HVACopediaBosch · Tankless water heater · Domestic hot water
Flow-activated hot water call
What happens, in order
- Hot tap opens and flow reaches the activation point (at least 0.45 GPM needed)
- Optional startup delay (menu A8) holds the burner back before it starts
- Fan starts, igniter sparks, gas valve opens and ionization rod proves flame
- Gas valve modulates the burner for the setpoint against flow and inlet temperature
- Solar mode if inlet water is already near the setpoint (within about 1 F) the burner does not light and the solar indicator shows
- When flow drops to the deactivation point (0.35 GPM) the burner stops
Timings and thresholds
| Item | Value |
|---|---|
| Activation flow | 0.45 GPM |
| Deactivation flow | 0.35 GPM |
| Burner start delay (A8) | 0-60 s |
Codes you may see during this
Applies to: Greentherm T9800 SE 160/199, T9900 SE 160/199
Models
Related
Take it to the job
Sequences of operation, error codes and tips work offline in the HVACopedia app.
Source: Bosch Greentherm T9800 SE (6720816815) and T9900 SE (6720813635), operation sections 6.4, 6.5. 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.