Why do flue lined chimney breasts not require ventilation?

[carbs for arbs]

Hi

 

I was discussing this with someone yesterday and we were both puzzled...  

 

It's normal practise to ventilate an unused fireplace/chimney, with a vent brick or grille at the fireplace and some sort of vent at the cowl.  Yet if that same space was being used, but with a flue liner going through it, why does the remaining space around the outside of the flue not need to be ventilated?  Especially considering the fluctuations of temperature going on inside the space...

 

Could anyone explain?  

 

Many thanks

 

[Squaredy]

Surely the purpose of the vent is to reduce the likelihood of damp in an unused flue by creating a through draft. If the flue is in use the problem is solved naturally.

[carbs for arbs]

I'm not massively familiar with stove installations.  But presumably the flexi flue is sealed to rigid flue at the stove, and likewise somewhere near where it exits the roof.  With a closure plate installed above the stove, and some kind of closure at the top where the flue exists (or where the cowl is if there's a chimney), and where/how is the cavity being ventilated?  I can see that the innards of the flue are ventilated by being open both ends, but the rest of the chimney cavity?

 

I do apologise if I'm missing something obvious!

[trigger_andy]

28 minutes ago, carbs for arbs said:

I'm not massively familiar with stove installations.  But presumably the flexi flue is sealed to rigid flue at the stove, and likewise somewhere near where it exits the roof.  With a closure plate installed above the stove, and some kind of closure at the top where the flue exists (or where the cowl is if there's a chimney), and where/how is the cavity being ventilated?  I can see that the innards of the flue are ventilated by being open both ends, but the rest of the chimney cavity?

 

I do apologise if I'm missing something obvious!

Just thinking out loud here. Above the Stove is a register plate. The flexi is hung off of the top hat at the chimney. I doubt think either are gas tight. With the flexi being heated to quite high temps the breast will be getting warmed as well and will naturally vent any moisture. 

[Squaredy]

10 hours ago, trigger_andy said:

Just thinking out loud here. Above the Stove is a register plate. The flexi is hung off of the top hat at the chimney. I doubt think either are gas tight. With the flexi being heated to quite high temps the breast will be getting warmed as well and will naturally vent any moisture. 

I think Andy has it spot on here.  The vent is for disused flues, which often get damp over time.  Any flue in use even if not daily will get a lot of drying and heat and therefore damp is not usually a problem.

[Dan]

I had an insulated double wall flue fitted in a thatched house. It had to be vented in the room above the stove and also 2 vent bricks in the chimney stack above the roof line. As per hetas regs at that time. 

[Guest Gimlet]

As a bricklayer I can tell you that the flue void where a liner has been fitted DOES need to be vented, top and bottom. 

For one thing, humidity from within the house will permeate the masonry and become trapped without venting, leading to chimney sweat when the stove is not in use and the flue cold. Also, soot deposits and cobwebs in an old brick flue remain a fire hazard so heat within that void needs to be vented out. On many older buildings wooden floor joists are often pocketed into the brickwork of the flue as well. If your liner insulation should fail or become detached of the liner itself become split or holed a hot-spot can be created around combustible material.

 

I've rebuilt dozens and dozens of chimney stacks, many on listed buildings and thatched properties and where clients checked their fire insurance requirements, it was demanded that all liners be insulated and the brick flue void vented at the register plate on the ground floor and at the terminal of the stack just below the pot. On listed thatched buildings some conservation officers also insist on fixing heat shields made from quarter inch aluminium plate around the stack where it is in contact with the thatch.

For ventilation I have always built in an air brick on the opposite side to the prevailing weather directly underneath the first corbel course of the terminal where it will be least visible and least likely to admit wind-blow rain.

 

Many people are not aware of stack heights required by fire regs either. On thatched houses the full height of the stack to the top of the pot must be at least 6'/1800 mm above the highest point of the adjacent thatch, and of that measurement no more than 600 mm can be the pot. So if you've got a low stack you can't just bed a massive pot on top.

I've seen lots of stove and liner installations in chimney stacks that were far too low. If they ever had a house fire their insurance would be void.

Edited April 2, 2023 by Gimlet