Flue outlets on multifuel stoves

[oilier than thou]

Most multifuel stoves have alternative flue positions, usually BACK or TOP. Why ? and is the output or efficiency affected by the choice. This one is the Hunter Herald 14 with wraparound boiler. It would seem that chimney sweeping would be simplest if I use the top outlet. Are there any other considerations that I've missed ?

Thanks John

[Dave Liquorice]

I guess it's so you can either go straight up and into the chimney or straight out through the wall to a flue.