Blocking off an unused (never used) chimney in a new house?

[dhutch]

Is it acceptable to block off the top and bottom of an unused chimney in a new house without providing ventilation to the space?

Looking online a lot of guides recommend fitting a ventilated cowl and the top and a louver at the bottom, stating that the soot and associated chemicals cause issues if sealed in, particularly in older older pre 1965 houses but obviously in this case there is no soot or chemicals present as its not been used.

The house was built by my parents in the late 80's and since then they have only used one of the two chimneys in the house. The room in question has never had a fire fitted and hence that flue/chimney is unused, for the last 25 years has been 'temporally boarded over' with plasterboard.
They would like to make this more permanent before redecorating as the current solution is not fully airtight at the bottom and the airflow leaves a mark along the edge of the carpet. The top could/would then have the pot removed and capped over with slate/lead or similar. There is a second flue alongside which has also never been used and would/could be capped off in a similar fashion at the same time.


Daniel

 

[dhutch]

No, no comments at all?

Obviously if you blocking off the bottom and leaving the top open you may well get damp issues, and if there is old soot etc in there you may get chemical issues along side that.

But if its fully sealed and waterproof at the top and bottom, its just a two-story unventilated cavity.


Daniel

 

[jeds]

It's impossible to say for sure without knowing the exact construction details. Going back to basic principals; the risk is that vapour entering the flue condenses on the surface. Two ways to stop that; dilute the vapour and keep it moving - that's venting - or, keep the temperature above the dew point. If the outside wall of the flue is sufficiently insulated then you might keep temperature sufficiently above the dew point. But even then you couldn't guarantee what happens when the heating is off. If it's not insulated then condensation is a dead cert. Personally I'd ventilate, It's safer.

 

[tony1851]

ventilated cowl and the top and a lover at the bottom,


Daniel

Never thought of that; a lover at the bottom might raise the temperature and so aid ventilation

 

[Alastairreid]

Are the flues on an external wall?

 

[dhutch]

It's impossible to say for sure without knowing the exact construction details. Going back to basic principals; the risk is that vapour entering the flue condenses on the surface. Two ways to stop that; dilute the vapour and keep it moving - that's venting - or, keep the temperature above the dew point. If the outside wall of the flue is sufficiently insulated then you might keep temperature sufficiently above the dew point. But even then you couldn't guarantee what happens when the heating is off. If it's not insulated then condensation is a dead cert. Personally I'd ventilate, It's safer.

Are the flues on an external wall?

Thanks for the reply.

The flue is in the center of the house, with rooms on both sides, and hence is heated until it passes above the 1st floor ceiling, where it emerges in a valley in the center of the house.

And the risks of condensed vapor are mainly damp patches on the walls?


Daniel

 

[Deltamale]

You can put a vented cap on the pot, but what you do with your 'lover' is up to you .

No seriously, cap the pot but do not put a louver in the room it served for the very reason you state, toxic fumes from old soot. If you put a louver vent in do it in the roof space. In that way you keep the stack dry in the area most likely to get wet if the rain gets in i.e. the stack itself.

Is the flue on an outside wall, if so you could fit a brick vent low down on the outside wall to vent the full height.

 

[jeds]

do not put a louver in the room it served for the very reason you state, toxic fumes from old soot.

It's a new chimney. Never been used.

 

[Deltamale]

Then not a problem with fumes and a hit-and-miss vent could be used as room ventilation if you wanted (though I have always found they just look terrible and get dirty so got rid of mine). If ventilation not wanted in room, put a brick vent in roof space to keep stack dry.

Still needs vented cap on pot, think of bucket left out in the rain and just how much water it fills up with after a heavy shower, that's what goes down the chimney and needs to be held at bay.

 

[dhutch]

If you put a louver vent in do it in the roof space. In that way you keep the stack dry in the area most likely to get wet if the rain gets in i.e. the stack itself.

Is the flue on an outside wall, if so you could fit a brick vent low down on the outside wall to vent the full height.

As the stack comes out in the middle of the valley there is very little visible in the attic space, I am not at the house myself right now so cant confirm if there would be room for an air brick or not. Obviously however one could be added to the base of the stack just above the roofline, but from the advice here I must say I am leaning towards capping the whole lot of 'air tight' as much as any brick structure will be.

The stack is in the middle of the house as above, so no chance of a air brick on the outside wall, but also far less chance of condensation etc.

I have always found they just look terrible and get dirty so got rid of mine.

Thats the reason we dont want to put a vent in the room, the house also has a whole-house mhvr unit fitted so the ventilation to the room is also undesirable.

Still needs vented cap on pot, think of bucket left out in the rain and just how much water it fills up with after a heavy shower, that's what goes down the chimney and needs to be held at bay.

Fully understand this, and certainly a vented cap must be better than it being open as it is currently, just trying to make sure it will be ok to totally cap it off, which it seems it will be.

Thanks

Daniel

 

[Deltamale]

I have on occasion seen a few stacks with a brick vent above the roof line but not sure about it.

Right so you can't fit a vent OK, so make sure the stack's pointing is good and the flaunching (the mortar on top of the stack around the pots) is sound with no cracks, renew if necessary. Then fit vented cap, better to have some ventilation then seal in any moisture that might get in.