Hello: I have no idea what the physics would be behind opening a door and having the heater fire up 😛 Aside from that, let’s untangle the problem a bit. Things that come to mind as possible problems are: damaged dip tube and cross connection. Also, if there is a recirculation line, it could play in too. Have a look in “tanklets” to get thoughts on how to check for those two/three problems and let us know what you find. 😉
Yours, Larry