How to Fix a Leaking Steam Boiler


A steam boiler heats a home by boiling water which rises through pipes as steam and escapes through radiators. As a boiler ages, it is vulnerable to rust and mineral build-up that can cause leaks. While a homeowner may be able to locate leaks, it is recommended to let a professional heating, ventilation and air conditioning (HVAC) technician do the repairs (or replacement if necessary), since special equipment is needed to fix the leak properly.

Things You'll Need

  • Manuals
  • Water meter
  • Jackhammer
  • Welding equipment


  • Check the sight glass on the side of the boiler weekly. The ball inside should be at 1/2 to 3/4. Check the glass for sediment that may cause decay inside the boiler. Add new water if necessary by opening the valve below the sight glass. Then check it again by shutting off the valve and watching water come back into the chamber. If it rises quickly, it's fine; if it trickles in, call a professional.

  • Blow down the system monthly by draining and refilling it. Check your manuals, or a licensed HVAC technician can show you how to do this quick process for your particular boiler. It involves draining and refilling the system to protect it from dry-firing. Also, before refilling, test the low water cut-off switch, which is operated by a float inside the boiler. That float can become clogged with rust or mineral build-up and could fail if the water level does drop too low.

  • Dry-firing can ruin the boiler and even cause a fire if flammable or combustible items are stored near it. Don't try to refill the boiler until it is completely cooled. Know where the fuel valve and electric switch are located so that in case something goes wrong, you can shut them off and call a professional.

  • Should the boiler spring a leak, the low-water cut-off device should shut it off before it loses all the water inside and dry-fires. Then it's time to check it for leaks. Leaks can occur at any of the pipe connections or even along pipes throughout the system. Look for rusty stains or little piles of mineral build-up on the floor or under connections

  • Leaks can mean that part or all of the system needs to be replaced, which requires special equipment. If the leak is under the floor, you'll also need a jackhammer to access it. In the event of structural damage such as internal rust or dry-firing, the unit will need to be either welded or replaced entirely, depending on the extent of the damage. It's easier, cheaper and safer to get someone who has the expertise and equipment to do the job properly.

Tips & Warnings

  • Improperly repaired connections or structural welding may cause the system to leak carbon monoxide, a tasteless, odorless gas that can cause illness and even death.

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