How to Install a Tankless Water Heater
Pluses include endless hot water and a compact design, but the installation means a lot of pipes in a small space.
Synopsis: Plumbing and heating contractor Tom Cardillo explains the process of installing an on-demand water heater, from sizing, to mounting and venting, to making all of the plumbing connections. The article includes the necessary wall-panel dimensions for mounting the heater, the different-diameter gas lines that may be needed depending on the heater’s size, and the length and diameter of the necessary vent and combustion-air piping. A sidebar includes a description of a condensate neutralizer and when it is needed to increase an on-demand water heater’s efficiency and to keep pipes safe from acidic water.
Promising endless hot water and taking up less space than a tank-type heater, a tankless water heater has a lot to like. Every year I install several for clients who are hoping for more hot water and savings on their energy bill. Unfortunately, switching from a tank-type water heater to an on-demand unit is not just a direct swap. You’ll need sufficient wall space for mounting the wall-hung heater, a different flue arrangement, and possibly a larger gas line. The installation means a lot of pipes in a small space, so it’s important to think through the layout, and work from the biggest pipes to the smallest.
How much heater do you need?
Before any installation, you have to size the water heater. My preference is to size the unit so there will be enough hot water, even if all of the hot-water fixtures and appliances are in use at the same time. To do this, I consider both the number of hot-water fixtures in the house and the temperature of the incoming water. Here in Rhode Island, we may have 40°F (or colder) incoming water during the winter. The Department of Energy suggests you should set your water heater at 120°F, so the temperature rise needed is 80° (40°+80°=120°).
The number of gallons per minute (gpm) that on-demand water heaters can produce goes down with greater temperature rise, so you have to get a larger heater in colder climates than you would in warmer areas. Many manufacturers have size calculators on their websites that make it easier to match the right model to your particular situation. The water heater in this installation is
a 19,000-Btu to 199,000-Btu model, which is normal for a house with three bathrooms.
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