Why Your Boiler Keeps Losing Pressure – A Homeowner Guide

One of the most common calls we get — especially in winter — is:

“My boiler keeps losing pressure and I have no heating.”

If your boiler pressure keeps dropping, your heating system is telling you that something isn’t sealed properly. A modern central heating system is a closed, pressurised system, so once it’s filled with water, the pressure should remain stable for months.

Topping it up once a year? Normal. Topping it up every week? There is a fault.

Left unresolved, low pressure will stop your heating and hot water and can eventually damage the boiler.

What Boiler Pressure Should Be

Most boilers should sit between:

1.0 – 1.5 bar when cold1.5 – 2.0 bar when hot

If pressure drops below about 0.5 bar, the boiler will lock out and stop working.This is why your heating suddenly disappears.

The Most Common Causes of Pressure Loss

1. A Water Leak Somewhere in the Heating System

This is the number one cause.

Your system is sealed.If pressure drops → water is escaping.

Leaks are often tiny and hard to notice. Check for:

• damp patches on carpets

• staining on ceilings

• rust marks around radiators

• wet valves under radiators

• pipe joints in cupboards or under the stairs

Important: under-floor pipe leaks are very common in older properties and you may never actually see the water.

2. A Faulty Expansion Vessel

This is the second most common reason and very typical in boilers around 6–10 years old.

The expansion vessel absorbs pressure changes when water heats up.If it fails:

1. Boiler heats up → pressure rises quickly

2. Pressure exceeds safe limit

3. Safety valve releases water outside

4. Boiler cools → pressure now very low

So you don’t see a leak inside the house — but the system is losing water externally.

Typical signs:

• pressure rises when heating is on

• pressure drops overnight

• you refill it repeatedly

• water outside the discharge pipe

This is extremely common on combi boilers.

3. The Pressure Relief Valve (PRV) Is Passing

Your boiler has a safety valve designed to release water if pressure becomes dangerous.

Sometimes it opens once and never reseals properly.

Result:The system slowly drains through the outside pipe.

Check outside your house:You may see a copper pipe going through the wall near the boiler location.If it’s dripping → that is very likely your pressure loss.

4. A Recently Bled Radiator

After bleeding radiators, pressure will drop because air has been released and replaced by water volume.

This is normal — but only once.

If pressure keeps falling after topping up, the system has another issue.

5. A Faulty Filling Loop

The filling loop (silver braided hose under the boiler) is used to top up pressure.

Sometimes:

• valves don’t fully close

• washers fail

• it lets water escape internally

This is less common, but we do see it.

6. A Heat Exchanger Fault (Serious but Less Common)

In some boilers, the internal heat exchanger can develop a pinhole leak.

Water then escapes into the condensate pipe and drains away unnoticed.

Signs:

• pressure drops quickly

• no visible leaks

• boiler gurgling

• condensate pipe dripping constantly

This requires repair or boiler replacement.

How to Temporarily Repressurise Your Boiler

(Temporary fix — not a solution)

1. Turn boiler off

2. Locate the filling loop under the boiler

3. Slowly open the valves

4. Watch the pressure gauge

5. Stop at 1.3 bar

6. Close valves tightly

If you do this more than once every few months, you need an engineer.Constant refilling introduces oxygen into the system and causes internal corrosion.

When You Should Call a Gas Engineer

You should book a repair if:

• pressure drops within days

• you are topping up weekly

• radiators need bleeding constantly

• boiler locks out regularly

• you see water outside the discharge pipe

• heating works briefly then stops

Ignoring it can lead to:

• pump damage

• heat exchanger failure

• full boiler breakdown

Many boiler replacements actually start as unresolved pressure faults.

Is It Safe to Use the Boiler?

Low pressure itself is not dangerous, but the cause might be.

For example:

• leaking pipework

• internal boiler corrosion

• failing safety valve

Do not keep repressurising daily — this can seriously damage the appliance.

How We Fix It

At Wirral Gas, a pressure loss investigation typically includes:

• system leak inspection

• expansion vessel recharge

• PRV testing

• discharge pipe check

• internal boiler inspection

• radiator valve inspection

In many cases, the repair is straightforward once diagnosed properly.

If your heating keeps cutting out or you’re constantly topping up pressure, we can help. We cover Birkenhead, Wallasey, Bebington, Heswall, Moreton, Upton, Greasby and nearby areas.

Visit our boiler repair page for more information on what we can offer you.