Mick, do you have a manual for your furnace? If not, you can try here:
It's down towards the bottom of the page - says
"Hydroflame FA-72, HF-77, FA-79, DC-82, and AC-82 series service manual- Dated Jan, 1985."
Try downloading that 1.8MB .pdf file and see if it shows you what you need to know. It may contain other info that you can use, too.