Boiler Losing Pressure in Corsham - What It Means and What to Do About It

Published July 2026 | Boiler Repair

You wake up on a cold Wiltshire morning, the radiators are barely warm, and when you glance at the pressure gauge on your boiler it's sitting somewhere close to zero. You top it up, the heating kicks back in, and you assume the problem's solved - until three days later when the same thing happens again. That nagging pressure drop is your boiler trying to tell you something, and ignoring it usually means a bigger repair bill further down the line.

Understanding What Is Actually Happening Inside Your Boiler

A sealed central heating system is exactly that - sealed. Water circulates through your boiler, your pipework, and your radiators in a closed loop. For this to work correctly, the system needs to hold pressure, typically between 1 and 1.5 bar when the boiler is cold. If that pressure drops repeatedly, water is escaping from somewhere, or something inside the system is failing to maintain it.

The pressure gauge on your boiler - usually a dial or digital display depending on the model - gives you a quick snapshot of system health. Below 0.5 bar and most modern boilers will lock out and stop working entirely, which is a safety feature rather than a fault. Worcester Bosch, Vaillant, Baxi, Ideal, and Viessmann all behave similarly in this regard. The boiler does not "lose" pressure by accident. Something specific is causing it, and identifying that cause is the starting point for any proper fix.

A one-off pressure drop is not always a crisis. If you bled your radiators last week, for example, you would expect to need to top up the pressure afterwards because releasing air from the system reduces it. What should concern you is repeated, unexplained pressure loss over days or weeks. That pattern points to an underlying problem.

The Most Common Causes of Boiler Pressure Loss

In our experience visiting properties across Corsham and the wider Wiltshire area, pressure loss almost always comes down to one of a handful of causes. Here are the main ones, and how to spot which one you are dealing with.

Leaks in the Pipework or Radiators

This is the most common culprit. Water is escaping the system somewhere - it just may not be obvious where. Small pinhole leaks in older pipework, particularly in houses with copper or microbore systems, can drip slowly enough that you never see pooling water, but they lose enough pressure over days to cause repeated lockouts. Check around radiator valves, under the boiler itself, and along any visible pipework in the airing cupboard or under the floorboards if accessible. Look for staining, rust marks, or white limescale deposits - these often mark the site of a very slow drip rather than an active flow.

A Faulty Pressure Relief Valve

The pressure relief valve (PRV) is a safety device that opens automatically if your system pressure climbs too high, releasing water to bring it back down. Over time these valves can weaken, wear, or get stuck in a partially open position. When that happens, the valve weeps water continuously - often down a drainpipe outside your property. Check whether there is a copper or plastic pipe on the exterior wall near the boiler that is wet, dripping, or discharging water. If there is, the PRV is almost certainly to blame. This valve must be replaced by a Gas Safe registered engineer.

A Failing Expansion Vessel

Inside most modern combi boilers there is a small pressurised vessel containing a rubber diaphragm. Its job is to absorb the expansion of water as it heats up, keeping system pressure stable. When the diaphragm perishes or the pre-charge pressure drops, the system cannot manage that expansion properly and the PRV starts releasing water instead. You end up in a cycle where pressure drops when the boiler fires, you top it up, it drops again. An expansion vessel fault is one of the trickier ones to diagnose without experience, which is why our engineers often run a GoFIX diagnostic check to distinguish it from other causes before opening anything up.

Air in the System

Air introduced into the system - typically from recent radiator bleeding, a poorly bled new radiator, or minor repair work - compresses differently to water and can cause misleading pressure readings. If you have recently bled radiators or had work done on the system, topping the pressure up to 1 to 1.5 bar and monitoring it over a week may be enough to confirm there is no ongoing leak.

Inhibitor Depletion and Internal Corrosion

Older heating systems, particularly those that have not been serviced regularly, can suffer from internal corrosion. As metal corrodes inside radiators and pipework, tiny pinholes develop. Products like Fernox F1 or Sentinel X100 inhibitor slow this process considerably, but if the inhibitor concentration has dropped - common in systems that are regularly topped up with fresh water - corrosion accelerates. A system that has never had a powerflush may be harbouring significant sludge and corrosion damage.

Solutions That Actually Work

The right fix depends on the cause, but here is a practical breakdown of what actually resolves boiler pressure loss rather than just managing the symptom.

For a one-off low pressure reading: Use the filling loop to repressurise the system. Most filling loops are a flexible braided hose or built-in valve beneath the boiler. Open it slowly, watch the gauge, and close it when it reads between 1 and 1.5 bar. Turn the heating on and check the pressure again after an hour. If it holds, you likely had a minor air issue.

For a leaking radiator valve: If you have identified a specific dripping valve and feel comfortable doing basic plumbing, isolating that radiator and replacing the valve is achievable without Gas Safe registration (it is not a gas component). However, if the leak is on the boiler itself or involves the heat exchanger, stop there and call an engineer.

For a faulty PRV: This must be replaced by a Gas Safe registered engineer. Attempting this yourself is illegal for gas appliances and potentially dangerous. Replacement typically costs between 80 and 150 pounds for the part and labour, depending on accessibility and boiler model.

For an expansion vessel fault: The vessel can sometimes be recharged using a bicycle pump if the diaphragm is still intact - an engineer can check the pre-charge pressure with a simple gauge. If the diaphragm has perished, the vessel needs replacing. This is commonly quoted at between 150 and 300 pounds including parts and labour for most standard combi boilers.

For system-wide corrosion: A powerflush removes sludge and corrosion debris from the system. In Corsham properties with older heating systems - particularly those in the town's stone-built period homes with original pipework - this can make a dramatic difference to both boiler reliability and radiator heat output. Expect to pay between 300 and 600 pounds for a full system powerflush, with price varying by the number of radiators.

When You Need a Professional and When You Can Sort It Yourself

You can do these yourself:

1. Repressurise the system using the filling loop (topping up with water is not a gas operation)
2. Bleed radiators to release trapped air
3. Check for visible leaks around radiator valves and pipework
4. Replace a radiator thermostatic valve (TRV) - these are on the water circuit, not the gas circuit

You must use a Gas Safe registered engineer for these:

1. Replacing the pressure relief valve
2. Replacing or recharging the expansion vessel
3. Any work inside the boiler casing
4. Investigating or repairing a leak at the boiler
5. Conducting a powerflush on a system connected to a gas boiler

If you are in Corsham and unsure whether your boiler engineer is registered, you can check the Gas Safe Register at gassaferegister.co.uk using their engineer ID. Any reputable local engineer will offer this number without hesitation.

What to Expect from a Boiler Repair Visit in Corsham

If you have called out an engineer to investigate ongoing pressure loss, here is roughly how a visit typically unfolds so you know what you are paying for.

The engineer will start by asking you how long the pressure loss has been happening and how often you have been topping up. This history matters - a system that needs topping up every two days has a faster leak than one that drops slowly over a month. They will check the gauge reading, repressurise if needed, and then inspect the system while it runs.

Common diagnostic steps include checking the PRV discharge pipe outside, running a leak detection inspection around all visible joints and valves, and checking the expansion vessel pre-charge pressure. If our engineers use the GoFIX diagnostic tool during this process, it helps them cross-reference symptoms against known fault patterns for your specific boiler model, which speeds up accurate diagnosis rather than guesswork.

A standard diagnostic call-out in the Wiltshire area typically costs between 60 and 100 pounds. If the fault is found during the visit and parts are available, many engineers will complete the repair the same day. Expect total repair costs to range from around 80 pounds for a simple fix to 300 to 400 pounds for an expansion vessel replacement, depending on boiler model and access difficulty.

For Corsham homeowners with older properties - particularly those with solid stone walls where pipework runs are harder to trace - leak detection may take longer and some engineers carry thermal imaging equipment to help locate hidden drips without unnecessary disruption to floors or walls.

Common Questions from Corsham Homeowners

How often should I need to top up my boiler pressure?

In a healthy sealed system, you should rarely if ever need to top up pressure outside of routine maintenance like radiator bleeding. Needing to repressurise more than once or twice a year suggests an underlying issue worth investigating. If you find yourself topping up every week or two, that points to an active leak or valve fault that will not resolve itself and is worth getting checked sooner rather than later.

Can low boiler pressure damage my boiler?

Running your boiler at very low pressure repeatedly does put strain on internal components, particularly the pump and heat exchanger. Most modern boilers lock out before they run dangerously dry, which protects them to a degree. However, the underlying cause of the pressure loss - whether it is a corroding pipe or a failing expansion vessel - can cause its own damage if left unaddressed over time. Catching faults early in Wiltshire winters is always cheaper than dealing with emergency breakdowns.

Is a boiler losing pressure dangerous?

Low pressure itself is not immediately dangerous in the way that a gas leak or carbon monoxide issue would be. The boiler will typically shut down safely before pressure drops to a level that causes harm. However, if the pressure loss is caused by a faulty pressure relief valve that is discharging water continuously, that discharge pipe should be clear and directing water safely away from electrical fittings or pedestrian areas. If you notice hot water discharging near anything electrical, treat it as an urgent call-out.

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

Costs in the Corsham and wider Wiltshire area vary by fault. Repressurising alone is something you can do yourself at no cost. A call-out and diagnosis typically runs between 60 and 100 pounds. Replacing a pressure relief valve commonly costs between 80 and 150 pounds all in. An expansion vessel replacement typically sits between 150 and 300 pounds depending on boiler model. A full system powerflush, where needed, is commonly quoted between 300 and 600 pounds for a typical family home.