HVACopediaRinnai · Boiler · Domestic hot water
DHW run limit and heating restart
What happens, in order
- Water flow turbine spins when a hot tap opens and the boiler serves domestic hot water
- Boiler keeps heating DHW until the draw stops
- If DHW runs longer than the limit in parameter 18, code 021 shows so the system does not sit on DHW with no central heating
- After DHW use, a pending heating call may wait: allow at least 3 minutes before heating restarts
- With Economy mode on, allow at least 5 minutes
Timings and thresholds
| Item | Value |
|---|---|
| Parameter 18 A (default) | 120 min |
| Parameter 18 b | 60 min |
| Parameter 18 C | 180 min |
| Parameter 18 d | unlimited |
| Wait after DHW before heating | 3 min |
| Wait with Economy mode | 5 min |
Codes you may see during this
21
Applies to: Rinnai I-Series Plus combi boilers
Models
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Source: Rinnai I-Series Plus diagnostic code troubleshooting guide (800000375), codes 021 and no-start-demand 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.