You are called to service a furnace that is not providing enough heat. Which one of the following is NOT a possible cause of the problem?

windy city oil field, karamay, xinjiang, China

Laying plastic pipes drainage to drain the water around the house of drainage system

Concrete cube or construction brick and trowel. Construction concept and mason

Aerial view of many colorful cars parked on parking lot with lines and markings for parking places and directions.

Worker in a carpentry shop measures plywood for cutting

Apartment Fusebox Installation by Caucasian Electrician in His 40s. Residential Home Electric System Theme.

Joyful young man construction worker holding mobile phone and smiling while standing by the table with laptop