W 18 HWarm-air outlet or circulated-air intake blocked
The heater cannot move enough air because an outlet or return-air opening is obstructed.
Plain-English answer
W 18 H indicates restricted airflow at the circulated-air intake or a warm-air outlet. Bedding, luggage, dust, or a closed outlet can cause overheating protection to intervene.
What to check first
- 01
Turn the Combi off and let it cool.
- 02
Check every accessible warm-air outlet and the circulated-air intake for blockage.
- 03
Move luggage, bedding, rugs, and other items away from the openings before restarting.
Stop and call a technician when
- Do not remove internal covers or bypass an overheat device.
- Seek service if the code returns with all openings clear.
Check the exact manual
Applicability: Truma Combi furnace and water-heater systems. The data plate and specification letter determine which procedure is correct.
Open Truma Combi error-code tech tip ↗Source reviewed August 2026Check the complete air path, not only the nearest vent
The warning identifies restricted circulated air. One open outlet does not prove that the system has enough airflow; storage and soft furnishings commonly block another part of the path.
Find the circulated-air intake
With the heater off and cool, locate the accessible intake grille and remove luggage, rugs, pet hair, or other material resting against it.
Walk every outlet
Check each warm-air outlet rather than only the closest one. Reopen adjustable outlets and move bedding or bags that can collapse against a duct opening.
Stop before internal work
If W18H returns after the visible air path is clear, record how long the heater ran and arrange service. Do not remove the appliance cover or bypass overheat protection.