How to displace water that wont drain out of rain gutters

[micky]

That works two ways.

A larger amount can make it trhough from top to bottom,

And, and I was goint to post about this, OP is there any chance your downspout is partially clogged? I'm not sure how to find out. If it were totally clogged, a garden hose from the top to the bottom with water pouring out might dislodge a clog. Maybe separate the straight part and look from one end to the other.

My AC has a pipe from the evaporator coil, the A-coil tray, to the sump, and everythign was fine the first 10 years I was here. Then I found water coming out the bottom of the furnace. Figured out it was condensate that was overflowing the tray. Cut off the plastic pipe and ran a garden hose from the laundry sink to the open piple and water poured out into the sump. Didn't seem like it was ever clogged but certainly wasn't now. Glued the pipe back together again. Same problem.

Pipe came out of the A-coil area, went down 1 inch, over to the wall, down theto the floor, and over to the sump. I rearranged it so it went down 6 inches at first, instead of 1, and everything worked again. No overflow. I have no idea why but it reminds me of your gutters.

[TimR]

The fan in your airhandler is past the coil area, so it is sucking air there, and that means it will be at lower pressure than outside.

Your drain should have a trap in it, but the trap needs to be low enough to overcome the low pressure of the fan.

See here:

I have seen a lot of commercial installations forget this. If the air handler is on a mount sometimes it is easily fixed but often you have to break a sump in the floor slab to get the drop.

[TimR]

And then I found this article that disagrees with what I've been taught but makes sense: