831 Indirect tank call running too long

What the control is telling you

The indirect DHW tank has been calling for more than 12 hours without cycling off.

Applies to: I-Series Plus Solo boilers — 150K BTU and smaller and 175K BTU and higher

Likely causes

• Indirect tank too small for DHW demand
• Tank thermistor mounted too low or near cold inlet
• Primary-secondary piping not used
• Supply temperature not above tank set temperature (Parameter 30)
• Faulty tank thermistor or wiring

Test and repair steps

1. Check that the tank is large enough for the hot water demand; a larger tank may be needed.
2. Check thermistor placement — if it is too low in the tank or too close to the cold inlet, the pump keeps running after most of the tank has recovered.
3. Confirm primary-secondary piping is used (low loss header, closely spaced tees, etc.).
4. Make sure the supply temperature to the tank is higher than the tank set temperature (Parameter 30).
5. Compare performance data 30 with a thermometer at the same spot; over 5° difference means continue, -40 means unplugged or bad connection.
6. Inspect the thermistor harness for loose, broken or damaged connections.
7. Measure tank thermistor resistance and compare against Appendix D in the source manual (p.55).
8. Replace the thermistor if damaged or defective; see code 371 for more on the indirect tank thermistor.