I’ll start by saying there are situations, especially in older homes, where you have no choice but to make your drain trap instead of using a manufactured trap. Ultimately though you never want to make your trap for an HVAC system. An HVAC system uses a 3/4 drain (which is much smaller than a sink drain) and a 3/4 trap. The water that drains through the trap is dirty it picks up dust and dirt from the coil and sits in the trap sometimes for long periods (even days) before it drains out stewing up all kinds of gunk and mildew.

If you make your own homemade trap, it will have 90-degree edges that more gunk will stick to and not flow out with the water. A homemade trap is more likely to have consistent drain problems, and in most cases vacuuming it out with a shop vac or purging it with nitrogen still doesn’t completely clean it so it clogs again not long after. Always use a machined trap that is smooth and rounded has fewer clogs and can be cleaned completely to help ensure a long time before another clog.