Actually, we just covered something similar in another thread.
Manny (Emmanuel) Scanlan came up with this, which appears to apply here as well.
**G2407.2 (304.2) Appliance/equipment location. **Equipment
shall be located so as not to interfere with proper circulation of
combustion, ventilation and dilution air.
Commentary for this section:
The provisions of Sections G2407 rely on the natural (gravity) movement of air; therefore, installations must allow unimpeded flow of combustion air from the source to the appliances being supplied. Other mechanical systems, appliances, or equipment in the same room or building can adversely affect the combustion air supply. For example,when placed too close to the combustion air openings, gas-fired appliances or equipment will restrict the free circulation of air.