locks out after you have restored the pressure.

Topping up a boiler losing pressure again and again is not a fix. Every time fresh water goes into the system it brings fresh oxygen with it, which speeds up corrosion inside the radiators and is one of the reasons systems end up full of sludge. It also means the leak, wherever it is, keeps doing damage.

Expansion vessels and relief valves on combi and system boilers are usually inside the boiler, so replacing them is a boiler repair for a Gas Safe engineer. If you want a feel for what the common repairs involve, our guide to boiler repair costs in St Albans covers the usual faults. A yearly boiler service is also the point at which an engineer checks the expansion vessel charge, so problems are often caught before they start costing you pressure.

Frequently asked questions