Boiler Pressure Keeps Dropping? A Step-by-Step Guide to Repressurising (And When to Call an Engineer)

A Gas Safe registered engineer explains what boiler pressure actually means, how to top it up safely, and when low pressure signals something more serious.

If your boiler pressure keeps dropping, you are not alone. It is one of the most common boiler problems we see, and in many cases you can fix it yourself in a few minutes.

But sometimes low pressure is a symptom of something more serious. This guide shows you how to tell the difference, how to top up safely, and when to call for help.

What Is Boiler Pressure?

Boiler pressure refers to the pressure of the water circulating through your central heating system. It is measured in bars.

Your boiler needs enough pressure to push water around all the radiators and back to the boiler. Too low and the system cannot circulate properly. Too high and safety devices will activate to protect the boiler.

This is different from gas pressure or mains water pressure. Boiler pressure is a closed loop: the same water goes round and round your heating system.

How to Check Your Pressure

Every boiler has a pressure gauge. It is either:

• Analogue dial: a round gauge with a needle, usually on the front of the boiler
• Digital display: shown on the boiler screen, often with a bar symbol

Some older boilers have the gauge on the pipework below the boiler rather than on the unit itself.

Reading the Gauge

The gauge shows pressure in bars. Many have a green zone (normal) and red zones (too low or too high).

• Below 1.0 bar: too low. Boiler may lock out.
• 1.0 to 1.5 bar: normal when cold.
• 1.5 to 2.0 bar: normal when heating is running.
• Above 2.5 bar: too high. Pressure relief valve may discharge.

What Pressure Should It Be?

For most combi and system boilers:

State	Normal Pressure
Heating off (cold)	1.0 to 1.5 bar
Heating running (warm)	1.5 to 2.0 bar

Pressure rises slightly when the heating is on because water expands as it heats. This is normal. It should drop back when the system cools.

How to Repressurise Your Boiler (Step by Step)

If your pressure is below 1.0 bar, you can usually top it up yourself. This is called repressurising.

What Is a Filling Loop?

The filling loop connects your boiler to the mains water supply. It allows you to add water to the heating system.

There are two common types:

• External filling loop: a braided silver hose with valves at each end, usually located below the boiler
• Built-in filling loop: integrated into the boiler, operated by a key or lever (common on newer models)

Step by Step: External Filling Loop

1. Locate the filling loop. It is usually under the boiler, a flexible braided hose connecting two pipes.
2. Check both valves are closed. Handles should be perpendicular to the pipe (across, not along).
3. Open the first valve slowly. Turn the handle so it aligns with the pipe.
4. Open the second valve slowly. Water will flow into the system. Watch the pressure gauge.
5. Stop at 1.5 bar. Close the second valve first, then the first.
6. Check for drips around the valves and hose connections.

Step by Step: Built-In Filling Loop

1. Locate the filling key or lever. Check your boiler manual for the exact position.
2. Insert the key or move the lever to open. You may hear water flow.
3. Watch the pressure. Stop at 1.5 bar.
4. Close fully and remove the key if applicable. Do not leave the key in place.

After Repressurising

Turn your heating back on and check:

• The boiler fires and runs normally
• Radiators heat up
• Pressure rises slightly when hot, then settles again when cold
• No visible leaks around the boiler, valves, or radiators

Why Does Pressure Keep Dropping?

If you are topping up regularly, something is causing water to leave the system. The cause depends on the pattern.

Pattern A: Slow drop over weeks

Most commonly a small leak at a radiator valve, a pipe joint, or a minor weep inside the boiler. These can be hard to spot because water can evaporate before it pools.

Pattern B: Drops to zero quickly

Usually a more significant leak, or the pressure relief valve discharging water outside.

Pattern C: Pressure rises too high when heating is on then drops when cold

Often an expansion vessel problem, or a filling loop left open.

1. Small Leak Somewhere in the System

Common leak points:

• Radiator valves (TRVs and lockshields)
• Towel rail joints
• Pipe joints under sinks or behind boxing
• Boiler internal components (pump seals, diverter valve, heat exchanger seals)

2. Pressure Relief Valve Discharging

If pressure gets too high, the pressure relief valve (PRV) releases water to protect the boiler. This discharges through a small copper pipe outside your property (often near an external wall, sometimes below a balcony).

Check outside:

• Wet patch, staining, or green marks below the copper discharge pipe
• Drips during heating operation

If the PRV has opened repeatedly, it may not seal properly afterwards, which can then cause gradual pressure loss.

3. Faulty Expansion Vessel

The expansion vessel absorbs the pressure increase when water heats up. If it fails or loses charge, pressure swings are common.

Typical signs:

• Pressure climbs rapidly when heating turns on
• Pressure approaches 3 bar
• PRV discharges outside
• Pressure then drops lower than before when system cools

This needs an engineer. Do not attempt to access or repressurise internal components yourself.

4. Recently Bled Radiators

Bleeding radiators releases air and a small amount of water. A small pressure drop after bleeding is normal. Top up once to 1.5 bar and monitor.

5. Auto Air Vent Weeping

Some systems have automatic air vents. If these stick open, they can release water slowly and cause gradual pressure loss.

Leak Checks You Can Do in 5 Minutes

You do not need to dismantle anything. These checks catch a large percentage of the obvious issues:

1) Check every radiator valve

• Run tissue around TRV head area and lockshield valve
• Look for green crusting, rust marks, or damp carpet
• Pay attention to towel rails (common leak point)

2) Check the boiler area

• Look underneath for moisture or staining
• Check visible pipe joints and isolation valves
• If the boiler is in a cupboard, check the base for water marks

3) Check the PRV discharge outside

• Find the small copper pipe that terminates outside
• Look for recent wet patches or staining beneath it
• If it drips when the heating is on, tell your engineer (this is useful evidence)

4) Check your filling loop valves are fully closed

• Both valves should be fully closed after topping up
• A slightly open valve can cause pressure creep and later discharge

When to Call an Engineer

Topping up occasionally is normal. Topping up repeatedly is not.

Call an Engineer If:

• You are topping up more than once a month. There is a leak or discharge that needs finding.
• The pressure drops to zero quickly. This indicates a significant leak or PRV discharge.
• Pressure rises too high when heating is on (towards 2.5 bar and above). Expansion vessel or filling loop issue.
• You see any water leaking from the boiler, pipes, or radiators.
• The boiler shows a pressure related fault code (see Manufacturer Help Hubs for model guidance).
• You topped up but the boiler still will not fire. There may be another fault.

Related Problems

Pressure issues often appear alongside other symptoms:

• Cold radiators or cold spots: may indicate sludge or circulation problems. See our Power Flushing Guide.
• Boiler noises: kettling, banging, or gurgling can relate to pressure, sludge, or trapped air. See our Boiler Noises Guide.
• Boiler will not fire: low pressure is one cause, but not the only one. Always check fault codes first.

Common Questions

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