Ventilation requirement for fireplace + electric fire and old flue liner

[JohnInBedford]

I’ve recently had a back boiler removed, and am going to get an electric fire and mantelpiece put over the front of the chimney breast.

What I’m trying to figure out is what ventilation I need. The chimney still has the liner in it from the back boiler, with a gas liner CG1 terminal on top. I believe I need to get a vent cap fitted on top of the chimney, which seems straightforward.

But how much venting do I need at the bottom, where the fireplace will be? The fireplace won’t be airtight, so is that enough to get some airflow through the flue and prevent any damp build-up? Or does it need a dedicated vent of some sort to be incorporated?

The fireplace fitter seems relaxed about the fireplace providing enough ventilation as-is, and also points out a vent that draws a lot of air through into the chimney will work against the electric fan heater. But it would be helpful to get input from someone who specialises in the chimney / flue / ventilation side of things.

(I’ve seen similar but not identical questions raised, but none that provide a clear-cut, specific answer to this scenario; but obviously happy to be signposted to one if I’ve missed it )

 

[RCr]

I'm not an expert, however as a CG1 terminal has a closed top, rain shouldn't get into the chimney. I'm therefore not sure you need a vent at the bottom as the flue will stay dry anyway.

Unless air from inside the house can permeate through the chimney wall and condense on the cold surfaces within?

 

[blup]

External vent, low down?