CSLB C-20 HVAC ContractorHeating SystemsEasy
Per the Uniform Mechanical Code, a furnace installed in a small utility closet with a volume of 40 cubic feet and no direct free communication with additional adjoining rooms is classified as being in which type of space?
- ACategory IV space
- BDirect-vent space
- CConfined space
- DUnconfined space
Show answer & explanationAnswer & explanation
Correct answer: C. Confined space
A confined space is defined as one with less than 50 cubic feet per 1,000 Btu/h of total input of all appliances installed in the space. A small 40-cubic-foot closet clearly falls under this classification, requiring engineered combustion air provisions such as ducted openings or outdoor air connections.
Why the other options are wrong
- A. Category IV refers to venting classification, not room volume.
- B. Direct-vent is an appliance venting type, not a room classification.
- D. Unconfined spaces have ample volume (50+ cu ft per 1,000 Btu/h) and often connect freely to other rooms.
Confined vs Unconfined Space
A confined space has less than 50 cubic feet of volume per 1,000 Btu/h of appliance input; an unconfined space has more, allowing natural infiltration for combustion air.
- Rule of thumb: 50 cu ft per 1,000 Btu/h input
- Confined spaces need engineered combustion air openings
- Determines whether ducted or direct openings are required
Memory trick: Tight closet, tight air — that's confined.