What is wrong if your pool heater starts with a resounding boom as if the gas flow starts well before ignition occurs?

on my old heater I found that it was installed in a metal shed. When the door was closed I got a boom because there wasn't enought combustion air. I istall screened vents by the front of the heater and now no more problems.

Those screened vents must be of a certain size see the heater mfgr's absolute rule on this - so many sq. in. per btu of heater size and installed one at a high elevation in the room, one at a low elevation and at opposite sides of the room. Have a qualified tech inspect and correct the ignition problem.

k