14 Thermal fuse or overheat sensor tripped

What the control is telling you

An overheat safety opened: the bi-metal overheat switch on the heat exchanger, or (non-SENSEI) a one-shot thermal fuse. This means the unit overheated and must be taken seriously.

Applies to: SENSEI, SENSEI RX/CX and non-SENSEI tankless

Likely causes

• Gas valve shutoff DIP switch left ON (SENSEI single DIP; non-SENSEI VC/KB SW2 #5)
• Wrong gas type, bad gas conversion, wrong orifice/venturi, or wrong manifold pressures
• Wrong SENSEI parameters (10 gas, 02 altitude, 13 model, 14 exhaust)
• Debris or blocked heat exchanger fins causing local overheating
• Cracked, separated or discolored heat exchanger, or scale needing a flush
• Short cycling, e.g. an aquastat with too small a delta T
• Damaged safety-circuit wiring, blown thermal fuse, or faulty PC Board

Test and repair steps

1. Note The bi-metal switch resets once cool and will keep tripping until the cause is fixed. Non-SENSEI thermal fuses are one-shot and must be replaced, which means removing the heat exchanger.
2. SENSEI/RX/CX The only DIP switch on the PC Board must be OFF (ON cuts the gas solenoid and shows code 71). Non-SENSEI VC/KB only: SW2 DIP #5 must be OFF. Check all other DIP settings too.
3. Confirm the gas type matches the rating plate. Non-SENSEI: if the unit was converted, make sure the conversion followed the model's procedure. Confirm clearances around the heater and vent terminals.
4. Check the overheat/safety-circuit harness for loose, damaged or broken wires. Ohm out the circuit to see if the bi-metal switch is open or a thermal fuse has blown.
5. SENSEI meter between conn "H" pin 14 and conn "D" pin 28 (black wires). RX/CX: conn "CN9" pins 1 and 11. Expect under 1 Ω, or 11–13 VDC.
6. SENSEI/RX/CX Check the venturi/orifice color (red = LP, blue = NG). Check parameters 10 (gas), 02 (altitude), 13 (model BTU) and 14 (indoor/outdoor). RX/CX: Param 10 only within 20 min of power reset.
7. Non-SENSEI – WARNING: Gas-qualified only. Use a manometer to check forced low and high fire manifold pressures (procedure is on the back of the front cover).
8. Non-SENSEI Check for short cycling (e.g. aquastat with a low delta T), which can heat the copper and trip the switch. Set the aquastat at least 20° below the heater set point.
9. Warning: Turn off electrical, gas and water before inspecting or removing the heat exchanger and burner.
10. Inspect the heat exchanger front, sides and back for cracks, separations, discoloration or damage, and replace it if found. Black discoloration where the copper meets the combustion box means blocked fins.
11. Look for heat damage on other parts in the cabinet. Check the burner manifold, burner and fins for debris. Non-SENSEI: replace the heat exchanger if debris caused the code 14.
12. SENSEI/RX/CX Flush the heat exchanger per the manual. Then separate the burner and primary heat exchanger, clean the fins, or replace the heat exchanger and all gaskets.
13. SENSEI Never reuse the burner gasket (use 101 Burner Gasket-Large). Clean and smooth the mating surfaces before refitting. Non-SENSEI: confirm vent clearances and lengths are within spec.
14. If code 14 remains after all checks, replace the PC Board.

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