Most Likely Causes (Ranked)
Boiler pressure should be 12-20 PSI when cold and 15-30 PSI when hot. If the pressure relief valve (PRV) is discharging, the system pressure is too high. Common causes: failed expansion tank (the bladder ruptures and the tank fills with water, providing no expansion room), failed auto-fill valve (continuously adds water), or closed system without adequate expansion.
Cast iron and steel heat exchangers corrode from the water side over 15-30 years, developing pinhole leaks. Oxygen entering the system (through failed air eliminators or uncapped radiators) accelerates corrosion dramatically. Once the heat exchanger leaks, the boiler typically needs replacement — the heat exchanger is the most expensive component.
The circulator pump moves heated water through the system. The pump seal where the shaft enters the pump body wears over 10-15 years. A failing seal weeps water that drips from the pump housing. Some pumps have a weep hole — check if it's actively dripping.
Threaded connections throughout the boiler system loosen from thermal cycling. Every heat cycle expands and contracts the pipes and fittings. Zone valve connections, supply/return manifold joints, and radiator valve connections are the most common leak points.
The expansion tank absorbs the pressure increase when water is heated (water expands ~4% from 60°F to 180°F). When the tank's internal bladder fails, the tank fills with water and can no longer absorb expansion. System pressure spikes during heating, triggering the PRV. Old-style expansion tanks (large, unpressurized) in the attic also develop leaks.
How to Fix It
Replace the Expansion Tank
$30-80 DIY / $150-300 Pro- 1Check the tank: unscrew the air valve cap and press the pin. If water comes out instead of air, the bladder has failed.
- 2Turn off the boiler and let the system cool. Close the fill valve.
- 3Drain a few gallons from the system (attach a hose to the boiler drain).
- 4Unscrew the old tank (it's threaded onto a tee fitting). Install the new tank.
- 5Pre-charge the new tank to match your system's cold pressure (usually 12-15 PSI) using a bicycle pump.
- 6Refill the system, bleed air from radiators, and test.
Replace the Circulator Pump
$80-200 DIY / $250-500 Pro- 1Turn off the boiler and power. Close isolation valves on either side of the pump (if present).
- 2Place a bucket under the pump. Disconnect the flanges (4 bolts typically).
- 3Install the new pump in the same orientation (note the flow arrow). Use new flange gaskets.
- 4Open isolation valves, bleed air, and test.
Tighten or Reseal Connections
$0-10 DIY- 1Identify the leaking connection. If it's a threaded joint, try tightening a quarter turn.
- 2If it still leaks, drain the system below the connection, disassemble, clean threads, and apply new Teflon tape or pipe dope.
- 3For compression fittings, tighten a quarter turn at a time.
When to Call a Professional
Boiler systems operate at high temperatures (180°F+) and pressure. Allow the system to cool before any repair.
If you smell gas near the boiler, leave the house immediately and call your gas utility. Don't operate any switches.
Never cap or plug the pressure relief valve — it prevents catastrophic boiler explosion.
Low water in a boiler can crack the heat exchanger when the auto-fill adds cold water to a hot cast iron section. Maintain proper water level.
How to Prevent It
Check boiler pressure monthly (should be 12-15 PSI cold). If it's climbing, check the expansion tank and auto-fill valve.
Bleed radiators annually to remove trapped air (air accelerates corrosion).
Have the boiler serviced annually by a licensed technician ($150-250).
Replace the expansion tank every 10-15 years proactively.
Frequently Asked Questions
Cast iron boilers: 20-30+ years. Steel boilers: 15-20 years. High-efficiency condensing boilers: 12-18 years. Proper water treatment and annual maintenance extend life significantly.
Most likely: failed expansion tank (bladder ruptured) or a stuck auto-fill valve that continuously adds water. Check the expansion tank first — press the air valve. Water means the bladder is failed.
Repair if: expansion tank, pump, or connection issue. Replace if: heat exchanger is leaking or the boiler is 20+ years old (efficiency improvements pay for the upgrade in 5-8 years).
Standard efficiency: $3,000-5,000 installed. High-efficiency condensing: $5,000-10,000 installed. The efficiency savings ($200-500/year) make high-efficiency worthwhile for most homes.