If you look at the manual for the boiler and compare heat input and outputs yoo shoudl be able to calculate it. I did this once an dfrom memory it was about 80% but it was a long while ago and I cant remember exactly. The manual may be downloadable from the glow worm site. If you are thinking of the bloiler scrappage scheme you have to be below70%?
Rob the plumbing lecturer
February 2010
Most of the Glow Worm Fuelsaver boilers are rated G - below 70% but a few are above this
Fuelsaver F is insufficient to be sure - here's a link to a table...
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix convector heaters.