Re: specs for trap in condensate drain?
Dave .... you might be over thinking this.
I suspect the reason you don't have a condensate trap is likely you have an up flow or blow through type of forced air system. In other words .... the blower is located at the bottom of the furnace and the air is blown upward across the heat exchanger and A coil.
During the heating season the trap would become dry .... negating the purpose of the trap ... the reason you usually won't find a "trap" on these types of systems..... it's also a problem with the draw through types of systems.
To design a trap would require knowing the air pressure of the system.
Even if you were to successfully and correctly design the trap .... you would need to continuously keep the trap primed throughout the heating season.
A simpler method would be to disconnect the condensate line and cap it off during the heating season ..... reconnect for the cooling season.
2 cents worth.
"" an ounce of perception -- a pound of obscure "