OWF installation on a grade

I'm in the middle of my install and I went for a walk tonight to run the dogs and clear my head. Of course I spent the time thinking up things I might be doing wrong! Today's hand-wringer is this: My home is on a pretty good grade so the furnace placement out back has the top of the water jacket vent 5 feet (or so) *below* the highest point where my heat exchanger is located. In an open system I think I might have a problem here with the system draining out. The easiest solution I could think of is to extend the vent pipe that 5+ feet so we don't have the drain problem. Thoughts? I'm installing a Woodmaster 4400 with a 80 ft run to a water-to-water heat exchanger supplementing/replacing an oil system.

Mike

OK, thanks!
My follow-on question then is why won't it overflow the vent when I'm trying to fill it. My Woodmaster rep said the same thing: It can't drain out. But his reasons-why don't make sense to my amateur engineer's brain. I guess we'll see what happens!

How big is your heat exchanger? Doing this would fill my HE in about 30 seconds....or you're filling everything except the HE...that must be it (good idea though with the clean-out setup).
In any case, my installation recommends a similar filling procedure with an inline filter and fill valve assembly located on the return line near the HE. My question though pertains to my furnace being downslope from the house...the top of the vent is 5-6' below the topmost point in the plumbing at the heat exchanger. As I see it, when I try to fill the system, it will do OK up until the water level hits the top of the vent and then will start to drain out there, not traveling back up the hill in the supply line.

I get it...thanks!