Why Does My Boiler Pressure Keep Dropping?

There isn’t one single cause of low boiler pressure. However, there are several common problems we look for when diagnosing a heating system that keeps losing pressure.

1. A Small Leak in the Central Heating System

One of the most common reasons for boiler pressure dropping is simply that water is escaping somewhere from the heating system.

It doesn’t necessarily have to be a large or obvious leak.

Small leaks can develop around:

• Radiator valves
• Pipe joints
• Exposed heating pipework
• Radiator connections
• Components around the boiler

Check around your radiators and any visible pipework for small amounts of water, damp patches, staining or signs of corrosion.

Some leaks can become more noticeable when the heating is running because the pipework and components are hot and under pressure.

However, never remove your boiler casing to look for a leak yourself. If there appears to be water coming from inside the boiler, it needs to be investigated by a suitably qualified engineer.

2. A Problem With the Expansion Vessel

Your boiler’s expansion vessel has an important job.

As the water inside your central heating system gets hotter, it expands. The expansion vessel is designed to accommodate this change and help keep the system pressure under control.

If the expansion vessel loses its charge or develops a fault, you may notice a particular pattern:

The pressure rises considerably when the heating comes on, then drops very low after the system cools down.

For example, if your boiler starts at a normal cold pressure but climbs towards the upper end of the gauge when the heating is running, something may not be working correctly.

Excessive pressure can cause the system’s pressure relief valve to operate and discharge water. When the boiler cools again, you’re then left with less water in the system and a low-pressure reading.

An expansion vessel is an internal boiler component and should be checked by a qualified heating engineer.

3. A Pressure Relief Valve (PRV) Problem

The pressure relief valve, commonly called the PRV, is a safety component designed to release water if the pressure inside the heating system becomes too high.

Normally, you shouldn’t have to think about it.

However, if the PRV has operated and doesn’t seal correctly afterwards, a small amount of water can continue escaping from the system.

Over time, that loss of water causes your boiler pressure to fall.

One clue can be water dripping from the boiler’s external discharge pipe, particularly after the heating has been running.

If you suspect a PRV problem, don’t attempt to repair or replace it yourself. The reason the valve has discharged also needs investigating rather than simply changing the valve.

What Does Your Boiler Pressure Gauge Tell You?

You don’t need to diagnose the boiler yourself, but watching how the pressure behaves can provide useful information.

Pressure gradually drops over several days

This can indicate a small leak somewhere in the heating system. Check visible radiator valves, joints and pipework for signs of water.

Pressure rises significantly when the heating comes on and falls when it cools

This can indicate an issue with the expansion vessel or pressure relief valve and should be investigated professionally.

Pressure falls below 1 bar and the boiler stops working

Many boilers can lock out or stop operating correctly when system pressure becomes too low.

You have to top the pressure up every few days

This isn’t something you should simply accept as normal. A sealed heating system shouldn’t continually need fresh water adding to it.

The underlying reason for the pressure loss needs finding.

How Do I Stop My Boiler Pressure From Dropping?

This depends entirely on what’s causing it.

If the pressure is simply low and there are no obvious signs of a larger problem, you may be able to repressurise the boiler yourself.

However, repressurising the system isn’t the same as repairing it.

If the pressure keeps disappearing, topping it back up is only treating the symptom.

How to Repressurise a Boiler Safely

The exact process varies between boiler manufacturers and models, so always check your boiler manufacturer’s instructions first.

For boilers with a filling loop, the general process is:

1. Switch the heating off and allow the system to cool.
2. Check the current pressure reading.
3. Locate the boiler’s filling loop or filling mechanism.
4. Open it slowly and allow water into the system.
5. Watch the pressure gauge carefully.
6. Repressurise to the level recommended by your boiler manufacturer.
7. Make sure the filling mechanism is completely closed afterwards.

For many domestic systems, the recommended cold pressure is approximately 1 to 1.5 bar, but don’t assume this is correct for every boiler.

If you’re unsure which valves operate the filling loop, don’t open valves at random. Check your boiler manual or ask a professional.

Can I Just Keep Topping Up My Boiler Pressure?

This is something we see regularly on boiler repair call-outs.

The homeowner knows how to use the filling loop, so every time the pressure drops, they simply top it back up.

The heating works again, so the problem appears to be solved.

But it isn’t.

If you’re having to repressurise your boiler every day or several times a week, water is being lost from the system or there’s another underlying fault that needs investigating.

Repeatedly introducing fresh water into a heating system isn’t a good long-term solution either. Fresh water introduces oxygen and minerals, which can contribute to corrosion and sludge formation within the heating system over time.

Rather than continually topping the pressure back up, find out why it’s dropping in the first place.

What We Commonly Find on Boiler Pressure Call-Outs

At Boiler Pro Solutions, we regularly attend homes where the owner has been topping up their boiler pressure for weeks or even months.

Sometimes the cause is relatively straightforward, such as a small leak around a radiator valve or connection.

In other cases, further investigation finds an issue with the expansion vessel, pressure relief valve or another part of the heating system.

This is why we don’t simply repressurise a boiler and consider the job finished.

Our approach is to look at how the pressure behaves, check for signs of water loss and diagnose the underlying problem before recommending the appropriate repair.

When Should I Call a Heating Engineer?

You don’t necessarily need an engineer because your boiler pressure has dropped once.

However, you should consider having the system professionally checked if:

• You have to repressurise the boiler regularly
• The pressure repeatedly falls below 1 bar
• Pressure rises excessively when the heating comes on
• Your boiler keeps locking out
• You can see water leaking from the boiler
• Radiator valves or pipework are leaking
• Water is coming from the external discharge pipe
• You’ve topped the pressure up but it quickly drops again
• The boiler is displaying a persistent fault code

If you’re unsure, it’s better to have the cause diagnosed rather than repeatedly resetting and repressurising the boiler.

What If My Boiler Is Showing a Fault Code?

Low pressure isn’t the only reason a boiler can lock out.

If your boiler has sufficient pressure but still refuses to fire, an error code can help narrow down the problem.

Baxi owners, for example, may come across an E133 fault, which relates to an ignition or flame failure rather than simply low system pressure.

If you have a Baxi 600 or 800 displaying this code, read our How to Fix a Baxi E133 Fault Code guide for the safe checks you can carry out before arranging a repair.

How Can I Prevent Boiler Pressure Problems?

Not every pressure problem can be prevented, but looking after your heating system can help identify smaller issues before they become larger repairs.

One of the simplest things you can do is have your boiler serviced annually.

During a boiler service, an engineer can check the operation and condition of the appliance and may spot developing issues that could otherwise go unnoticed.

It’s also worth occasionally checking your pressure gauge rather than waiting until the boiler stops working.

If the pressure is gradually changing over time, investigating it early is usually better than discovering the problem when you need your heating most.

Boiler Losing Pressure? Get the Cause Properly Diagnosed

If you’ve topped up your boiler but the pressure keeps dropping, don’t keep reaching for the filling loop.

There’s a reason the system is losing pressure.

Boiler Pro Solutions provides professional boiler repairs across Bolton and surrounding areas, diagnosing problems such as pressure loss, leaks, boiler lockouts, ignition faults and heating failures.

We’ll investigate what’s causing the problem, explain what we’ve found and advise you on the most sensible repair.

If your boiler is losing pressure repeatedly or has stopped working altogether, book a boiler repair with Boiler Pro Solutions and we’ll get to the bottom of the fault.