The temperature difference between the U1 and U6 sensors is too great.
1. Check the electrical resistance of the two sensors corresponds. If one or both sensors does not have the correct resistance value, it must be replaced.
2. Check that the heating water flow is not too low. If temperature difference between U1 and U8 is higher than 55°F at maximum input, the heating water flow rate is too low. The heating water flow rate must be corrected.
