How efficient is my oven at heating my apartment?

What if you left a (properly vented) gas oven door closed while using it for heating? You can roast a turkey for hours with no ill effects.

Ever feel headachey and lethargic while you were using it? That was the initial stage of CO poisoning.

Carbon Monoxide

Apartment building with giant central hot water heater that provides hot water to all apartments. They pay for it as part of rent (condo fees, etc) but it is not metered or billed by usage.