Heartrate Monitors

Hey all, I just picked up a Beurer heartrate monitor (chest strap style with calorie counter) (PM40) a few weeks ago, and was wondering what you guys thought. Are they accurate? I plugged in my data (height, body mass, age etc.), and have seen some surprising numbers. When I first bought it out, I used it overnight to check calories, etc., while I was asleep, too. Occasionally, my HR seems to have spiked to 230-240 BPM. In retrospect, I guess the monitor doesn't deal well with extremely low numbers, and may have represented a low sleeping RHR as very high...
Additionally, on days when I do 1000C worth of cardio in the gym, plus some anaerobic workout, my daily calorie count is 7000C-8000C. Can this be right? I think my basal metabolic heartrate is around 2400. I'm a 30-year old male, 1m82/6'2”, 90kg/200lbs. Until I started back to the gym a few weeks ago, I guess I had moderate physical activity – rugby practice twice a week. Addionally, I'm a teacher, so I generally spend the day on my feet, walking through the classroom (with a surprisingly low blood pressure considering all the little jerks I put up with :p )