140 Overheat switch tripped (heat exchanger overheating)

What the control is telling you

The bi-metal overheat switch on the right side of the heat exchanger has opened, indicating overheating. It self-resets once cool but will keep tripping until the root cause is fixed.

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

Likely causes

• Air in the system (incomplete deaeration)
• Boiler pump not running, miswired, reversed or seized; blown fuse
• Closed valves, frozen CH circuit or indirect tank pump issue
• Scale build-up / dirty plate heat exchanger
• Wrong gas type, orifice or A0/A3 parameter
• Blocked vent or clogged heat exchanger fins
• Damaged switch wiring or faulty PC Board

Test and repair steps

1. Treat 140 as a genuine overheat condition. Re-run the deaeration process from the Installation and Operation Manual until no air is heard.
2. Check the pump 120 VAC at pump and boiler pump terminal, correct install and flow direction, not seized. Powered but idle = replace pump; no voltage = check fuse; fuse OK but no voltage = replace PC Board.
3. Confirm the gas supply matches the rating plate, Parameter A0 is correct, and Parameter A3 (altitude) is set for the site.
4. 175K and higher confirm the correct gas orifice is fitted in the gas valve, groove side out (grey = propane, black = natural gas, white = natural gas high altitude above 2000 ft).
5. Check vent termination clearances and inspect the vent system for damage or blockage.
6. Inspect the overheat switch harness for loose, damaged or broken wiring.
7. Inspect the heat exchanger for hot spots (scale) and for cracks, separation or discoloration on all sides; replace it if any are found.
8. Confirm all CH circuit valves are open and the CH circuit is not frozen.
9. With an indirect tank, confirm the tank pump is plumbed and wired correctly (the main system pump does not run during an indirect tank call).
10. Test the switch circuit at CN8 unplugged, pin 15 (black) to pin 4 (black) should read under 2 Ω; plugged in, under 1 VDC.
11. Flush the plate heat exchanger per the Installation and Operation Manual.
12. If 140 persists after these checks, replace the PC Board.
13. Remove the primary/secondary heat exchanger assembly, separate the burner, and clean burner and fins; replace exchanger and gaskets if not cleanable. Never reuse the burner gasket; keep mating surfaces clean.