Why do flue lined chimney breasts not require ventilation?

[Oxbow16]

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

[Dave Jones]

new build pumice lined chimney stacks are insulated with vermiculite so do not require ventilation.

 

Same with using the tin liners, they are insulated.

[Oxbow16]

Thanks for the reply.  In our cases we both have old houses, so are referring to flue lining old chimneys with 316 or 904.  They are quite thin steel and can't imagine they would be all that insulating.  But regardless of how insulating it is, there is still a chimney void - quite large I'd imagine in some old houses - that is unventilated.  Anyone know why that is not considered a problem?  

 

Cheers

[Dave Jones]

was referring to new build, there is no lined stack with an insulated liner. you can put a fake chimney breast in for aesthetics etc

 

Old houses ive seen with capped off pots often have a couple air bricks in the stack at pot level. The houses are leaky enough to provide air flow i guess.

[karl mcilwain]

On 29/03/2023 at 09:11, Dave Jones said:

was referring to new build, there is no lined stack with an insulated liner. you can put a fake chimney breast in for aesthetics etc

 

Old houses ive seen with capped off pots often have a couple air bricks in the stack at pot level. The houses are leaky enough to provide air flow i guess.

 

[karl mcilwain]

During a refurbishment I’ve removed an old gas fire from a Victorian house. It had a tin gas flue which I left in place. During the winter I’ve noticed that there is brown water coming from inside the flue and dripping on the fireplace plinth. It doesn’t necessary happen every time it rains, in fact the first time I noticed it was middle of winter when I had the heating on. The tenant is complaining about this. 
is their anything I can do to stop the water coming out of the flue liner or reducing it. I have tried taping a heavy duty black bag under the flue but with no joy.

Thanks for reading

[Dave Jones]

Condensation more than likely.

 

get on the roof, take the pot off and rip the liner out. Seal the chimney with a slab and if you feel inclined chop in an air brick.

[karl mcilwain]

Thanks but was hoping to keep the flue liner incase needed at a late date

[Dave Jones]

23 minutes ago, karl mcilwain said:

Thanks but was hoping to keep the flue liner incase needed at a late date

 

put a fan (like once of the crappy through the wall bathrooms ones) into it at the hearth and let it run for 10 mins every 12 hours to get a bit of airflow to dry it out and keep the condensation out.