If its a Baxi Brazilia wall heater you are talking about with a whitish cover that lifts off 4 studs these DO NOT have any safety device fitted to them.Quite strange really as all other appliances of that era had to have them.Sounds like your problem is flue related possibly snow in the flue or iced up.Go outside and sweep all the snow from it and then try again.
Tony S, February 2009