HVACopediaRinnai · Combi boiler
021 DHW continuous operation time exceeded
What the control is telling you
Domestic hot water has run longer than the maximum continuous time allowed by Parameter 18, so the boiler stops dwelling in DHW without serving central heating.
Applies to: I-Series Plus boilers — 150K BTU and smaller and 175K BTU and higher (Combi)
Likely causes
- Parameter 18 set too short for the application
- Hidden DHW leak or fixture left running
- Long continuous hot water draw
Test and repair steps
- Check Parameter 18 (max DHW continuous run, in priority or simultaneous mode): A = 120 min (default), b = 60 min, C = 180 min, d = unlimited. See the Installation and Operation Manual for programming.
- Look for leaks or running fixtures on the DHW side.
- Note DHW demand is sensed by a flow turbine; a very small leak will not spin it, so look for a draw large enough to register as flow.
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Source: Rinnai Troubleshooting I-Series Plus Boiler Diagnostic Codes (800000375), p.3. Summarized in our own words; check the original manual for your model.
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