HVACopediaNoritz · Tankless water heater · Domestic hot water
On-demand heating cycle
What happens, in order
- Hot water is drawn and flow reaches the minimum flow
- Premix fan starts and mixes air and gas through the venturi
- Gas valve opens and igniter / ignition rods spark
- Flame rod proves flame; no flame gives code 11, flame lost repeatedly gives code 12
- Unit modulates on flow, set temperature and inlet water temperature to reach set temp fast
- Flow stops and the burner shuts off
- Fan speed normally reads 100 on MM49; above 100 means a restriction in intake or exhaust (code 90)
Timings and thresholds
| Item | Value |
|---|---|
| Minimum flow to stay on | usually about 0.5 gpm |
| BTU demand | gpm x temperature rise x 500 |
Codes you may see during this
Applies to: Noritz premix tankless
Models
More Noritz tankless water heater sequences
- Water route: main and bypass servo by set temperature · Domestic hot water
- Auto Learning recirculation · Pump/purge
- Other recirculation modes · Pump/purge
- Freeze prevention heaters and pump · Freeze protection
Related
Take it to the job
Sequences of operation, error codes and tips work offline in the HVACopedia app.
Source: Noritz Premix R2 training (EZ, NRCR, CDV) and Residential R2 training, maintenance monitor and component pages. Written in our own words; timings can differ by model and software version, so check the manual for your exact unit.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.