Boiler Losing Pressure in Horsham - What It Means and How to Fix It

Published August 2026 | Boiler Repair

This guide walks you through what it means when your boiler keeps losing pressure, why it happens, and how to safely top the system up and track down the root cause. It's written for homeowners in Horsham and across West Sussex who want to understand the problem properly before calling an engineer - or who want to try the safe bits themselves first.

Before You Start - Safety First

Before you touch anything on your boiler, there are a few safety rules you need to know. These aren't just standard disclaimers - they matter.

First, never attempt to repressurise a boiler that is showing an active fault code, making unusual noises, or leaking water visibly from the unit itself. Topping up the pressure on a boiler with an active internal leak will only delay the problem and mask what could be a serious fault.

Second, the boiler should be switched off and allowed to cool before you touch the filling loop. Working on a hot system is unnecessary and increases the risk of burns if water escapes under pressure.

Third - and most importantly - if you smell gas at any point, stop immediately. Do not touch any switches or electrical fittings. Leave the property, leave the door open if it's safe to do so, and call the National Gas Emergency Service on 0800 111 999. Any work involving the gas supply or internal sealed components of a boiler must only be carried out by a Gas Safe registered engineer. This is a legal requirement in the UK, not a recommendation you can choose to ignore.

What You Will Need

Topping up your boiler pressure is one of the few boiler-related tasks a homeowner can safely carry out themselves. You won't need much.

Tools and materials:

• Your boiler's user manual (check the front panel for the model number if you've lost it)
• A flat-head or crosshead screwdriver - some filling loop valves have slotted heads
• A small towel or shallow bowl to catch any drips
• A phone torch - the filling loop is often tucked in a dark corner underneath the boiler

What you don't need: No specialist plumbing tools are required for a basic represerisation. If you're doing more than that - checking for leaks, testing the expansion vessel, inspecting the pressure relief valve - you'll still mostly be looking with your eyes rather than working with tools.

Time estimate: The represerisation itself takes around 5 to 10 minutes. Diagnosing why your boiler keeps losing pressure is the part that takes longer. Budget 20 to 30 minutes to check for obvious causes including visible leaks, a weeping pressure relief valve discharge pipe, or signs of expansion vessel failure.

Step-by-Step Instructions

Step 1 - Check your boiler's pressure gauge

Find the pressure gauge on your boiler. On most modern combi boilers - including popular brands like Worcester Bosch, Vaillant, Ideal, and Baxi - this is a small dial or digital display on the front panel. For a cold system (boiler off and fully cooled), pressure should read between 1 and 1.5 bar. Below 1 bar and the boiler will likely have locked out or be refusing to fire. Some boilers display a specific fault code when this happens - Worcester Bosch units commonly show codes like EA229 or similar low-pressure warnings. Make a note of any codes before you reset anything.

Step 2 - Locate the filling loop

The filling loop is the short braided hose or rigid pipe that connects the mains cold water supply to your central heating system. On most modern combi boilers, it sits underneath the boiler, sometimes tucked inside the casing. It typically has one or two valves - these might be slotted screw heads, quarter-turn levers, or push-fit connectors depending on the age and make of your boiler. Our engineers regularly visit homes across Horsham where homeowners have spent ten minutes just finding the filling loop before the job even starts - if it's not obvious, check your manual or search your boiler model number online for a diagram.

Step 3 - Connect the filling loop if needed

Some older systems - or installations where the loop was removed after commissioning - use a removable external filling loop. This is a short flexible hose with a connector at each end. If this applies to your setup, connect the hose between the two designated ports and check both ends are secure before you open any valves. Most modern installations have a permanent internal loop and you can skip straight to the next step.

Step 4 - Open the filling loop valves slowly

With the boiler switched off, open the valve or valves on the filling loop. You'll hear water beginning to flow into the heating circuit. Watch the pressure gauge carefully as you do this. The needle or digital reading should start rising within a few seconds. Keep the valve only partially open - if you open it fully, pressure can overshoot before you have time to react and close it again.

Step 5 - Stop at the correct pressure

Close the valves when the gauge reads between 1.2 and 1.5 bar. Do not let it go above 2 bar. Overpressurising is a very common mistake, and if pressure gets too high the system's pressure relief valve will open and discharge water through a pipe outside the property. If that pipe starts running with water after you repressurise, you've gone too high - bleed a radiator slightly to bring the pressure back down, then close the bleed valve and check the gauge again. Once you're in the correct range, close all filling loop valves fully.

Step 6 - Restart the boiler and monitor

Switch the boiler back on using the manufacturer's instructions. Many boilers reset themselves automatically once pressure is restored. Others require you to hold down a reset button - this is usually clearly marked on the front panel. Give the system 15 to 20 minutes to run and bring the heating up to temperature, then check the pressure gauge again. When hot, a reading of 1.5 to 2 bar is normal and expected - pressure rises slightly as water expands with heat. If it's climbing above 2.5 bar while running, that points to an expansion vessel problem rather than a simple pressure loss.

Step 7 - Keep a record and monitor over the following days

This is the step most homeowners skip, and it's the most important one for working out whether you have an ongoing problem. Note down the date, the cold pressure reading after represerising, and then check again after 24 hours and again after a week. If the pressure drops by more than 0.3 bar without you having bled any radiators in the meantime, there's an underlying issue that needs investigating. That record of dates and readings will be useful to an engineer if you do need to book a callout.

What to Do If This Does Not Fix It

Represerising the system fixes the symptom, not the cause. If pressure keeps dropping, here's where to look before calling someone out.

You've recently bled radiators: This is the most common and least worrying explanation. Bleeding releases trapped air - and some water - from the system, which drops the pressure. Simply top it back up. This isn't a fault.

Visible leaks on radiators or pipework: Walk around every radiator in the house and check both valves at either end. Look for white limescale deposits around joints, rust streaking down the radiator, or damp patches on the floor or carpet underneath. Even a very slow drip from a valve can cause the system to drop pressure over several days. In older properties across West Sussex, pinhole corrosion leaks in copper pipework are a fairly common culprit, particularly in homes where the heating water hasn't been chemically treated.

The pressure relief valve discharge pipe: Go outside and find the pipe that exits the wall near where your boiler is located. It's usually a copper pipe, 15mm or 22mm in diameter. If this pipe is weeping or actively dripping, your pressure relief valve is opening to discharge excess pressure. This typically happens when the expansion vessel has failed, causing pressure to climb too high when the system heats up and then fall again when it cools. A new expansion vessel typically costs between 150 and 300 pounds fitted, depending on the size of your system.

Expansion vessel failure: The expansion vessel is a small pressurised tank - inside or adjacent to the boiler - that absorbs pressure changes as water heats and cools. When its internal membrane fails, the system loses the ability to manage pressure fluctuations. Our engineers use the Voltrade GoFIX diagnostic tool during callouts to check expansion vessel charge pressure and assess system behaviour quickly, which makes it much faster to confirm whether the vessel needs replacing or just re-pressurising through its Schrader valve.

When to Stop and Call a Professional

There are clear situations where you should stop and get a Gas Safe registered engineer involved. These aren't judgment calls - they're situations where DIY attempts can cause harm or significantly increase repair costs.

You can smell gas. Call 0800 111 999 immediately. Don't touch switches, don't use your phone near the boiler, get out.

Water is leaking from inside the boiler casing. A leak originating inside the unit - from the heat exchanger, a seal, or an internal pump fitting - will get worse if you keep adding water to the system. Switch off and call an engineer.

Pressure drops back to zero within hours of represerising. This suggests a significant leak somewhere in the system that needs locating and repairing before the heating is run again.

Persistent fault codes you don't recognise. Fault codes vary significantly between manufacturers - Worcester Bosch, Vaillant, Viessmann, and Ideal all use different numbering systems. An experienced engineer can read and interpret these in minutes.

Your boiler is over 12 to 15 years old and pressure keeps dropping. Older boilers with repeated pressure loss can indicate general system deterioration rather than a single fixable fault. Many Horsham homeowners with boilers installed before 2012 find that repair costs start approaching the price of replacement. An honest engineer will tell you which makes more sense. A standard diagnostic callout in the Horsham area typically costs between 80 and 120 pounds, with most repairs falling in the 150 to 400 pound range depending on the parts required. Always verify Gas Safe registration before booking - it's a legal requirement and takes two seconds to check on the Gas Safe Register website.

Questions About This Process

Why does my boiler keep losing pressure even after I top it up?

If your boiler drops pressure repeatedly - say once a week or more - there's almost certainly an underlying fault rather than just a one-off occurrence. The most common causes are a leaking radiator valve, a pinhole leak in the pipework, a failed expansion vessel, or a pressure relief valve that's opening when it shouldn't. A single pressure drop after bleeding radiators is normal and expected, but repeated drops without bleeding are always worth getting looked at. The problem won't resolve itself and can shorten the lifespan of the boiler if left unaddressed.

Is it safe to repressurise my boiler myself?

Yes, in most cases. Adding water to the system via the filling loop is a routine homeowner task that doesn't involve any gas work. As long as the boiler is switched off, there's no visible internal leak, and you stop adding water at the correct pressure, it's a safe job to carry out yourself. What you should never attempt is anything involving the gas pipework, the heat exchanger, or the sealed internal components of the boiler - that work legally requires a Gas Safe registered engineer and attempting it yourself is both dangerous and illegal.

How much does it cost to fix a boiler that keeps losing pressure in West Sussex?

It depends entirely on the cause. If it's a leaking radiator valve or a simple pipework joint, a plumber might charge between 80 and 150 pounds for the callout and repair. A failed expansion vessel typically costs between 150 and 300 pounds fitted. A leaking heat exchanger on an older boiler can push costs to 400 pounds or more - at that point, replacement often makes more financial sense than repair. Labour rates across Horsham and West Sussex are broadly in line with the wider South East average, so those figures represent what you'd typically expect to pay locally.