Treato found 16 discussions about Cold Symptoms and Cats Allergy on the web.
Best Answer: When the toilet is flushed, the toilet is robbing pressure from the shower, which is usually close to the toilet. As the COLD water pressure goes down from the toilet flush, the HOT water pressure is higher, therefore the water gets hotter as the COLD pressure goes down.
If the plumber had the shower turned on as one of the hot water faucets, it could backfeed cold water into the hot water system as there's no pressure in the hot water system to prevent it.