Tank vs tankless water heaters: an honest comparison

How each one works

A tank water heater keeps a reservoir — typically 40 to 75 gallons — hot all the time, reheating whenever the temperature drops. When you open a tap you draw from that reservoir. When it is empty you wait for recovery.

A tankless unit heats water only as it flows. Open a tap and a burner or element fires, bringing the water to temperature on its way through. Nothing is stored, so nothing is kept hot for no reason.

Capacity vs flow rate — the distinction that matters

This is where expectations most often go wrong. A tank is limited by capacity: it can supply a lot at once, but only until it runs out. A tankless is limited by flow rate: it never runs out, but it can only heat so many gallons per minute to a given temperature rise.

So 'endless hot water' is true — and it does not mean 'unlimited simultaneous hot water.' Two showers and a dishwasher at once can exceed a unit's rated flow, and the result is water that arrives cooler than you asked for rather than water that stops.

The relevant question is not household size in the abstract; it is how much simultaneous demand you actually have, and what temperature rise your incoming water requires. Groundwater here is cold in February, and temperature rise is exactly what limits flow.

Where the cost difference really sits

The units themselves differ in price, but the larger gap in a retrofit is usually the supporting work. A gas tankless unit fires at a much higher input rate than a tank while it is running, which frequently means a larger gas line back to the meter. Venting is different — typically stainless or PVC rather than the existing flue. Many units also need an electrical circuit for controls.

None of this is exotic, and none of it is optional. It is simply the difference between the sticker and the installed cost, and it is the part worth having spelled out in the quote before you decide.

Maintenance, honestly

Tanks need the anode rod checked — it is a sacrificial component that corrodes so the tank does not — and benefit from periodic flushing of accumulated sediment.

Tankless units need descaling. Hard water leaves mineral scale on the heat exchanger, and scale directly reduces the flow rate the unit can heat. A tankless that is never descaled will gradually stop meeting the flow it was sized for, which reads to the homeowner as the unit going bad.

Neither is maintenance-free. Anyone who tells you otherwise is selling.