New Water Heater No Hot Water at Commissioning Decision Tree

Why this matters

A brand-new water heater that will not make hot water on the commissioning call is almost never a defective unit. It is an installation or first-fire sequence fault: power or gas not fully on, a heater dry-fired or air-locked, a setpoint left at the shipping default, a crossover from the new connections, or a safety device that has not been reset. Calling the supplier for a warranty swap before working this tree wastes a day and frequently returns the same "defective" unit that was never actually faulted. This tree walks the new install in commissioning order so the most common first-fire misses are caught before the unit is condemned.

Symptom presentation

• No hot at any fixture immediately after install; cold flows normally.
• Heater shows no flame/no heat (gas) or no element draw (electric).
• Lukewarm only, or hot that runs out instantly: setpoint or crossover, not a dead heater.
• Electric breaker trips on energize, or the high-limit reset has already tripped on first fire.
• Gas unit will not light or lights and drops out.

Quick checks

1. Confirm the unit was filled and purged of air BEFORE energizing. A dry-fired electric element fails in seconds; an air-locked tank will not heat evenly. Open a hot fixture until it runs solid water with no sputter.
2. Verify setpoint: many units ship at a low default or a "vacation" position. Set to the intended service temperature.
3. Electric: confirm 240V at the heater terminals, both legs, and that both thermostats and the high-limit are calling.
4. Gas: confirm the gas valve is on, the appliance gets correct manifold pressure, the pilot/igniter sequence completes, and the flue/venting is connected so the unit is not failing on a blocked-vent or flame-rollout safety.

Isolation tree

Branch A: electric, no heat.

• Tank not filled before energizing: a dry-fired upper element is open. Confirm tank full, then test element continuity; replace any dry-fired element.
• No 240V or one leg only: a tripped or single-pole-feeding breaker, a loose terminal, or a wiring error. A 120V-only feed will not heat properly.
• High-limit (ECO) tripped on first fire: usually a consequence of dry-firing or a stuck thermostat. Find the cause before resetting.
• Both elements good, power good, but cold: thermostat not calling or wired wrong; verify upper-then-lower staging.

Branch B: gas, no flame.

• Gas not fully on or air in the new gas line: bleed the line, confirm manifold pressure under load.
• Ignition sequence fails: pilot not lit, igniter/flame-sense fault, or thermocouple/thermopile not proving on a standing-pilot or millivolt unit.
• Vent/safety lockout: a blocked-vent switch, flammable-vapor sensor (FV lockout on many residential gas heaters), or flame-rollout switch open from an install issue.
• Setpoint at minimum/pilot position.

Branch C: heats but no hot at fixtures (distribution/crossover).

• New hot/cold connections reversed, or a crossover through a single-handle fixture lets cold backfeed the hot line. Close the cold main, open a hot fixture; sustained flow indicates a crossover.
• Heater isolation valve left closed on the hot outlet.
• Air still trapped in the new piping.

Branch D: heats briefly then cold (capacity/dip tube).

• New dip tube damaged/omitted, short-circuiting cold to outlet.
• Undersized unit for the demand, or a thermostat set too low.

Confirming diagnosis

Fill/air confirmation: the unit must run a solid stream from a hot fixture before energizing. If it was energized dry, an electric element is the prime suspect and a continuity test confirms an open dry-fired element.

Power/gas confirmation: a clamp/meter reading of 240V both legs (electric) or correct manifold pressure plus a completed ignition sequence (gas) confirms the energy side is healthy and redirects to thermostat/element or vent/safety.

Crossover confirmation: with the cold main closed, sustained flow from a hot fixture proves cold is backfeeding the hot line through a fixture or a reversed connection. Verify the new hot/cold piping orientation.

Safety-lockout confirmation: a gas unit that proves gas and ignition but stays locked out points to a vent, rollout, or vapor-sensor safety that the install tripped; resolve the install cause, then reset.

Remediation

• Replace any dry-fired electric element and correct the fill/purge procedure for next time.
• Correct power feed (both 240V legs, tight terminals) or gas supply (bled line, correct manifold pressure).
• Reset safety devices only after the root cause (dry fire, blocked vent, reversed gas/vent) is fixed.
• Correct reversed hot/cold connections and clear crossovers; open the hot isolation valve.
• Set the service temperature and verify outlet temperature at a fixture after a full recovery cycle.

References

• International Plumbing Code (IPC) 2021 Chapter 5 Water Heaters (installation, temperature, relief)
• Uniform Plumbing Code (UPC) Chapter 5 Water Heaters
• ANSI Z21.10.1 / CSA 4.1 Gas Water Heaters, Volume I (ignition, FV lockout, venting safeties)
• NFPA 54 / ANSI Z223.1 National Fuel Gas Code (gas supply and venting)
• ASSE 1017 Temperature Actuated Mixing Valves (outlet temperature control)