Gas Piping, Combustion Air, and Appliance Installation
Gas piping
- Size with the longest-length method using tables for 0.60 specific gravity and 0.5 in. w.c. pressure drop, and include every appliance on the line.
- Natural gas: about 7 in. w.c. supply, 3.5 in. w.c. manifold. LP: 11-13 in. w.c. supply, 10 in. w.c. manifold.
- Sediment trap at least 3 in. long, downstream of the shutoff and ahead of the controls.
- Appliance connector maximum 6 ft, not concealed, not through walls.
- Bond CSST per the manufacturer. Find leaks with bubble solution or an electronic detector, never a flame.
Combustion air
- Unconfined space: 50 ft3 per 1,000 Btu/h of total input.
- Two permanent openings, each 1 in2 per 1,000 Btu/h, minimum 100 in2, one within 12 in. of the ceiling and one within 12 in. of the floor.
- Vertical ducts: 1 in2 per 4,000 Btu/h. Horizontal ducts: 1 in2 per 2,000 Btu/h.
- Louvers cut free area: metal about 75%, wood about 25%, so size openings upward.
- Derate gas input about 4% per 1,000 ft above sea level.
Worked example
A 100,000 Btu/h furnace and a 40,000 Btu/h water heater sit in a 25 x 20 x 8 ft room (4,000 ft3). Allowed input = 4,000 / 50 x 1,000 = 80,000 Btu/h, so the space is confined. Each opening must be 140,000 / 1,000 = 140 in2; with 75% free metal louvers, specify 140 / 0.75 = about 187 in2.
Traps
- Ignoring the water heater when sizing pipe or air openings.
- Measuring supply pressure with the appliance off.
- Forgetting louver free area.
- Assuming an undercut door supplies combustion air.