Why would putting a lid on a pot which is cooking reduce heat loss?
Water will boil faster with the pan lid on. Keeping the pan lid
on traps some of the heat and steam in the pan which would
otherwise escape, thereby increasing the temperature, and bringing
the water to a boil more quickly. To keep a full pot from boiling
over, be sure and keep the side of the lid farthest from you
cracked just slightly, so a little of the steam can escape.