One very important thing to do during a home inspection is turn on appliances and get them going, and test all of the plumbing fixtures, baths and kitchen. Doing so forces water into the system, and therefore displaces air. That air needs to have someplace to go so that as the water is drained there is not a disruptive glub glub (technical words to be sure). Theoretically this displaced air exits through the roof.
Unless there is another way out!
This particular house uses a condensate pump to eliminate the water the air conditioner collects from the house.
That water is drained into the little box and it exits through that brown tube going out the top.
These boxes are often discharged outdoors, but sometimes into a floor drain, laundry tub or into the plumbing itself.
ANYWAY IS FINE, IF DONE RIGHT!
The gentleman who set up this system was thinking, but not quite far enough.
He installed a trap! This trap fills with water and prevents sewer gases from getting back into the house. You all know this.
That trap is perfectly done. But here is the problem. It is January! The AC has probably not been used since early October or so. Any water in that trap has evaporated. When the plumbing is flushed with lots of water, as I did during the inspection, sewer gases are displaced and they came out through the top of that tube!
And those gases came out in a big way!
What a great wake up call! I was happy to catch this during a home inspection. My clients were too!
What could have been done instead?
This is the arrangement in my house. It services a water softener.
Water softeners regenerate every three days, and send out lots of brine water each time. When we first installed it the plumber did not provide enough ability for displaced air to exit, and water came out everywhere. I had him come back and install a second vent on top of the stack, and an air admittance valve. That special valve can be seen on the vertical tube to the right. It lets air in, but not gases out.
However, our softener broke many years ago. I will remove it should we ever move, but it is there for now. But it no longer sends water into the system, filling that trap.
Those two openings in the PVC to the left needed to be capped! Why?
So we don't get the "Sudden Smell" that happened during that home inspection!
So I capped them!
If the purchaser of our house, should that day come, and wants the softener, I will keep it and this plumbing in place and repair the unit. It doesn't make sense to remove it now.
My recommendation: at first glance, some things seem to be done properly. That condensate drain above had a proper trap! And it works, but only when there's water present. During the "off season" that opening must be covered, or physics will do its job. Physics doesn't fail... and sometimes physics smells really bad!