How Much Does It Cost to Fix a Boiler That Keeps Losing Pressure?

The short answer

A boiler that loses pressure faster and faster is leaking. Somewhere.

The trick is finding where before you spend money. Most of the time it's one of three things: a failed expansion vessel, a passing pressure relief valve, or a weeping radiator valve or union.

If a pro has to trace a real leak and repair it, budget $200–$900.

The gap between those numbers is big because the diagnosis is where the work lives. A visible dripping valve is a quick swap. A pinhole under the kitchen floor is a whole afternoon.

What it costs in 2026

Let's break the numbers down so you know where your money goes.

That covers a decent filling loop or top-up kit, a radiator bleed key set, a bottle of leak sealer if you want to gamble on a pinhole, and maybe a new gauge if yours is stuck. Most of this is one-time tool cost you keep.

Pro side, $200–$900. The call-out and diagnostic time is usually $150 to $250 of that. The rest depends on the fault.

A relief valve swap: the part is $30 to $60, but with draining down and refitting you're looking at $250 to $400 total.

An expansion vessel replacement: $400 to $750 depending on whether it's internal to a combi (more labor) or an external add-on vessel (cheaper to fit).

A radiator valve replacement: $180 to $350 per valve including labor.

A hidden pinhole leak: this is the wild card. Trace plus repair can run $500 to $1,200 if they have to lift flooring, which is why the top of the range exists.

• Professional repair typically runs $200–$900. This is not a DIY job, so plan on hiring a pro.
• Relief valve replacement (pro): $250 to $400
• Expansion vessel (pro): $400 to $750
• Radiator valve (pro, per valve): $180 to $350
• Hidden pipe leak trace and repair (pro): $500 to $1,200

How we built these numbers

The cost ranges reflect 2026 US pricing for parts and licensed HVAC or plumbing labor, blended across regions. City rates run higher, rural lower.

This is a pro visit, so the pricing reflects labor plus a service call, not a parts-only DIY estimate. Your final number depends on access, the fix needed, and local rates.

The likelihood order comes from what field techs report finding most often on sealed boiler systems: expansion vessel and relief valve faults lead, mechanical weeps follow, hidden pipe leaks trail because they're less common but pricier.

Your actual bill depends on where the water's going and how hard it is to reach. Use these as a planning range, not a quote.

What's actually causing it

Worsening is the key word. A pressure loss that stays steady is often just settling air after recent work. One that speeds up is a leak growing.

The single most common cause techs find is a waterlogged expansion vessel. The vessel has a rubber diaphragm with air on one side. When the air charge leaks away, water has nowhere to expand into, so pressure spikes when the boiler's hot and drops when it cools.

You'll see this as a gauge that reads 2.5 bar or higher when hot, then sits at 0.5 or lower cold. That swing is the tell.

Second most common is a passing relief valve. Once grit or scale gets on the valve seat, it weeps water out the discharge pipe even at normal pressure. Invisible unless you check outside.

Then there are the honest mechanical leaks: radiator valve gland nuts, tail unions, the pump gasket, the auto air vent. These weep slowly and often evaporate off warm metal so you never see a puddle.

The nasty one is a pinhole in buried pipework. No visible source, steady loss, and it only gets worse. That needs a pro with a pressure test to find.

What the pro will do

There is no do-it-yourself path we recommend for this repair. The safe and lasting fix comes from a licensed pro who can diagnose the true cause and complete the work correctly.

Booking a qualified tech protects your safety, your warranty, and your home. Call a licensed professional to handle it end to end.

Can this wait?

For a low-urgency slow drop, yes, you have time. As long as you keep the pressure above about 1.0 bar cold, the boiler will keep running.

But waiting isn't free. Every top-up you do adds fresh oxygenated water, and oxygen is what corrodes the inside of the system. Constant refilling quietly rots the heat exchanger and radiators from the inside.

If you're topping up more than once a month, and the gap between top-ups is shrinking, that's a leak on a growth curve. Get it looked at before winter loads it up.

The one time you don't wait: pressure hitting zero, water stains spreading, or the boiler locking out and refusing to fire. That's a same-week call.

When these show up

Pressure faults love the first cold snap. The boiler sits idle all summer, then fires hard in October, and any weak vessel or scaled valve gets exposed under load.

That's also the worst time to need a pro. Everyone's calling at once and the wait stretches to a week or more.

If your pressure was creeping down through the summer, book the fix in early fall. You'll get a faster appointment and you won't be sitting in a cold house waiting for a slot in December.

Safety basics

Boilers hold hot water under pressure and, if gas-fired, burn fuel. Respect both.

Kill the power before you remove any cover, even if you're only looking. And if you ever smell gas, leave first and call the emergency line, don't investigate.

Don't overfill. Past 1.5 bar cold you're just pushing water out the relief valve and wasting it. Aim for 1.2 bar cold and stop.

If pressure drops to zero, switch the boiler off rather than let it try to fire dry. A dry run cooks the pump.