Boiler Losing Pressure in Crawley - What It Means and What to Do

Published July 2026 | Boiler Repair

In the first 10 minutes

A boiler losing pressure is one of the most common call-outs our engineers see across Crawley and the wider West Sussex area. The good news is that a single pressure drop does not automatically mean an expensive repair. But you do need to understand what you're looking at before you do anything.

The pressure gauge is usually a small dial or digital display on the front of your boiler. Normal operating pressure when the system is cold sits between 1 and 1.5 bar. When the heating is running, it typically rises to around 1.5 to 2 bar, which is normal. If the needle has crept below 1 bar, or the display is showing a pressure warning or fault code, that is why your boiler has stopped working or is struggling to fire.

Here is what to do in the first few minutes:

1. Do not panic. A low-pressure warning is not a gas emergency. You do not need to evacuate the property.
2. Check the gauge reading. Note whether it is below 0.5 bar (locked out), between 0.5 and 1 bar (struggling), or hovering just under 1 bar (borderline).
3. Look around the boiler itself and your visible pipework for any obvious drips or wet patches on the floor or nearby walls.
4. Check your radiators. If you recently bled one, you may have accidentally let pressure out of the sealed system.
5. If you smell gas at any point, stop what you are doing, leave the property, and call the National Gas Emergency Service on 0800 111 999 immediately.

Assuming there is no gas smell, you are dealing with a plumbing and heating issue, not a gas safety emergency. You have time to assess it properly.

Within the first hour

Once you have confirmed the pressure is low and there is no immediate safety concern, the next step is working out why it dropped.

Boiler pressure drops for a handful of common reasons. The most frequent one our engineers find in Crawley homes is a slow leak somewhere in the central heating system. This might be at a radiator valve, a joint under the floorboards, or at the boiler itself. Over time, even a very small drip can cause the pressure to fall enough to trigger a lockout.

The second most common cause is a failed expansion vessel. The expansion vessel is a small pressurised tank inside your boiler (or occasionally mounted externally) that absorbs the extra volume of water as it heats up and expands. When the membrane inside it fails, the vessel can no longer do its job, and the system either loses pressure when cold or overpressurises when hot. This is particularly common in Worcester Bosch, Vaillant, and Baxi boilers that are more than eight years old.

A third possibility is a faulty pressure relief valve (also called a PRV). If the PRV is discharging, you will typically see a small copper pipe outside the property, usually pointing downward near an external wall, occasionally dripping or weeping water. That is the PRV releasing excess pressure or, in some cases, failing and venting when it should not.

Within this first hour, do a walk around your home. Check under radiators for damp patches, look at the floor around any visible pipework, and check under the kitchen sink if the boiler is located in a kitchen. Use your phone to photograph anything that looks wet or corroded - this information is useful when you speak to an engineer.

If you are confident with the filling loop procedure (it is covered in most boiler manuals and there are manufacturer guides for models including Ideal Logic, Worcester Bosch Greenstar, and Vaillant ecoTEC), you can repressurise the system yourself to restore heating while you arrange a repair. Connect the filling loop, slowly open both valves until the gauge reads around 1.2 bar, then close both valves and check the gauge holds steady. If it does not hold, there is an active leak and you should not keep topping it up without getting it investigated.

Same day

If you have repressurised the boiler and the pressure holds, you are not out of the woods. A boiler that needs topping up more than once every few months has an underlying problem. Booking an engineer should be your priority before it fails completely in cold weather.

If you repressurised the boiler and the pressure dropped again within an hour or two, or if you cannot locate the filling loop, or if you are at all unsure about what you are looking at, call a qualified engineer today. Do not attempt any internal boiler work yourself. By law, any gas work must be carried out by a Gas Safe registered engineer - this is not optional guidance, it is a legal requirement under the Gas Safety (Installation and Use) Regulations 1998.

When you contact an engineer or use the Voltrade GoFIX diagnostic tool to describe your fault, have the following ready:

• Your boiler make and model (usually on a label inside the front casing)
• The current pressure gauge reading
• How often the pressure has dropped (first time, or recurring)
• Whether you have spotted any drips or wet patches
• Whether the boiler is showing a fault code and what it says

A responsive engineer should be able to attend the same day or the following morning for most Crawley postcodes. If they cannot, ask whether there is a temporary fix you can make safely while you wait.

The repair visit

When an engineer arrives, the first thing they will do is a system pressure test to confirm where the loss is occurring. For a simple repressurise and check where no fault is found, you are typically looking at a call-out charge of between 60 and 100 pounds, depending on the company and whether it is a standard or emergency call-out.

If a leak is found at a radiator valve or compression fitting, fixing it typically costs between 80 and 180 pounds including parts and labour, depending on access difficulty.

A failed expansion vessel is one of the most common findings in West Sussex boiler repairs. Replacing or re-pressurising an external vessel is typically 150 to 250 pounds. If the vessel is internal and requires partial boiler disassembly, that can rise to 200 to 350 pounds.

A pressure relief valve replacement typically costs between 120 and 220 pounds. If the PRV has been weeping for a while and has caused secondary corrosion damage, additional work may be needed.

The job itself typically takes between one and three hours for most repairs. The engineer will need access to the boiler and, in some cases, to floor or ceiling spaces if pipework needs tracing. Clear a working area around the boiler beforehand and make sure you have parking available - in many parts of Crawley town centre and surrounding roads, parking access for a van matters to how quickly an engineer can set up and get started.

After the repair, the engineer should repressurise the system, run the boiler through a full heat cycle, and confirm the pressure remains stable before signing off. Ask for written confirmation of the work done and any parts used, and check that the engineer's Gas Safe registration number is on the invoice.

The following week

In the seven days after a repair, keep an eye on the pressure gauge each morning. A healthy sealed system should hold its pressure consistently - you should not need to top it up at all under normal circumstances. Note the cold reading (before the heating fires) and the hot reading (while it is running) to get a baseline for your system.

Bleed your radiators if any are cold at the top. Air in the system can cause the pressure to fluctuate slightly and is worth clearing out after any system work. Bleed from the upstairs radiators first, working your way down to the ground floor. Keep a cloth ready as a small amount of water may come out with the air.

Check the external PRV discharge pipe again - that small copper pipe on your outside wall. It should be completely dry. If it is still weeping, contact your engineer.

Long term

Preventing repeat pressure loss comes down to routine maintenance and understanding your system's age and condition. Boilers in Crawley and across West Sussex are subject to the same hard water challenges as much of the South East. Limescale build-up in the heat exchanger and pipework puts additional strain on components and contributes to premature failure of expansion vessels and seals.

An annual service is the single most effective thing you can do to catch pressure-related faults before they become breakdowns. A Gas Safe registered engineer will check the expansion vessel pressure, inspect seals and valves, and flag anything that is showing early signs of wear. Annual services typically cost between 60 and 120 pounds and can extend boiler lifespan significantly.

If your boiler is over 12 to 15 years old and the expansion vessel or PRV has failed, it is worth having an honest conversation with your engineer about whether repair or replacement makes more economic sense. A new A-rated boiler from a brand like Worcester Bosch, Vaillant, or Ideal, installed and commissioned, typically costs between 1,800 and 3,200 pounds depending on the model and any associated pipework changes. Modern boilers are considerably more efficient than older units, and the fuel savings over time often offset the installation cost within five to eight years.

Consider fitting a system filter (a magnetic filter) if you do not have one already. These capture iron oxide sludge that circulates in older systems and causes wear to pump seals and heat exchangers. They are inexpensive to install and dramatically reduce the rate of component deterioration.

Timeline questions

How quickly will my boiler lose pressure if there is a leak?

It depends on the size of the leak. A pinhole drip at a radiator valve might cause the pressure to drop by 0.5 bar over several weeks, while a more significant leak at a joint can cause a noticeable drop within hours. If your boiler is losing pressure within a day or two of being topped up, treat that as an urgent fault rather than routine maintenance.

Is it safe to keep topping up the boiler pressure myself?

Topping up occasionally, perhaps once or twice a year, is generally considered acceptable as a small amount of pressure loss can occur naturally through micro-aeration. If you find yourself topping up every few days or every week, stop. Repeatedly adding cold water to a pressurised system in the presence of a leak is not a safe long-term fix and can accelerate corrosion and component wear. Get it diagnosed properly.

What does it cost to fix a boiler losing pressure in Crawley?

The cost varies depending on the cause. A simple repressurise check where no fault is found typically costs between 60 and 100 pounds for the call-out. Expansion vessel repairs or replacement commonly fall between 150 and 350 pounds. A pressure relief valve replacement is typically 120 to 220 pounds. If a hidden pipework leak requires tracing and repair, costs can range more widely depending on access. Always ask for a written quote before work begins.

Does boiler pressure loss mean I need a new boiler?

Not automatically. Pressure loss is often caused by a single failed component - an expansion vessel, PRV, or a small leak - and these are repairs, not reasons to replace the whole unit. However, if your boiler is over 12 years old, has had repeated component failures, or the cost of repair approaches 50 percent of a replacement unit, a new boiler may be the more sensible investment. A qualified engineer can give you an honest assessment based on the specific make, model, and condition of your current system.