Why Your Boiler Keeps Losing Pressure: The 4 Causes Ranked by Frequency in London 2026

A boiler that keeps losing pressure is the single most common heating call we take in London between October and April. The pattern is almost always the same: the homeowner notices the pressure has dropped below 1 bar, tops the system back up to the green band at 1.2–1.5 bar using the filling loop, and three or four days later the pressure has dropped again. By the third top-up most callers are asking the same question — what is actually going on, and is this something you can fix in one visit?

The short answer is almost always yes. After fourteen years of attending pressure-loss faults across every London borough, on every major boiler brand sold in the UK — Worcester Bosch, Vaillant, Ideal, Baxi, Viessmann, Glow-worm, Potterton, Main and Alpha — we have catalogued the causes by frequency. Four faults cover 95% of the call-outs. The other 5% are exotic: a microleak in a buried underfloor heating manifold, a failed inhibitor-dosing pot venting under back-pressure, a primary heat exchanger crack on a much older boiler. The four below are the calls we run every week.

How a Sealed System Loses Pressure

Every domestic combi or system boiler installed in the UK since around 1998 runs as a sealed pressurised system. The principle is simple: the heating circuit is filled to roughly 1 bar at the boiler when cold; when the water heats up it expands; an expansion vessel inside the boiler casing absorbs the extra volume; the pressure rises modestly (to about 1.8–2.2 bar at full operating temperature); and when the system cools, the pressure drops back. There is no header tank in the loft, no constant water replenishment — the system is closed.

Closed systems lose pressure for exactly two reasons: water is leaving the system somewhere, or air is leaving the expansion vessel and forcing water out through the pressure relief valve as a downstream consequence. Both end with the same observable symptom — the gauge sits below 1 bar in the morning, the boiler refuses to fire from cold, and the filling loop tap gets opened again.

The 5-Minute Diagnosis Before You Call

You can rule out half of the possibilities yourself in five minutes before the engineer arrives. Do these four checks in order:

1. Step outside and look at the pressure relief valve discharge pipe. It terminates at low level on an external wall, usually a brass or copper pipe pointing down with a downturned elbow, 200–400 mm above ground. Look for water dripping actively, a limescale stain down the brickwork, or moss growth on the wall directly below. If any of those are present, the PRV has been discharging — which makes the diagnosis either Cause 1 or Cause 2.
2. Watch the pressure gauge with the heating off, then turn it on and watch it again. Cold pressure should sit at 1.0–1.2 bar. When the heating comes on, pressure should rise gently to 1.5–2.0 bar over 10–15 minutes. If it spikes above 2.5 bar within a few minutes, the expansion vessel is failed (Cause 1).
3. Walk every radiator and feel the unions, valves and towel-rail joints. Run a tissue around each compression joint and lockshield. Any damp spot, even a faint one, is a leak (Cause 3).
4. Open the airing cupboard, the boiler cupboard and any visible pipework. Look for water staining on the floor, on the underside of any joint, or — in older installs — a chalky white deposit on a soldered joint that has been weeping over months.

The five-minute check tells the engineer where to start. It also tells us whether to bring a replacement expansion vessel, a PRV, leak-detection kit or a torch and tissue paper on the first visit.

Cause 1: Failed or Waterlogged Expansion Vessel (40%)

The expansion vessel is a sealed steel container inside the boiler casing, typically the size of a 1-litre paint tin, with a rubber diaphragm splitting it in half. One side holds compressed air at a factory pre-charge — usually 0.75 to 1.0 bar. The other side connects to the heating water. When the system heats up and water expands, the diaphragm flexes against the air, compressing it and absorbing the expansion. When the system cools, the diaphragm relaxes back.

The air side leaks slowly over time — typically 8–12 years on a domestic boiler in London, sometimes longer on Vaillant ecoTEC and Viessmann Vitodens, sometimes shorter on the older Worcester Greenstar 24Ri and the early Ideal Logic ranges. When the air charge drops below about 0.4 bar, the diaphragm has no room to flex; system water heats and expands into a vessel that cannot absorb it; system pressure climbs sharply to 3 bar; the pressure relief valve opens to dump the excess; water discharges outside; the system cools; the pressure now sits well below 1 bar.

The diagnosis on site takes about 15 minutes. The fix is one of two options: re-pressurise the vessel through its Schrader valve at the back or side of the boiler (a temporary measure that buys 3–9 months on a vessel with marginal air retention), or replace the vessel entirely. On most modern domestic boilers the replacement takes 90–120 minutes including drain-down, isolation, swap and re-fill. Parts cost £80–£180 depending on brand; total visit cost £180–£320.

Cause 2: Leaking Pressure Relief Valve (30%)

The pressure relief valve is a simple spring-loaded brass valve that opens at 3 bar to dump water through the discharge pipe to the outside wall. It is designed to be a last-resort safety device — most domestic boilers fire and run for a decade without ever opening their PRV. When the PRV starts to weep continuously, it is for one of two reasons: the upstream expansion vessel has failed and the PRV is doing its job (Cause 1 above), or the PRV's brass seat has scaled, scored or pitted enough that it no longer seals at rest. Either way, the visible symptom is the same: the discharge pipe outside the wall is wet, dripping or limescale-streaked.

If the expansion vessel charge tests good, the diagnosis is a failed PRV itself. The replacement on most boilers takes 30–60 minutes, parts £40–£90, total visit cost £140–£240. The catch: a PRV that has been weeping for months has often dropped enough water from the system that radiators and the heat exchanger have started corroding, which means an inhibitor recharge belongs in the same visit. The full visit with a system flush and inhibitor dose runs £240–£380.

Cause 3: Hidden Leak in Radiator Union, Towel-Rail or Buried Pipework (20%)

The third most common cause is the one homeowners most fear — a leak somewhere they cannot see. In practice the leaks we find break into three sub-categories. The easiest is a weeping radiator union or towel-rail compression joint, which is a visual find on the walk-round above and a £40–£90 fix. The middle ground is a weeping airing-cupboard joint, an unvented-cylinder relief discharge that is vapourising into the cylinder cupboard floor, or a coupling on the boiler's primary flow that is producing a microleak masked by case insulation. The hardest are buried-pipe leaks under a solid floor, behind a tiled bathroom wall, or inside a chimney chase.

The London-specific factor here is age. Victorian terraces in zones 1 and 2 — Hackney, Tower Hamlets, Lambeth, Southwark, Camden, Islington — still run a lot of original 1950s and 1960s copper, which has lasted 60+ years but is now starting to fail at the soldered fittings under floors and in chases. 1930s semis in the outer boroughs — Ealing, Hounslow, Bromley, Croydon, Enfield, Barnet — have a similar issue with the 1970s-era refits that replaced the original gravity system. On both, the failing fitting is often inside a wall or a floor void where there is no visible water until a ceiling stain appears below.

Diagnosis costs scale with access: £80–£200 for a confirmed visible-joint repair; £180–£280 for an acoustic leak detection survey that pinpoints a buried leak without lifting the floor; £350–£600 for a buried-pipe access and repair, more if the floor is a herringbone parquet or a tiled wet-room. We always quote the diagnosis cost on the phone before dispatch, separate from the repair cost — the survey often costs less than half the day's mistaken repair.

Cause 4: Stuck Open Auto Air Vent (10%)

The least common but most insidious of the four. The auto air vent (AAV) is a small brass cylinder, usually mounted on the boiler pump head, that bleeds air automatically as the system runs. Inside is a small float on a needle valve. When air is present, the float drops and the valve opens; when water reaches the chamber, the float rises and the valve closes. When the float fails — usually because debris or limescale has worn the seat — the valve stays open under water, and the AAV bleeds a continuous fine mist of system water along with the air.

The give-away symptom is the AAV cap area: water staining, limescale streaks, or a slow drip when the boiler is running. The fix is a 20-minute swap for a replacement AAV, parts £20–£40, total visit cost £120–£180. The diagnostic is sometimes done at the same visit as a Cause 2 PRV swap because the two failures cluster — both are wear-out faults on a boiler that has done 8–12 years of service.

The Topping-Up Trap

Every London plumber we know has stood in a kitchen watching a homeowner reach for the filling loop tap for the fifteenth time in a fortnight. The instinct is correct in the short term — the boiler will not fire below 0.5 bar so you need to top up to keep heat in the property. The problem is what daily topping-up does over a heating season. Every refill introduces fresh oxygenated mains water into a system that is designed to run anaerobic. The oxygen accelerates corrosion at the radiator bottoms, where sludge starts to form within weeks of consistent topping-up. The sludge then settles in the lowest-flow areas of the heat exchanger and the pump volute, both of which are expensive to replace.

The rule of thumb: if you have topped the system up more than twice in a month, book the diagnosis call. The £140–£320 to fix the underlying fault is far cheaper than the £600–£900 replacement heat exchanger or the £300–£500 powerflush that follows a year of compounded sludge. The how-to-repressurise-boiler guide covers the safe top-up procedure for the situation between booking and fix — it is not a long-term plan.

What is Different About London Boilers

Three things make the London install base behave differently from the rest of the UK. First, the water is hard — Thames Water's east-London supply runs at 270–300 mg/L calcium carbonate equivalent, harder than almost anywhere else in England and Wales. Hard water deposits scale on the inside of the boiler heat exchanger, on the PRV seat, on the AAV needle valve, and on the diaphragm in the expansion vessel. Every one of the four causes above is statistically more frequent in London than the national average — because the contributing fault mechanism is the same: limescale.

Second, the property stock is old. Roughly 38% of London's housing stock pre-dates 1939 — Victorian, Edwardian, interwar — and the heating systems installed into those properties were rarely original. Most were retrofitted in the 1970s and again in the late 1990s or 2000s, which means the actual installed boiler and pipework dates vary widely within a single block of flats or a single terrace. We have attended pressure-loss calls in Hackney where the boiler is 4 years old and the pipework feeding it is 50 — and the leak was in a fitting older than the engineer.

Third, the install density inside the Congestion Charge zone means that out-of-hours response times for emergencies are longer than the outer boroughs, but the access to specialist parts is faster — most central London plumbers can collect a Worcester or Vaillant expansion vessel from a Plumbase or Wolseley within 30 minutes during business hours. If your boiler dies on a Friday afternoon inside the C-Charge zone, the diagnosis-to-repair gap is usually shorter than it would be in Bexley or Havering.

When to Call a Gas Safe Engineer

Call the same day for any of these:

• Pressure dropping below 0.5 bar more than once a week.
• The pressure relief valve discharge pipe outside is actively dripping or running.
• The boiler is locking out on low pressure faults (fault codes EA, F22, F75 on common UK boilers).
• Visible water inside the property at any radiator union, towel-rail joint, airing cupboard or under-floor inspection point.
• Boiler short-cycling — switching itself on and off every few minutes — which often indicates sludge or air in the heat exchanger as a downstream consequence of repeated topping-up.

Call within a week for slow drift below 1 bar over 2–4 weeks. Call within a month for any system that has been topped up more than twice in 60 days.

Under the Gas Safety (Installation and Use) Regulations 1998, any work that requires opening the boiler casing has to be performed by a Gas Safe registered engineer. That includes all four of the causes above except the visible-radiator-union sub-case of Cause 3. Search the engineer's name on gassaferegister.co.uk before the visit — the registration number on the ID card and the invoice is the legal proof.

If you are in London and a pressure-loss fault is the symptom, our boiler repair line runs 24/7 and the diagnosis call costs less than the fourth top-up of your fortnight. The cause is one of the four above. The fix is usually inside a single visit.