Blocking up two fireplaces.

[Jasontenn]

I have a 1930’s semi which I am currently renovating. There are two fireplaces, one in the living room and one in the dining room. Both these rooms share the same chimney which comes down through the main bedroom and then branches off at celling height in the living room into the two separate rooms.

I am removing the fireplaces from both (the living room had an old gas fire and dining room an electric fire and blocking them both up.

My question is, do I need to put an air brick or vent of some description and if so does it have to be in both blocked up fireplaces or can I get away with it being in just one of them?

 

[Harry Bloomfield]

My question is, do I need to put an air brick or vent of some description and if so does it have to be in both blocked up fireplaces or can I get away with it being in just one of them?

Yes, and both needed.

 

[mrrusty]

This comes up so often. The question really is "what do I want to achieve". A chimney will have a natural updraft if open at the top due to the venturi effect of the wind over the open stack, so in winter, if the stack is open at the top, you draw air at 20 degrees and about 60% humidity through the vents, up a chimney which could be 2-3 degrees at the top and well below the dew point, causing liquid condensate in the chimney. In most cases, not enough to cause a problem as long as the chimney is used occasionally with a fire in the bottom to regularly dry it again.

So block the top completely, as you normally should for an unused internal chimney. Now there is no air movement, so what is the point of vents at the bottom? where is the vented air going to go?

The only time a vent could conceivably be useful is if there is penetrating damp into the chimney. Now it would make sense to just cowl the top, and have a hit and miss vent at the bottom, so when there is little risk of condensation due to introduced air, the air movement through ventilation could help in evaporating any penetrating damp.

So the question really starts with "what am I trying to achieve"

 

[ree]

OP,
Best practice in the building trades IOW's how its done by experienced trades people is to first sweep both flues then install hit&miss vents in the blocking, and insert redundant flue cowls into the stack pots - ie. each flue is vented bottom & top.

Do not leave the chimney pots open or birds etc. might enter.
Remove any flex SS flue liner - also remove any register plate.
Due diligence by qualified professionals for gas & electrics.