Could you possibly store your powder and primers inside some place thats climate controlled?
Gas water heater if not of the sealed combustion type put out alot of water vapor when buring,most of this vapor is vented out the roof but not all. Plus you have humidity entering to room from outside.
Do not seal the room up unless your water heater is sealed combustion that gets its make up air from outside via a coecentric vent. Most of these type heaters are tankless.
Be careful around that water heater with that powder. I dont know your set up but I would hate for you to have a powder spill or the heater malfunction with powder close by.
Stay safe my friend!