Tankless water heaters. Episode sixty of the Gas Service Animated series.

A tankless water heater is a two-hundred-thousand-Btu appliance hanging where a forty-thousand-Btu tank used to sit. That one sentence is most of what goes wrong with them. Endless hot water is real — but the gas line, the vent, and the water all have to be re-thought, not reused.

Start with the gas-sizing trap. A one-ninety-nine tankless draws roughly five times the gas of the tank it replaces — about two hundred cubic feet an hour instead of forty. That's not a reconnect; that's a new load calculation. Re-run the longest-length method for the whole system, because the branch you just piped can be fine while the trunk upstream goes undersized. Three-quarter pipe only carries that load on short runs — by a hundred feet of longest length, you're in one-inch territory on the standard tables.

Starved gas shows up in disguise. Ignition-failure lockouts. Flame loss at high fire. Or the classic: it only fails when the furnace and dryer run at the same time. The test that settles it is a manometer on the unit's inlet port at full fire with every other gas appliance running — static pressure can look perfect while dynamic pressure collapses under load.

Inside, the burner modulates. A modern unit runs from around thirteen thousand Btu up to its full one-ninety-nine — about a fifteen-to-one turndown — matching the fire to the flow and the temperature rise. And it needs a minimum flow to fire at all — around four-tenths of a gallon a minute. A dribbling faucet gets cold water by design; check the aerator and fixture screens before you condemn the heater.

Now the vent, and the categories matter. A non-condensing tankless is Category three: the fan pressurizes the vent and the flue gas is hot. That means sealed, corrosion-resistant stainless special gas vent — every joint gasketed, because a leak is flue products in the living space. And it means never, ever the old B-vent or masonry chimney it used to share.

A condensing tankless is Category four: still positive pressure, but the second heat exchanger pulls the flue so cool it condenses — which is why plastic vent is allowed, per the manual, sloped back to the unit. That condensate is acid, around pH three to five, so it gets a corrosion-resistant drain and, where required, a neutralizer cartridge before the house plumbing.

Water is the third system. Hard water bakes scale into the heat exchanger, and the cure is built in at install: isolation valves with service ports, and a periodic descaling flush with the pump and the solution the manual calls for. Skip it and the unit derates itself — error codes, temperature swings, and a heat exchanger that dies young.

So the tankless method is three re-checks. Gas: longest-length math, then dynamic pressure at full fire. Vent: the right category, the right material, sealed and sloped. Water: minimum flow at the fixtures and a descaling routine. The error code tells you which subsystem; the manual tells you the test. Same discipline as every appliance — just less forgiveness.

Tankless in one breath. It's a two-hundred-thousand-Btu appliance: re-run the gas sizing and prove dynamic pressure. Category three is sealed stainless; category four is plastic with an acid condensate to neutralize. Respect the minimum flow, and descale on schedule. Next: the modulating-condensing boiler.