13/10/2020
Why a flue should be lined when fitting a modern stove:-
This is why a modern stove should not be fitted to an existing (traditional) brick or pot lined class 1 flue without a stainless liner.
For the sake of figures let’s say the room needs 4kW’s of heat. Be that’s electric, gas etc
The old open fire is 20% efficient, so 4kW into the room and 16kW of heat escapes up that big brick or pot lined flue and keeps it warm. As it’s warm nothing will condense and it draws well.
The modern stove is 80% efficient. So 4kW into the room. 1kW up the flue.
So instead of 16kW escaping up the flue there’s only 1kW of heat using a modern and efficient stove. That’s a saving of 15kW 😊. At full output that’s 15kWh of energy wasted every hour if you had an open fire 😳
That’s a tremendous saving, so what’s the catch?
That super efficient modern stove is only putting 1kw of heat into the flue, at full output!! It runs cool compared to an open fire. Put that into a big brick or pot lined flue and tars and resins (from wood) will condense to form creosote on the cold walls and condensation (also from solid fuel) will form.
A liner is much smaller and will heat up quicker to an acceptable operating temperature. Better still if it’s back filled with vermiculite. There can be down sides to back filling tho, it’s not always the best answer.
You can draw an analogy with having a shower in a cold bathroom and condensation sticks to the walls, windows, mirror, toilet etc. Open the window when you’re done and given time that will all evaporate. With the stove in the big brick or pot lined flue/chimney and once those tars and resins deposit in the flue they won’t go until mechanically removed via sweeping or an expensive chemical treatment (£250 at time of writing)
The converse is having a shower in a hot bathroom, the steam doesn’t condense on the warm surfaces. This is the modern efficient stove running at the correct temperature in a stainless liner (or twin wall insulated standalone flue system)
The difference lining correctly a class1 (traditional brick or pot lined) flue makes is astonishing.
To give an example a stove used daily over winter in a small two bed old cottage running in a class 1 (brick) flue when swept the soot/ tar would fill a bucket and the vacuum. That same stove was then lined and the soot/ debris would probably fill two pint glasses!!
