Pressure Relief Valve Dripping in London: Causes and What to Do

What Is a Pressure Relief Valve and Why Does It Drip?

If you have noticed water trickling from a copper pipe on the side of your boiler — usually running down the wall or draining outside — you are most likely looking at a pressure relief valve dripping. In London, this is one of the most common boiler complaints we attend, particularly in Victorian and Edwardian terraces across Islington, Hackney, and Southwark, where ageing heating systems work hard against the capital's notoriously hard water. A PRV drip is never normal, and it is never something to put off.

The pressure relief valve (also called a safety relief valve) is a last-resort safety device fitted to your sealed central heating system. Its job is to open automatically if system pressure climbs above 3 bar, releasing water to bring the pressure back down and protect your boiler, pipework, and radiators from damage. Under normal operation the valve should stay firmly shut. When it drips — even occasionally — it means pressure is reaching or exceeding that threshold repeatedly, which points to an underlying fault that needs diagnosing.

The pipe you see dripping is the PRV discharge pipe. It is designed to dump water safely, so a small trickle might look harmless. But the act of repeatedly opening and closing causes wear on the valve seat, and eventually the valve will either fail open (giving you a constant leak) or fail shut (removing your only overpressure protection). Neither outcome is acceptable.

Why Is Your Pressure Relief Valve Dripping?

There are three main causes of a pressure relief valve dripping in London homes. A Gas Safe engineer will work through them in roughly this order.

1. Failed or waterlogged expansion vessel

This is the cause in the majority of cases. Your boiler has an expansion vessel — a small pressurised tank, usually inside the boiler casing — that absorbs the increase in water volume as your system heats up. Inside is a rubber diaphragm separating a water side from an air-charged side. When the diaphragm perforates or the air charge bleeds away over time, the vessel can no longer absorb expansion. Pressure spikes every time the boiler fires, and the PRV opens to compensate. London's hard water accelerates diaphragm deterioration, making this especially common in properties supplied by Thames Water across south and east London. You can read more about the symptoms in our guide to expansion vessel problems.

2. Filling loop left open or stuck open

The filling loop is the connection used to top up your system when pressure drops. If it is left partially open — easy to do by mistake after repressurising your boiler — mains cold water continuously trickles in, driving pressure above 3 bar. The PRV then opens to release the excess. Check your filling loop: it should be fully closed (both valves) when not in use.

3. Faulty PRV

Less commonly, the valve itself is at fault. PRV seals degrade over time, particularly in older systems or after a period of repeated opening. A worn seat can weep at normal operating pressures of 1.5–2 bar, where the valve should be firmly shut. In this case, the valve needs replacing — but an engineer will always verify system pressure is not the root cause before fitting a new one, otherwise the new valve will develop the same problem.

How Urgent Is It and What Does Fixing It Cost?

A pressure relief valve dripping does not require a 2 am emergency callout, but it should be dealt with within a day or two — not weeks. Left alone, the situation tends to escalate. Each time the PRV opens and closes, it deposits mineral scale on the valve seat (London's hard water makes this worse than almost anywhere else in the UK). Eventually the valve cannot fully reseat and the drip becomes a trickle, then a steady flow. At that point you have a water damage risk as well as a boiler fault.

There is also the issue of your boiler's behaviour. Many modern combi and system boilers — Worcester Bosch, Vaillant, Ideal, and Baxi being the most common in London properties — will lock out on a high-pressure error code once pressure climbs high enough. If your boiler is repeatedly turning itself off, a chronic PRV drip and the overpressure behind it may be the reason.

Costs in London are straightforward for this type of repair. An expansion vessel replacement — the most common fix — runs £200–£350 fitted, including a new vessel and labour. Most engineers carry standard vessel sizes in their van, so it is usually a same-visit repair. A PRV replacement alone is cheaper, typically £80–£150. If both need doing, a good engineer will combine them for less than the sum of two separate visits. Our team waives the £60 call-out fee on jobs that run over two hours, which applies to most expansion vessel replacements.

What Happens at the Engineer Visit

When a Gas Safe engineer attends a pressure relief valve dripping call, the diagnostic sequence is quick. They will check your system pressure cold and hot, inspect the filling loop, test the PRV discharge pipe for continuous flow or intermittent release, and assess the expansion vessel using a Schrader valve gauge. In most London properties this takes 20–30 minutes and gives a clear picture of the fault.

If the expansion vessel is the culprit, they will drain down the relevant section of the system, remove the old vessel, fit a new pre-charged replacement, and repressurise the system to the correct charge — typically 0.5–1 bar below your cold fill pressure. They will then run the boiler through a heat cycle and recheck pressure. If it stabilises in the 1.5–2 bar range at operating temperature, the job is done.

It is also worth knowing that a dripping PRV is sometimes the first visible symptom of a system that has been slowly losing and regaining pressure for months. If your pressure gauge has been fluctuating and you have been topping it up more than once every few months, mention this when you book — the engineer will check for micro-leaks in the system as well as the expansion vessel.

Properties in areas like Hammersmith, Richmond, and Twickenham with older floor-standing boilers or gravity-fed systems may also have external expansion vessels fitted in the loft or airing cupboard. The same faults apply, but the vessel is easier to reach and replace. If you are not sure where yours is, an engineer will locate it quickly during the visit.

Frequently Asked Questions