Piping, Sheet Metal, and Materials Installation Standards
Refrigerant piping
Use ACR copper, deburr and ream, and braze with 5 to 15 percent silver alloy while purging dry nitrogen through the line to prevent internal oxidation. No flux on refrigerant joints. Use 45-degree flare fittings where flares are specified and torque to manufacturer values. Follow manufacturer line sizes rather than eyeballing, slope horizontal suction lines toward the compressor, and trap oil at the base of tall risers. Insulate the suction line with closed-cell material and protect the vapor barrier.
Condensate and gas
Condensate drains slope at least 1/8 inch per foot, include a trap, and discharge indirectly; a secondary pan or float switch protects the ceiling. Gas piping is Schedule 40 black steel sized from NFPA 54 tables using equivalent length, with a shutoff valve, union, and sediment trap at each appliance.
Sheet metal
SMACNA construction standards set gauge by duct size and pressure class. Seal joints and seams to the specified Seal Class, using mastic or listed tape only. Reinforce at SMACNA spacing, hang rectangular duct at standard intervals, and avoid flex duct where hard duct is practical.
Worked example
A 120,000 Btu/h connected load - 80,000 furnace plus 40,000 water heater - on 40 feet of Schedule 40 black iron at 0.5 inch w.c. drop. The 3/4 inch column at 40 feet is around 120,000 Btu/h, right at the limit with no allowance for fittings. Add equivalent length for elbows and use 1 inch pipe for margin.
Traps
Brazing without nitrogen; using plumbing copper instead of ACR; no condensate trap; missing sediment trap; cloth tape as duct seal; compressed flex duct.