Why would pool equipment explode?

I assume from your question that by pool equipment you mean filter. A filter might explode if it had air trapped in it and was not released. Air will compress. A filter has air in it, the pump is turned on and water begins to fill up the filter and circulate through to the pool. The filter can not fill completely because the air is at the top of the filter. The air gets compressed further and further until it can compress no further and the top blows off the fitler. This scenario would be due to a fair amount of air in the filter (not small quantities).. The air should eventually be pushed through to the pool wwhere you will see it bubble up until the air is out of the system and the filter is full or subsatantially full. Sometimes for many reasons this does not take place and Boom! Another reason for a filter exploding is if you have a filter with a band clamp around the center holding (clamping) the top half of the filter to the bottom half of the fitler. The cause I have seen for this is that the washer that is on the bolt to secure/tighten the clamp ring with has been dropped/lost. The nut pulls through the hole and the top of the fitler flies off. If you are talking about a heater exploding then of course I would consider gas as the first culprit. If it is any other piece of equipment then please let me know and I will see if I can be of further help. Steve Dunn Commercial Pool Systems, Inc. Refillling the filter with water should be done with the valve in the "backwash" position to safely purge air from the filter.