Vent Chimney flue externally?

[Benus333]

Hi
I've just had a bricked up fireplace opened up to put a range cooker in. There will be an extractor hood above the cooker, built in flush and ducted out to vent externally. This won't help to ventilate the flue which will be sealed at the bottom now.

Is there any reason why I can't have an airbrick to vent the bottom of the flue externally so I can avoid having an internal vent spoiling the look inside? Or does it need to be warm internal air venting the flue. There is a cowl on top to stop rain and allow ventilation.

 

[KenGMac]

Benus333, good evening.

Was there an internal air brick on the face of the old bricked up chimney ?

Siting an external air vent will work, but not a well as an internal air vent with as you point out "Warm" air which will tend to rise faster than heavier external cold air will. hoe about you consider fitting 2 air bricks externally??

One consideration is that in all such chimneys, over time a great deal of dust, dirt soot will detach from the internal walls of the Flue and drop to the bottom of the stack, at times this detritus can build up to a degree that the air brick is simply blocked, further how do you get at this build up and clear it out?

Ken.

 

[Benus333]

Thanks for the reply Ken. There was a missing brick at the bottom internally, covered by a grille. The builder has now installed the extractor which vents externally via flat ducting to a double airbrick. He has not connected the ducting to the airbrick with an adaptor, it is just pushed up against it. (I asked whether the ducting needs to be mortared in to make a seal but he said it will be fine.)

The ducting doesn't cover the whole area of the air brick, so there is in theory air moving in and up the chimney I hope. It's quite a tall chimney (3 floors, high victorian house) so I'm wondering whether it's best to add an internal vent. The builder came recommended by a well connected tradesman I know personally, so I trusted what he was doing. Now I'm not so sure...

 

[bobasd]

why dont you post pics of inside and outside showing any work so far and the air bricks?

 

[Benus333]

Good idea Bobasd! The external air brick is just about visible in the dark. It's a standard clay double air brick. The ducting is 220x90mm flat ducting. Thanks. It would be pretty straightforward to add an extra vent into the plasterboard between the extractor and the new lintel, which is 40cm above, but obviously would affect aesthetics.

 

[bobasd]

thanks for the pics.

i can't see any detail in the brick photo - what i was looking for was a context showing the height and position of the air brick that is venting the bottom of the flue?

i cant see where the flat ducting leaves the chimney breast either or where it goes, and how its terminated?

 

[Benus333]

The ducting comes out of the extractor and elbows 90 degrees out through the back of the chimney breast and external wall through the air brick. It isn't connected to the air brick with an adaptor, it is just shoved right up against it. The height of the airbrick is about 400mm above the extractor.

This is the only airbrick - as the area of the ducting is half that of the airbrick, and as there is no 'seal' anyway, air can get into the flue from the outside and ventilate the flue when the fan is not in use. Whether that is sufficient to prevent damp is what I'm wondering. I'm also wondering if the possibility that any moist extracted air that gets into the flue is going to be enough to cause problems. The builder said it will be fine and the fan is powerful enough to drive it away.

It's a single skin stone wall and the back of the breast is brick. A brick was removed for the ducting to pass through.

 

[bobasd]

I still cant visualise what you've got but it sounds wrong.




Hopefully someone else will pick this up and help you better than me.

 

[Benus333]

Thanks for trying!

 

[Nige F]

Good idea Bobasd! The external air brick is just about visible in the dark. It's a standard clay double air brick. .

that is no good for a powered vent to try and blow through. The holes you see diminish in size when they get to the inside. The airbrick wants taking out and replaced with modern plastic ones -2 will fill the hole. Then the vent duct will have a good chance of blowing out.

 

[KenGMac]

Benus333.

Agree with Nige F above.

What will is going to happen is that the discharging warm very moist air venting from the open end of the ductwork will to a varying extent be deflected back into the flue, that is not where you want to "dump" warm very, very moist air, if you do the air not venting to the air externally will condense on the inside of the old flue, this will in turn cause all sorts of issues in the future.

As all above, get the builder back, have him install a correct duct termination to the ducting, make sure he fits a proper weight loaded flap cover.

You can then consider the ventilation of the flue itself, suggest you consider installing at least one and possible two air bricks, but site them below the duct terminal, why? warm air rises, so if the flue vents are at a lower level there is minimal chance of sucking warm damp air to where it is not wanted.

Ken.