It can be vented either horizontally or vertically. Home Depot up north had a cut away of the unit showing the unit and how it is to be mounted and connected. Do not know if they have it down here. Check out this web sight
http://www.boschhotwater.com/BoschHo...3/Default.aspx
I used the Aqua Star 125FX. The FX has an internal fan that forces the exhaust out, thereby allowing for the horizontal venting.
The unit does quite well for a home. And it takes the same amount of time to get water to you shower as it does now. There is I believe 115,000 BTU kicking in when you turn on the faucet. Think of it as a big Bunsen burner kicking in. And as soon as the faucet is turned off it quits.