Oh dear things are getting complicated.
That clay vent only provides about 50% surface area ventilation due to the thick pieces of clay between the holes.
So, you tend to find that a modern plastic air brick of half the size is equivalent in actual air space.
Or knock it out and cover the top half of the gap with a fine mesh (aluminium plaster patch is the right size and saves buying a whole roll of ss mesh). You just need to fashion a support frame, or mould the mesh corners into the gap and silicone around it. Then a coat of paint if you like.
Keep it simple.
That clay vent only provides about 50% surface area ventilation due to the thick pieces of clay between the holes.
So, you tend to find that a modern plastic air brick of half the size is equivalent in actual air space.
Or knock it out and cover the top half of the gap with a fine mesh (aluminium plaster patch is the right size and saves buying a whole roll of ss mesh). You just need to fashion a support frame, or mould the mesh corners into the gap and silicone around it. Then a coat of paint if you like.
Keep it simple.
