What it is

A tankless (on-demand) water heater has no storage tank. Water runs through a heat exchanger only while a hot tap is open, and a burner (or electric elements) heats it on the fly. Open the faucet, it fires; close the faucet, it shuts off. The payoff is endless hot water and no standby loss; the catch is that everything happens in real time, so flow rate and temperature rise are the whole game, and mineral scale in that heat exchanger will choke it.

This covers gas condensing/non-condensing tankless units, which are by far the most common in service work. Whole-home electric tankless exists but draws enormous amperage and is less common.

How it works

When you open a hot tap, water starts moving through the unit. A flow sensor (a little turbine/paddle wheel) detects the flow and tells the control board to start. The board needs a minimum flow rate to fire — typically around 0.4–0.6 GPM — below that, it won't light, which is why a trickle at the faucet stays cold.

Once it's firing, the board watches an inlet thermistor (incoming water temp) and an outlet thermistor (leaving water temp) and modulates the gas valve and combustion blower to hold the setpoint. If you open a second tap and flow doubles, it ramps the burner up; if incoming water is freezing in January, it ramps up to hold the same outlet temp at a lower total flow. The unit is constantly trading flow vs. rise — it can only add so many BTUs per minute, so the colder the inlet, the lower the gallons-per-minute it can deliver at your target temperature.

Condensing tankless units add a secondary heat exchanger that pulls extra heat out of the flue gas, which is why they vent in plastic and make acidic condensate that needs a drain (and often a neutralizer), same as a condensing furnace.

In the field

Descaling / flushing is the single most valuable thing you do to a tankless, and on hard water it's an annual job. Scale (calcium carbonate) builds inside the heat exchanger, insulates it, restricts flow, and eventually triggers fault codes or destroys the exchanger. Most units have a maintenance reminder for a reason.

The flush, on a unit with isolation/service valves installed:

  1. Shut off the gas (or kill power), close the hot and cold isolation valves to the house.
  2. Connect a pump and a bucket to the service-valve ports — discharge hose on one, return hose on the other, both into a 4–5 gallon bucket of descaling solution.
  3. Circulate descaler (white vinegar or a commercial food-safe descaling solution) through the heat exchanger for 45–60 minutes. The pump pushes solution up through the exchanger and back to the bucket. Vinegar is cheap and food-safe; commercial descalers work faster.
  4. Drain and flush the solution out, then close the descaler ports and run clean water through the unit (open the isolation valves, run a hot tap) for several minutes to rinse all the acid out.
  5. Clean the cold-water inlet screen / filter — scale and debris collect there and restrict flow.
  6. Restore gas/power, confirm it fires and holds temperature.

If the unit has no isolation valves, the homeowner skipped the install kit — recommend adding the service valves so future flushes take 10 minutes instead of a plumbing teardown.

Normal values & targets

  • Minimum activation flow: roughly 0.4–0.6 GPM (varies by model). Below it, the unit won't fire.
  • Rated capacity: sold by GPM at a given rise. A ~199,000 BTU/h residential unit delivers roughly 5–7 GPM at a 45°F rise, but only ~3–4 GPM at a 70°F rise (cold winter inlet). Always size by rise, not just GPM.
  • Temperature rise math: GPM × 8.33 × rise(°F) × ~1.0 = BTU/h needed; divide unit input (× efficiency) by that to get deliverable GPM at a given rise.
  • Descaling interval: annually on hard water (>7 grains/gal), every 2–3 years on soft water.
  • Setpoint: commonly 120°F; many units cap user adjustment and use a remote controller.

Common faults & what they mean

  • Hot tap runs cold at low flow. Below minimum activation flow — aerator clogged, low-flow fixture, or a partially closed valve. The unit literally can't sense enough flow to fire.
  • Lukewarm or fluctuating temperature, error codes. Scaled heat exchanger restricting flow and heat transfer. Descale it. Persistent scale faults after a flush mean it's badly scaled or the exchanger is damaged.
  • "Cold-water sandwich." Hot, then a slug of cold, then hot again when you cycle a tap quickly. It's the water that sat in the exchanger/lines cooling off between draws — inherent to tankless. A small buffer tank or recirculation smooths it out; it's not a fault.
  • Ignition/flame faults on a gas unit. Gas supply too small (tankless needs a big gas line and enough meter capacity), blocked vent/intake, or combustion problem. Undersized gas piping is a top install miss — these units demand high BTU input.
  • No hot water at all, error on display. Read the fault code. Common: flame failure, flow sensor, overheat (often scale), or vent/condensate blockage on condensing models.

Tech tips & gotchas

  • Gas sizing kills more tankless installs than anything. A 199K BTU/h unit can starve a 1/2" gas line or overload a small meter. Verify the pipe size and meter capacity against the input before you ever blame the unit for ignition faults.
  • Scale is the silent killer. A homeowner who's never flushed a 5-year-old tankless on well water probably has a partially blocked exchanger. Flushing can bring a struggling unit back; let it go too long and you're replacing the exchanger or the whole unit.
  • The cold-water sandwich is a customer-education issue, not a repair. Explain it. If they hate it, recommend a recirc system or a small buffer tank.
  • Don't forget the condensate on condensing models — it's acidic, needs a trapped drain, and often a neutralizer. A frozen or plugged condensate line will fault the unit in winter.
  • Always install isolation (service) valves if they're not there. It's the difference between a quick annual flush and a miserable one.

Safety / code notes

  • Gas tankless units are fuel-burning appliances: venting per the listing and fuel-gas code (IFGC §503), adequate combustion air, and proper gas-pipe sizing per the fuel-gas code sizing provisions. Their high input makes pipe sizing critical.
  • Condensing models discharge acidic condensate — trap, drain to an approved point, and neutralize where it'll contact vulnerable materials (IMC §307 for condensate).
  • A T&P relief valve is still required on most tankless installations per the listing and plumbing code (IPC water-heater provisions) — don't assume "no tank" means "no relief."
  • Use only food-safe descaling solution (white vinegar or a product listed for potable systems). Rinse thoroughly so no descaler remains in the potable water before returning to service.