Air brick question

[nonymouse]

Do I need one? I have two chimney breasts in my living room, one is still opened and used occasionally - the other is not. Neither is it capped off at the top so I presume it's either filling with rain water or moisture. So in my infinitie wisdom I bought a vent that apparently circulates 10"=B2 of air (whatever that means!) with vents that can be closed (presumably if it becomes too windy). The chimney breasts are around 30 ft tall.

I have a reasonable amount of diy expereince but understand little about how buildings actually work and would really appreciate some advice from someone better qualified than me (which is pretty much anyone actually).

Thanx - Bear

[Codswallop]

"nonymouse" wrote: Do I need one? I have two chimney breasts in my living room, one is still opened and used occasionally - the other is not. Neither is it capped off at the top so I presume it's either filling with rain water or moisture. So in my infinitie wisdom I bought a vent that apparently circulates 10"² of air (whatever that means!) with vents that can be closed (presumably if it becomes too windy). The chimney breasts are around 30 ft tall.

I have a reasonable amount of diy expereince but understand little about how buildings actually work and would really appreciate some advice from someone better qualified than me (which is pretty much anyone actually).

Thanx - Bear

[ =========== ]

In my opinion there should be an air brick in the chimney breast that has been bricked up.

[nonymouse]

The chimney hasn't been bricked over at the top or capped - does this make any difference?

[Brian Reay]

I have a reasonable amount of diy expereince but understand little about how buildings actually work and would really appreciate some advice from someone better qualified than me (which is pretty much anyone actually).

Last time I checked, the advice was to have an air brick is the top (outside) wasn't capped and the lower end (inside) was bricked up.

Brian

[Codswallop]

It would help to dry out the bricks and mortar (which will be wet when it rains) if there is a current of air ascending the chimney. Otherwise you run the risk of damp problems and deterioration of the masonry.

[nonymouse]

Interesting - I already have mortar problems on the chimney breast which is open. Nearing the roof, the mortar has gone soggy and one or two bricks have had to be replaced. The chimney which is not used however, has been fine with no such problems - although the adjacent wall has suffered detioration of the plaster (gone very soft). Is there any suggestion of what size of vent is required?

[Codswallop]

By vent, do you mean a ventilated chimney-top cowl? This might help:

[Weatherlawyer]

No you need to stop rain entering the hole at the top without shutting it off completely. This means cementing a slab on the vent and having some overhang so rain runs off and leaving a small vent in the mortar, preferably on the side out of the predominant wind direction.

But going up there to do that, one may as well take the chimney down below the roof and close the roof. In the meantime all the warm air in the rooms below, is being vented up the chimney.

[Andrew Gabriel]

A chimney needs venting top and bottom, so there's a through-draft. It is not normally necessary to cap off a chimney (although it might slightly extend the life of the exposed outdoor part). Keeping it vented at the top is much more important than capping off certainly. If the chimney is on an outside wall, I would suggest venting the bottom to the outside rather than the room, so it doesn't draw heat from the room. Ideally, the vents shouldn't be closable because someone might close them ;-)

[Brian Sharrock]

I agree with Andrew; consider: a brick lined vertical hole open to the top is frequently called; - 'a well'. Keep your 'well' dry and not damp -or wet- by permitting air to be funnelled through it, the top is thirty feet(?) above the bottom which will give a 'suction' effect.

[Peter Taylor]

Hang on Andrew. I accept this is standard text book advice, but I've never actually been able to understand the thinking behind it. What's likely to happen if you block up a flue top and bottom? Condensation? To my mind it just creates a sealed cavity, probably like the rest of the wall, and condensation in cavities doesn't cause problems. Perhaps that's because the cold surface is on the outside skin.

I'd feel happier with your advice if the ventilating air was cold, with low RH. But, to my mind, allowing warm humid air from inside a room to travel up a cold flue, depositing its vapour on the way, is likely to make the problem worse. And if the flue is an internal one, with warm brickwork all round, it's unlikely that much condensation will form anyway, and ventilating it is just a way of losing heat.

Agreed. It could be added that the chimey pot should be capped with a ventilating insert to prevent rain and birds coming down the flue.

Peter

[Mark]

But cavities are not sealed; at least not the older type built with fire places and chimney breasts. They are open at the bottom were the air-brick is in the outer brick leaf and also at the top in the loft. Also the hearth, and the external chimney breast were it exits the roof often have no DPC, or even if it does they are often bridged by tar and soot deposits. So sealing a chimney breast top and bottom is IMHO a bad idea.

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[nonymouse]

Many thanks for your input but I have a further dilemma.

My house was built circa 1901 and has two chimney breasts. Presumably, each would have originally had an open fire on the ground level and a further one on the first floor (one of which on the ground floor is still in operation). In the attic, these two breasts meet from either side of the house before exiting through the roof.

The fireplace which is still in use exits through one of the four chimney pots.

Having read all of the suggestions, I had decided to place one air brick (well, technically not an airbrick but a bunch of holes drilled through the brick with a vent in front) in the bricked up chimney breast on the ground floor. A second air brick would be placed directly above it in the attic.

...and now my problem:

how can I tell whether there is simply one cavity in the chimney breast between the ground floor, first floor and attic meaning placement of the vents is not critical; or two separate cavities meaning that the attic air brick may simply ventilate the first floor flue but not the ground floor?

I have trawled the Internet for this information but have not been able to answer my own question...

[meow2222]

IIUC youve got 4 fireplace and 4 chimney pots, so you'll have 4 flues, and need a vent on each floor. Venting to exterior rather than interior will reduce heat loss.

NT