Hang on. I would seriously check the heater though. Typically it should be checked and cleaned every year or 6 months for a old heater. Best to be safe and sorry. I had to do it with my cooker just recently. It's very old and very easily to get to a stage where it could release that nasty undetectable stuff.

As for the Mac D thing. Perhaps it's gotten worse because you're not there to supervise >:)