Why does my unstop coal-effect gas fire pass sour fumes on rainy days and is within a cure? I have a Portway start coal-effect fire with a 6-inch chimney flue...

I have a Portway start coal-effect fire with a 6-inch chimney flue next to the correct cowl on top.
However, on damp/foggy days, we get a strong downdraft even when the fire is departed on full for more than 15 minutes. This downdraft brings powerful nausiating fumes and eventually sets off our carbon monoxide alarm. This effectivelly renders our fire useless on the sunshine we need it most.
Can anybody give support to?
How do we find someone to check out...
Answers:    You probably need something higher than your chimney pipe to help prevent the downdraft. A hasty search found this example:
http://www.superiorchimney.net/html/vacu...

Someone at the hardware store can probably pass you more specific help.

Do this since you use your stove again, as Carbon Monoxide can be deadly.

This could also be a sign that you hold other air leak in your home. If the upper air is coming into your house via the stove chimney, it's leaving your house somewhere else. It could be leaky window on the downwind side of your house, or it could be dryer or bathroom vents.

This same site have a whole page that deal with your issus:
http://www.superiorchimney.net/html/chim...