My overflow drain has gas

[BT05]

I only get this problem whenever there is not enough air entering my standpipe or there is a dramatic change in air pressure inside my canopy. Usually when I whip the door open is causes a flush, then settles down shortly after. Doesn't happen if I open the door properly though.

I have a so called "stockman standpipe" which is just the "hofer" with the coupling reversed. So try your standpipe out in the stockman form, it's more sensitive but the amount of air drawn in can be controlled easily. With the Hofer, air is drawn from the drainage holes in addition to the hole in the end cap. Unless the holes are aligned properly, one will always be drawing too much/too little air causing the flushing effect. The stockman doesn't have this problem since the holes are always submerged and enclosed by the coupling.

Then again, this might not apply as my drainage plumbing doesn't drop straight down into my sump but it's worth a shot.