Should you use the rear, or top, flue exit on your stove?
Many stoves come with an interchangeable flue pipe connection so you can decide on whether to use the top or rear connection. Usually the stove comes set up with the flue collar on top. The flue collar being the part which holds the flue pipe in place on the stove. To swap over the connection then one usually just unbolts this spigot; you then unbolt the round plate off the back of the stove which covers and seals the rear connection and swap the two around.
When to use the rear connection on a stove
If your stove is going to be installed into a recess, especially if that recess is small, then using the rear flue will help to push the stove further into the room. This in turn allows more heat from the stove to pass into the room. But of course you may not have the space in front of the recess to allow for this.
Please bear in mind that there are a couple of potential disadvantages of using the rear exit on your stove, as follows:
- You may be unable to access the tee to sweep your chimney. If the stove is in a tight space already then there is unlikely to be enough space to get sweeping rods up through the base of the tee. This means that you would have to take the stove out to sweep the chimney, which makes it less likely that you will have it swept as regularly as needed, and makes it a bigger job to do of course. Sweeping your chimney keeps your chimney clear and reduces your risk of chimney fire.
- It may be hard to light your stove before the chimney has warmed up. A cold chimney will have a lot less draught (or suck) than a warmed one. This is because a main part of the reason for this draught is that the air in the house and chimney is warmer, and therefore less dense, than the colder air outside. When using the rear exit on a stove there is even less draw on the flue gasses making it harder to light before the chimney has warmed up and starts 'sucking'.
When to use the top connection on a stove
If your stove is freestanding then, depending on the style of stove of course, it can often look most elegant to use the top connection so that the whole installation rises up directly from the floor as it were. Another advantage of the top connection is that the flue gases have a straighter path to travel - hence the chimney draught will not be reduced as much as using the rear connection. Usually it should be fine either way but if you have a chimney with borderline draught, or a flue system where you need to use quite a few bends, then using the top exit can help.
But will using the rear flue will pass more heat to the room?
You should get a little more heat gain from the stove to the room by using the rear flue exit as the flue gases do have a little further to travel. So if it is possible, and you have a limit on the amount of single skin you can use anyway, then it should definitely be considered.
Whilst the 90 degree tee at the back of the stove will be passing heat to the room this does not mean that the room will get hotter. This is because I would not recommend using more than 2m of straight single skin flue pipe off a stove as it will cool the flue gasses too much otherwise. When using a 90 degree tee then I would recommend reducing the straight lengths accordingly. The reason is that you actually do not want to get rid of all the heat in a chimney - it is the driving force behind the flue draught, and also by keeping the flue gasses hot you reduce deposits in the chimney.
To warm up a reluctant chimney you can leave the air vents on your stove a little open so that air flows up the chimney to warm it. You can light a few sheets of scrunched up newspaper in the stove to pre-warm the chimney - this can act as a kick-starter to help get the fire going.
As an aside it is interesting to note that when stoves are tested in the KIWA Gastec labs that they:
are always tested top flue as this is the shortest outlet route for the escaping flue gases, therefore will produce worst case performance test data (due to higher flue gas temperatures).