Tank or tankless: which water heater replacement?

The two machines solve different problems

A tank water heater stores hot water and keeps it hot. Its limits are capacity and recovery: you can draw a fixed number of gallons before the temperature falls, and then you wait. A tankless unit heats water as it flows and never runs out, but it is limited by flow rate. It can supply a certain number of gallons per minute at a certain temperature rise, and beyond that the water gets cooler rather than running out.

That difference is the whole decision. If your complaint is that the third shower is cold, tankless solves it directly. If your complaint is that the unit is old, tankless solves nothing you were asking about.

Temperature rise is the detail people miss. A unit rated for a generous flow in a warm climate delivers considerably less in a cold one, because incoming groundwater in winter can be twenty or thirty degrees colder and the unit has to do more work per gallon. Sizing has to be done against your winter inlet temperature, not a headline number.

Why the installation, not the equipment, drives the cost

A gas tankless unit fires at a far higher input than a tank of the same household size, because it has to make the heat on demand rather than over an hour. That usually means the existing gas branch is too small and has to be upsized back to a point where the supply can carry it, and occasionally that the meter itself is undersized.

It also cannot use the old chimney. Condensing tankless units produce acidic condensate and cool exhaust, so they vent through dedicated stainless or polypropylene pipe, usually out a sidewall, and they need a condensate drain with neutralisation in many jurisdictions. Add isolation valves for descaling, and often a small electrical circuit for the controls.

None of that is exotic. All of it is labor and materials that a tank changeout does not involve, and it is why two quotes for the same equipment can differ by a wide margin depending on what the house makes necessary.

Running cost, honestly

Tankless units avoid standby loss, the energy a tank spends keeping water hot that nobody is using. That is a real saving and it is also a modest one in absolute terms, especially against a modern well-insulated tank. Federal efficiency guidance puts the improvement in a range that pays back over many years at typical usage, and longer in a household that uses little hot water.

The genuine advantages that do not show up on a bill are the ones worth weighing: no run-out, a longer service life if the unit is descaled on schedule, and reclaiming the floor area a tank occupied. The genuine disadvantages are the descaling requirement, which is not optional in hard water, a short delay and a slug of cool water at the start of a draw, and poor behaviour with very low flow fixtures that never trigger the burner.

The cases where the answer is easy

Replace with a tank when the old one simply aged out, the household never runs short, and nothing about the space has changed. This is most replacements, and it is not a compromise.

Go tankless when you routinely run out, when the heater is in a closet or under stairs where the floor area is worth reclaiming, when you are already renovating and the gas and vent work is cheap to fold in, or when the unit sits somewhere a tank failure would be catastrophic and the flood risk of eighty stored gallons is the thing you want gone.

If the tank you are replacing failed suddenly, read our page on a water heater leaking from the bottom first. A conversion is not an emergency job, and deciding it under water pressure at nine in the evening is how people buy the wrong machine.

What this site publishes

The labor figure above is computed from published wage data times the stated hours and is labor only, with no equipment, gas piping materials, vent, or permit fees. We publish no equipment prices and reproduce no third-party cost tables. Contractors on record appear alphabetically and are never ordered by any commercial relationship.