Hello
Yes, there are a couple of reasons for having a vertical rise.
1. When the stove shuts down especially if there is a power failure, then any smoke in the stove and vent pipe will draft out safely and not come back into the room.
2. Any type of cross wind will not blow the smoke into the stove and room if there is a vertical rise.
When installing your venting keep in mind that the EVL should be 15 or less.
Remember:
If you are going vertical more then the recommended install of approx. 5 ft for 3†vent then keep track of the EVL and go to 4†if you are over 15!!
Here is a good definition of EVL !!
Also see here for more info
http://nevelsstoves.com/pellet-stove-venting.html
Pellet stoves have to push the exhaust air through the vent pipe with the stove’s exhaust fan. With less restriction your pellet stove will breathe easier and burn more efficiently. First let’s clarify the word restriction. Restrictions come from several different items.
1. 45 degree elbows
2. 90 degree elbows
3. Horizontal distance
4. Vertical distance
5. Elevation where you live.
A rule of thumb equation we are using has been adopted by most pellet manufactures. The equation is called the sum of Equivalent Vertical Length (EVL). All of the above mentioned venting restrictions have been assigned EVL values as follows:
1. Each 45 degree elbow = 3 EVL
2. Each 90 degree elbow and Tees with cleanout = 5 EVL
3. Each foot of horizontal run = 1 EVL
4. Each foot of Vertical run = 0.5 EVL
5. Elevations above 3000 ft with an EVL of 7 must adapt to 4 inch vent pipe.
If your installation is below 3000ft, we would need to do some math. The rule of thumb equations is that if the sum of the EVL is 15 or greater, then the pellet vent pipe would be increased to 4 inch diameter pellet vent pipe.