Hot-Water Boiler Repair

Understanding how a hot-water boiler works and maintaining one can make it trouble free longer. This Fix-It Guide on hot-water boiler repair tells how a hot-water boiler works, what often goes wrong, how to identify the hot-water boiler problem, and what parts and tools you will need to fix it. It then gives simple step-by-step instructions for how to troubleshoot an expansion tank, how to flush a hot-water boiler of rusty water, and how to bleed a radiator. You may need to refer to switch repair, fuse repair, and circuit breaker repair. Also see the Steam Boiler Fix-It Guide for steam boiler repair.

How Does a Hot-Water Boiler Work?

Hot Water Boiler Repair

Components of a typical hot water boiler.

A hot-water heating system consists of a boiler to heat (but not boil) water, a network of pipes that distributes the water, and radiators that heat various rooms. A hot-water heating system uses the same type of boiler that a steam heating system uses. Controls include a combination gauge (or altitude gauge) that lets you check water temperature and pressure and lets you know when the boiler needs water or is malfunctioning. In some systems, a pressure-reducing valve takes care of the water automatically. The typical system also has an expansion tank that must be properly charged with air to prevent the water from boiling. Newer systems usually locate the expansion tank on the basement ceiling near the boiler; they also include a purge valve to release water and let in air as needed. Older systems sometimes have the expansion tank in the attic and include a gauge glass similar to that on a steam boiler.

What Can Go Wrong with a Hot-Water Boiler?

Because they have few mechanical parts, hot-water heating systems usually perform reliably for many years. The most common problems are with the expansion tank or a circulator rather than the boiler. Here are some symptoms of problems: A hot-water system may produce no heat or poor heat and leaks can occur. Some radiators may not heat while others do. Pipes may make a clanging noise.

Fix-It Tip

Place a pan of water on top of a radiator to add humidity to the air during the winter, if needed.

How Can I Identify a Hot-Water Boiler Problem?

  • If the unit produces no heat, raise the thermostat, check switches, fuses, circuit breakers, and the water level. Also check the burner’s safety controls (see below).
  • If the unit doesn’t produce enough heat, check the combination gauge, then the expansion tank (see below). Next, flush the boiler (see below). If water leaks from a heating system pipe, try repairing it yourself.
  • If only some radiators in the system heat up, bleed air from the cool units (see below). If you suspect that the circulator requires repair, call an experienced service person.
  • If the pipes suddenly start clanging, the circulator may need professional service or replacement.
  • If there is a chronic banging noise, check the slope of all return lines; they must slope toward the boiler to work correctly.
  • If a single radiator warms only slightly, but evenly, water may be trapped inside. Make sure the radiator slopes toward the return; if it does not, insert a wooden shim under the end opposite the return.

Fix-It Tip

Water pressure in a hot-water boiler is automatically maintained by a pressure-reducing valve. Periodically check the combination gauge and call an experienced service person if the valve needs repair or replacement. If your system has no pressure-reducing valve, you can manually feed the boiler by opening the feed water valve and closing it again when pressure reaches 12 pounds per square inch. High water consumption is caused by a leak in the supply or return piping or in the boiler itself.

What I Need for Hot-Water Boiler Repair?

Most components of a hot-water boiler system are available through larger plumbing supply houses. Check your local telephone book. For basic tests and repairs you’ll need these tools:

  • Screwdrivers
  • Pipe wrenches

Fix-It Tip

Here’s how to read a combination gauge. The moving pointer shows actual pressure. The fixed pointer indicates the minimum pressure. If the moving pointer drops below the minimum, the system needs water. The lower temperature gauge shows water temperature. Maximum boiler water temperature is set by moving a pointer along the sliding scale of an aquastat. Don’t try to adjust the aquastat.

What Are the Steps to Hot-Water Boiler Repair?

Troubleshoot a hot-water boiler expansion tank:

  1. Check the pressure-relief valve. If water is spurting from it, there is too much water and not enough air in the tank.
  2. Touch the side of the tank; the bottom half should feel hotter than the top. If the top is nearly as hot or as hot as the bottom, the tank is filled with water and requires bleeding. Once the system is cool, attach a hose to the tank’s purge valve and drain 5 to 10 gallons of water from the system.
  3. Return all valves to normal settings and start the boiler. Check the system’s pressure on the combination gauge. If it is not within normal operating range, call for service.

Flush a hot-water boiler of rusty water:

  1. Shut off power and open the drain cock and the air vents on the highest radiation units.
  2. If the boiler has a manual feed, open it.
  3. When the water runs clear, close the drain and vents and wait until pressure reaches 20 pounds per square inch (psi) of pressure.
  4. Bleed each radiator until the pressure reaches 20 psi, then drain off the water. If the pressure falls below 12 psi, add more water.

Bleed a hot-water boiler radiator:

  1. Open the vent with a screwdriver or special tool supplied with the radiator.
  2. When water (instead of air) comes out, close the vent.