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How to Install a Combination Boiler and Hot Water Heater

Replace two devices, your water heater and boiler, with a single combi boiler unit that’s more efficient and compact.

In this video, This Old House plumbing and heating expert Richard Trethewey replaces a water heater and boiler with a single unit that’s much more efficient.

A combination boiler and hot water heater, also known as a combi boiler, is a heating system that provides both heat and hot water to a small to medium size house. It brings together the functionality of boiler and a hot water heater into one compact unit that’s much more efficient. In this video, plumbing and heating expert Richard Trethewey shares how it works and how to install one to replace your existing system.

Can You Install a Combi Boiler Yourself?

For safety reasons, it’s best to bring a professional heating contractor in to help you install your new combination boiler. The process requires running new gas, electrical, and plumbing, along with drilling holes through the house wall for vents, which requires precision and technical expertise.

Combi Boiler Installation in 10 Steps:

  1. Mount a plywood panel to the basement wall to serve as a mounting panel for the new combination boiler.
  2. Run new gas, electrical, and plumbing lines to the plywood panel.
  3. Screw the metal mounting brackets to the plywood panel, then hang the combination heater onto the brackets.
  4. Drill two 4-inch-diameter holes through the house wall for the exhaust vent and air intake vent. Be sure the air intake is above the snow line, and that the exhaust vent is 12 inches from a window and 12 inches from the air intake.
  5. Use PVC pipe and fittings to create the exhaust and air intake vents. Run the piping from the boiler through the exterior wall.
  6. Connect hot and cold water lines to the combination boiler to deliver water to the heating system and the domestic water fixtures.
  7. Connect the gas line and new electrical cable to the combination boiler.
  8. Mount an outdoor temperature sensor onto the north side of the house to help regulate the production of hot water.
  9. Run a PVC condensate line with limestone neutralizer from the combination heater to a nearby utility sink drain or floor drain. Use a condensate pump to drain away the excess condensation.
  10. Turn on the combination boiler, and it should immediately start making hot water.
  11. Remove and discard the old boiler and water heater.

Tools You’ll Need