First — figure out exactly where the water is coming from
Boilers don't "just leak." Water is coming from one specific component. Identifying which component is 90% of the diagnostic. Before calling, look at:
- Where on the boiler is the water? Top, side, bottom, base pan?
- Is it continuous or intermittent? Continuous suggests pressure-related (relief valve, PRV). Intermittent suggests cycle-related (heat exchanger, gasket).
- Is it clean water or rusty/discolored? Rusty = system water from inside (worse). Clean = could be condensate or fresh fill water.
- How much per day? A few drops vs. a cup vs. a steady drip vs. a puddle.
Snap a photo of the leak location and a sample of the water before you call. Saves diagnostic time, which sometimes saves you the service-call fee.
The 6 causes, by frequency
1. Failed expansion tank (35% of cases) — $340–$520 to fix
The single most common boiler-leak source we see. When the expansion tank's internal diaphragm fails, system pressure can't be absorbed and climbs past the relief valve setpoint (30 PSI). The relief valve discharges water as a safety measure — usually into a discharge tube near the boiler base.
How to identify: water at the base, near the relief valve discharge line. Often appears after the boiler runs for a while (when pressure peaks). System gauge reads above 25 PSI when hot.
The fix: replace the expansion tank. Routine. Same-visit. See our pressure troubleshooting guide for the full diagnostic.
2. Pressure-reducing valve (PRV) stuck open (20% of cases) — $380–$580
The PRV controls how much water fills the system. When it fails open, water continuously enters at full city pressure (60+ PSI). Pressure climbs, relief valve discharges, water under boiler.
How to identify: same symptoms as failed expansion tank, but pressure climbs even when isolation valve closed.
3. Pump flange or piping leak (15% of cases) — $180–$420
Cast iron pump flanges develop gasket failures around year 10–15. Pipe fittings — especially where copper meets steel — can corrode and weep over decades.
How to identify: visible water at a specific joint or fitting. Often white or greenish mineral residue at the leak point.
The fix: replace the gasket or fitting. Routine.
4. Section gasket failure (cast iron only, 10% of cases) — $480–$780
Cast iron boilers are sectional — multiple cast iron sections bolted together with gaskets between them. After 20–30 years, the gaskets deteriorate and seep water.
How to identify: water appearing between sections of an older cast iron boiler. Usually slow seepage, not active drip.
The fix: replace the gasket pack. 3–4 hour job. Worth it on a 20-year-old CGa or Burnham; replacement math may win on a 30-year-old.
5. Failed low-water cutoff or sight glass (8% of cases) — $220–$420
Older boilers have a sight glass (visible water-level indicator). If the gasket fails, water weeps from the sight glass fittings.
The fix: replace the gasket or glass assembly.
6. Heat exchanger leak (5% of cases) — $1,400–$3,800 or replace unit
The bad one. If the heat exchanger itself is leaking, you're looking at a major repair or replacement. Cast iron HX cracks (rare, terminal). Aluminum HX corrosion (some Ultra and older mod-con models). Stainless HX seam failures (rare).
How to identify: water from inside the boiler cabinet (not from external piping). Sometimes accompanied by condensation, low system pressure that keeps dropping, or unexplained boiler shutdowns.
The fix: heat exchanger replacement (if under warranty, parts are covered, ~$1,400 labor only). Out of warranty, the math usually favors a full unit replacement — see our replacement decision guide.
What to do right now if you see water
- Turn the boiler off at the emergency switch or breaker. Heat will stop within minutes; you can run it intermittently if needed for warmth.
- Close the water supply to the boiler (typically a small valve on the fill line).
- Place towels or a shallow pan under the leak to protect flooring while waiting for service.
- Snap photos of leak location and any visible system gauges.
- Call us. Most leaks are same-week service. Major heat-exchanger leaks may need an urgent visit.