Flue sizes and queries

[ruperthebare]

Hi,
I have a medieval cottage in Devon with an inglenook fireplace.
I have just removed a Franklin Stove which was old and juicy on the wood and am replacing with a new wood burner. The Franklin used an 8" flue, stove pipe to the register plate, then approx 18' of flexible 8" straight up to the top.
The new stove has a 5" flue.
My questions are as follows: should I
a) join a piece of 5" stove pipe from the new stove using an adaptor to the existing 8" flexible flue at the register plate
or
b) drop a new flexible 5" flue down the 8" and connect the two 5"
is 5" increasing to 8" too much of a increase? Will it compromise the stove operation/ performance?
Does a smaller flue give a better draw?
If I go with option b) will one flue within another cause any problems?
Thanks in anticipation
Robert

Edited September 27 by ruperthebare
Correction

[JohnMo]

What does your installation manual say on the subject?

[Temp]

We have a wood burner with a pipe around 125m diameter connected to a 220mm diameter pumice block chimney. The increase in diameter doesn't seem to cause any issues. 

 

How old is the existing 8" liner? I would see what condition it's in and try reusing if it looks OK.  

[ruperthebare]

I haven’t bought the stove yet as I need to re-tile the hearth first. The manufacturer (Ecosy 5kw) of the stove I’m after said there shouldn’t be any problems as long as the size increases, which it does.

The liner is about 5 years old and still looks pretty good nik albeit a bit sooty, but I’ll get it swept before use.

I had read somewhere that you shouldn’t increase in size by more than 2" or you may get adverse effects on the stove operation or the draw, that’s really why I’m asking.

Thanks both, Robert

[G and J]

We have three Clearview stoves, an 8kW connected to a 6’ insulated flue, a 5kW connected to an 6’ insulated flue, and another 5kW connected to am 8’ clay liner (this installation would not be allowed now).  All work exactly the same, the control on the fires set how they work as long as they have a decent draft.

 

When the fires are burning properly the flow rate is very small, which is what delivers their efficiency.  We had a Dovre previously that simply couldn’t work like that and asa result used twice the wood.  So I assume that the flue temperatures are much lower with my current fires than they would have been with an open fire or an inefficient stove.  I think that means that whatever size liner you have if burning properly they won’t get that hot, so I see no issue in continuing with your 8’ flue.  That’s certainly what I’d do in your position.

[ruperthebare]

Thanks. I’ve come to a similar decision