Do I have to ventilate a capped chimney, isn't it just like a cavity wall?

[foxhole]

Thanks yes looks like that's it.

But... so why do lined chimneys blocked with register plates at bottom not need air bricks?

If you’ve lined the chimneys, then you are using it to transfer heat up the chimney which does the same thing as ventilation.

 

[ChrisKingston]

If you’ve lined the chimneys, then you are using it to transfer heat up the chimney which does the same thing as ventilation.

Within the flue, but what about the void around the flue inside the chimney?

 

[Iwilldoit]

Within the flue, but what about the void around the flue inside the chimney?

That's what foxhole was saying, imagine it like having a long radiator pipe in the chimney space

 

[mrrusty]

Redundant flues should be ventilated at the bottom

They really should not! (or at least internally) I am involved with installing "chimneys" often through flat roofs. (actually Pneumatic Tube stations in hospitals, which often have the tube routed out of a ward through the roof and cross to somewhere else outside). These will be 110mm and 160mm tubes. I can guarantee you that if we don't intentionally valve off the tube so it doesn't have an open end exposed to the room, we end up with a condenser. The warm humid air from the room goes in to the open end, rises and condenses in the tube as it passes to the cool outside, and runs back down.

If the room is pressurised (e.g. manufacturing Pharmacy) we have to weight the valves to above room pressure to make sure they are closed when the system is dormant.

This is exactly the same as a chimney - an opening to a warm room which allows room air to rise to a cold area. Same problem - you get condense where the warm rising air hits the dew point.

 

[ree]

OP,
Redundant flues should be vented at the bottom.

 

[ChrisKingston]

OP,
Redundant flues should be vented at the bottom.

Yes I will leave it ventilated! Perhaps with one of those C-caps on top of the stack to reduce the heat loss.

Thanks everyone for the advice. I guess bricks are less waterproof than I thought.

 

[mrrusty]

Redundant flues should be vented at the bottom.

Show me the science.

OP
redundant flues should NOT be ventilated at the bottom internally.

In fact when we need to actually install a vertical ventilation pipe, we have to fit condense traps to catch the condense caused by the air passing from warm area to cold area https://nhbc-standards.co.uk/2018/8-services/8-1-internal-services/8-1-12-extract-ducts/

 

[ree]

.

 

[George_engineer]

Show me the science.

OP
redundant flues should NOT be ventilated at the bottom internally.

In fact when we need to actually install a vertical ventilation pipe, we have to fit condense traps to catch the condense caused by the air passing from warm area to cold area https://nhbc-standards.co.uk/2018/8-services/8-1-internal-services/8-1-12-extract-ducts/

I'd not be too worried about a bit of condensation if the chimney is ventilated. Worse is to trap it in there if/when water penetrates into the chimney or the humid air permeates through from the house.

Ventilation ducting is a bit different due to the intentional extraction of humid air and the higher flow rates.

But my highest preference is to ventilation from the outside to avoid the issue as I showed in my diagram.

 

[mrrusty]

But my highest preference is to ventilation from the outside to avoid the issue as I showed in my diagram.

Exactly!