100 Air intake/exhaust blockage or full condensate trap

What the control is telling you

The control has detected a restriction in the intake or exhaust path. A backed-up condensate trap inside the boiler can also restrict airflow and trigger it.

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

Likely causes

• Clogged or frozen condensate drain backing up the trap
• Dirty inlet air filter
• Blocked intake/exhaust terminations or debris in vent piping
• Vent too long, wrong materials, poor joints/gaskets or collapsed pipe
• Insufficient combustion air with indoor air
• Fan wheel/housing restriction or stuck check valve
• Debris in burner or heat exchanger fins
• Failed fan motor or PC Board

Test and repair steps

1. Confirm condensate drains freely; a clogged or frozen drain backs condensate into the trap and chokes airflow.
2. Clean the inlet air filter located under the front cover, top right of the cabinet.
3. Warning never run the unit with vent piping disconnected; the next check is a brief test only — reconnect immediately.
4. If the code occurs while onsite temporarily detach the exhaust from the top of the unit and call for heat/hot water. Code still appears = likely the unit (go to fan check); code clears = likely the venting.
5. Venting confirm approved materials, correct high altitude parameter, enough combustion air if using indoor air, clear terminations, proper clearances, and equivalent vent length within limits.
6. Inspect all vent piping, joints and terminations; clear debris, check gaskets, and ensure spray foam hasn't crushed the pipe. Use the vent maker's lubricant; bad joints cause noise, flue leaks and code 100.
7. Caution — fan check: power off, remove the fan motor and gas valve assembly, inspect fan wheel, housing and air duct for restrictions, and confirm the check valve behind the fan housing moves freely. Reassemble and test.
8. Warning shut off power, gas and water before inspecting the heat exchanger and burner. Do not blow out the burner with compressed air, and replace all gaskets on reassembly.
9. 150K and smaller remove the heat exchanger assembly, separate the burner, clean debris/insects from burner (incl. back side) and air chamber, and clear fins with 120 psi air; replace exchangers if not cleanable.
10. 175K and higher heat exchanger stays in place; remove the burner door assembly to inspect (see Appendix G: Abnormal Combustion Noise).
11. With power and gas off, replace the fan motor assembly if code 100 continues.
12. If a new fan does not fix it, replace the PC Board.