True. The idea is to apply heat only to the top of whatever you're cooking, not heat up the entire oven. With the broiler element hitting 1000 deg F pretty quickly and staying there, the entire inside of your oven would heat up pronto with the door closed. Also, you should be standing there watching whatever's under the broiler while it's cooking, so if it catches fire you can deal with it asap - easier to see if the door's open. There's probably some sort of combustion deal where if your food catches on fire with the door closed, the flames could surge when you open the oven door from the additional O2, but not really sure about that.
Doesn't really apply for gas oven broilers where it's a drawer underneath the bottom gas jets that slides out though. With those you just have to keep constantly checking progress.
― Jaq, Saturday, 30 January 2010 04:15 (fourteen years ago) link