Water heating

There is one equation behind everything in this section, and once you have it, most tankless marketing stops working on you.

GPM = usable BTU ÷ (temperature rise × 8.34 × 60)

A BTU raises one pound of water by one degree Fahrenheit. A gallon of water weighs 8.34 pounds. Run a heater for a minute and you have the flow rate it can deliver at a given temperature rise. Everything else — fixture counts, "supports five outlets", family-size recommendations — is that equation with the arithmetic hidden.

Why the headline flow rate is always wrong for you#

Manufacturers quote maximum flow at a small temperature rise, usually somewhere around 35 to 45 °F. That is a legitimate operating point and it describes a heater in Miami.

Your temperature rise is set by two things: the temperature of the water arriving at your house, and the temperature you want out of the tap. Groundwater runs about 75 °F in south Florida and about 42 °F across the northern tier. A 120 °F setpoint therefore means a 45 °F rise in one place and a 78 °F rise in the other — and the same heater delivers nearly twice the flow in the first case.

This is why household size is the wrong first question. A family of four in Georgia and a family of four in Minnesota need substantially different heaters, and no fixture-count chart can tell you that. our tankless sizing calculator does the arithmetic with regional groundwater built in.

Gas or electric changes the whole project#

The two fuel types are not variations of the same purchase. They fail differently, cost differently and get installed differently.

Gas units reach whole-home capacity easily — 150,000 to 200,000 BTU is routine — and cost their money in venting and gas line work. An indoor unit needs Category III stainless flue at $40 to $70 a foot, plus a gas line usually upsized to 3/4 inch. An outdoor unit skips the venting entirely, which is why the outdoor propane models look so cheap, and takes on a freeze-protection dependency instead.

Electric units are cheap to buy, need no venting at all, and consume electrical capacity most houses do not have spare. A 27 kW whole-home unit draws 112.5 amps at 240 V across three double-pole breakers. That is over half a 200 A service before you have turned on the range. The full arithmetic is in the electrical requirements for electric tankless units, and it is the reason a lot of electric tankless projects become service upgrades.

What tankless does and does not save#

Tankless heaters eliminate standby loss — the energy a tank spends keeping 50 gallons hot around the clock whether anyone is home or not. That is a genuine saving and it typically runs 10 to 20% of water heating energy.

What tankless does not do is heat water more cheaply. A therm of gas is a therm of gas. The efficiency difference between a modern condensing tankless unit and a modern tank is smaller than the marketing suggests, and once you account for a higher purchase price and a more expensive installation, payback periods commonly run past ten years. the ten-year cost comparison against a tank shows the full arithmetic including the cases where it never pays back at all.

The reasons to go tankless that survive scrutiny are usually not financial: endless hot water, a recovered closet or garage bay, and a twenty-year service life against a tank's ten to twelve.

What we look at in a review#

Four things, in this order.

Output at your temperature rise, not the number on the box. The installation requirement — venting, gas line size, breaker count, service capacity — because that is where budgets actually break. The activation flow rate, which decides whether the heater drops out when someone runs a low-flow tap, and which is the source of most cold-water-sandwich complaints. And the heat exchanger warranty, which is the clearest statement any manufacturer makes about how long they expect the appliance to last.

Scale is what actually kills a tankless heater#

Storage tanks tolerate hard water reasonably well because scale settles to the bottom of a large vessel where it is a nuisance rather than a blockage.

Tankless heaters do not have that luxury. Water passes through a narrow heat exchanger at high temperature, which is the condition that precipitates calcium carbonate most aggressively. Scale forms on the exchanger wall, insulates it, and forces the burner or elements to work harder for the same output.

The symptoms arrive gradually and get misattributed: flow rate drops slightly, temperature becomes less stable, the unit cycles more, and eventually an error code appears. Owners usually conclude the heater was undersized.

Descaling is annual as a baseline, and six-monthly above about 10 grains per gallon of hardness. It takes about an hour with a pump, a bucket and a few litres of descaling solution, and it needs isolation valves — which is why fitting a service valve kit at installation is worth the $80 it adds.

If your water is hard and you are not willing to do this, a tank heater is the more honest choice.

Recirculation, and the cold water sandwich#

Two related complaints that come up constantly with tankless heaters, and both are plumbing problems rather than heater problems.

The wait for hot water is a function of how much cold water sits in the pipe between the heater and the tap. A tankless unit adds a couple of seconds to that while it fires, but the bulk of the delay is pipe volume and it exists with a tank too. A recirculation loop solves it; a demand-controlled pump on a button solves it without the standing energy cost of a continuous loop.

The cold water sandwich is specific to tankless. Turn the tap off and back on quickly and you get a slug of cold water — the heater stopped firing, the water in the exchanger cooled, and it takes a moment to catch up. Units with a small buffer tank largely eliminate it; units with a low activation flow rate reduce it. It is worth knowing about because it is the single most common source of tankless regret, and it is a design characteristic rather than a fault.