Chimney balloons - how to measure & fit ?

[medianewbie]

I have a very cold Victorian sandstone house. Main fireplace has had insert removed. Had it swept but it blows back (poss wrong pot cap on?). Can't afford a log burner fitting (v high chimney would need scaffolding). Need to block it up as draught horrendous. How do I measure for a chimney balloon - any tips about which kind is best please ?

Our chimney sweep recommended an old pillow in a black plastic bag, which has worked v well for us (attach something to dangle down so you don’t forget it’s up there!) Any of the things you can buy he reckoned were either impossible to measure accurately / not as effective or not worth the money

We also do the pillow in a bin liner trick which works absolutely fine. That was because I couldn’t work out what size of a proper thing I needed to get…

I saw this on Dragons' Den and thought it was genius. https://chimneysheep.co.uk/collections/chimney-sheep

Thank you ! Old pillow in plastic bin liner = cheap & easy to replace. Perfect.