33 Heat exchanger thermistor fault

What the control is telling you

The heat exchanger temperature thermistor or its circuit is reading abnormally.

Applies to: SENSEI Series; SENSEI RX/CX Series; Non-SENSEI Series

Likely causes

• Loose, broken or damaged thermistor harness
• Scale build-up on the thermistor
• Thermistor resistance out of spec
• Failed thermistor

Test and repair steps

1. Inspect the thermistor harness from thermistor to PC board for loose, broken or damaged connections.
2. Caution: Shut off the water supply and drain the system before continuing.
3. With the hot water lines empty, remove the thermistor and clean off any scale or deposits.
4. With a meter set to read 20K ohms, measure across the thermistor plug; resistance should drop when the bulb is warmed and rise when cooled.
5. Typical values 11.4–14K ohms at 59°F, 6.4–7.8K at 86°F, 3.6–4.5K at 113°F, 2.2–2.7K at 140°F, 0.6–0.8K at 221°F.
6. If readings are good, reinstall the thermistor, checking that its small O-ring is intact and lightly lubricated.
7. Restore water supply and check for leaks at the thermistor.
8. Fire the unit again; if code 33 returns, replace the thermistor.

Settings for these units

• RSC supply connection sizes · SENSEI
• RSC water quality limits · SENSEI
• RSC clearances to combustibles · SENSEI
• RSC temperature settings available · SENSEI
• RX supply connection sizes · SENSEI RX/CX
• RX water quality limits · SENSEI RX/CX
• RX water treatment and flushing by hardness · SENSEI RX/CX
• RX clearances to combustibles · SENSEI RX/CX