What is the required grill size for three hot water boilers firing at 500 MBTUH each, two hot water heaters firing at 270 MBTUH each, and a domestic water heater firing at 30 MBTUH, assuming 80% free air space?

To determine the required grill size for the specified appliances, we need to first calculate the total BTU input and then consider the required free air space for venting.

1. Calculate the total BTU input:
   • Three hot water boilers at 500 MBTUH each contribute:

 \(3 \times 500 = 1500 \, \text{MBTUH}\).

• Two hot water heaters at 270 MBTUH each contribute: (2 \times 270 = 540 , \text{MBTUH}).
• One domestic water heater at 30 MBTUH contributes: (1 \times 30 = 30 , \text{MBTUH}).

Adding these together gives: (1500 + 540 + 30 = 2070 , \text{MBTUH}).

2. Since the appliances require proper combustion and ventilation, we need to convert this total BTU into a required free air size considering the 80% factor for free air space. The general guideline used in the industry is that 1 square inch of grill opening is needed for approximately 1,000 BTU (this can vary based on specific local codes