Tankless Water Heater Installation

What makes it different from a tank swap

A tank heater warms water slowly and stores it. A tankless heater heats water only while a tap is open, so it has to make all that heat at once. That's why the supply side matters so much.

Gas units are rated roughly 140,000 to 199,000 BTU, depending on how much hot water the house needs. Many older homes have a gas line sized for a tank, and it can't feed that. HomeGuide prices a larger gas supply line at $350 to $2,000.

Whole-house electric tankless units run on 240 volts and range from 14 to 32 kW. A.O. Smith notes your electrician may need to upgrade the panel or add new circuits. Plenty of homes with a modest panel can't support a whole-house electric model, so get the electrician's opinion before you buy one.

Venting: condensing vs. non-condensing

Venting is the part people underestimate.

• Non-condensing units send hot exhaust straight outside, which wastes some of the heat. They need venting rated for that hot exhaust, as the manual specifies.
• Condensing units pull extra heat out of the exhaust and use 2-inch venting. A.O. Smith says that can make installation easier. This is where you see PVC vent kits. Condensing units also produce acidic condensate, so plan a drain line for it, following the manual.

Never mix and match vent pipe or vent kits between brands. The installation manual lists the exact vent materials, lengths and terminations allowed for that model, and the inspector will check against it. HomeGuide prices vent replacement at $100 to $600, and venting a new location for the first time at $500 to $1,500.

Picking the spot

Tankless units mount on a wall, so they fit in a closet or on an exterior wall. A.O. Smith says to choose a location near both the water inlet and the gas or electrical connection. Every foot of new gas line, vent run or circuit costs money, so the old tank's spot is often the cheapest place even if it isn't the prettiest.

Outdoor models skip the indoor venting entirely, which is a big savings where the climate allows. Check freeze protection in the manual before you go that route.

Installation steps (the overview)

This is the sequence a pro follows. It's here so you can follow the quote and ask good questions, not as a DIY script.

HomeGuide says labor typically runs 3 to 10 hours, and a brand-new install can take up to 10 hours.

Installation cost at a glance

For the full price picture, see tankless water heater cost.

Installation kits and recirculation

An installation kit usually means the isolation valves with purge ports, pressure relief valve and fittings. It's the single best add-on, because it makes yearly descaling a half-hour job instead of a plumbing project.

A recirculation pump cuts the wait for hot water at far taps. Some tankless models have recirculation built in and some need an external pump wired to the unit. Check compatibility in the manual before buying one. Our recirculation pump guide covers the options.

Mistakes to avoid

• Reusing the tank's gas line without checking its size. An undersized line starves the burner at full fire, and you get lukewarm water or error codes.
• Using the wrong vent material. It fails inspection at best and leaks exhaust at worst.
• Skipping the isolation valves. You'll pay for it at the first descaling.
• Ignoring hard water. A.O. Smith says gas tankless heaters build scale in the heat exchanger if the water isn't treated. If you're on hard water, read about hard water and plan for a softener or regular descaling.
• Buying an electric whole-house model before checking the panel.

Is it a DIY job?

For most people, no. Gas piping, venting and 240-volt circuits all carry real risk and usually need a permit. Where you can save is in the prep: clear the area, decide on the location, and get the electrician or gas fitter to quote the supply side before you pick a model.