How could you suddenly lose all hot water pressure without losing any cold water pressure?

Answer 1

Primarily this is caused by some other user of hot water. Like the cloths washer or dishwasher turning on. They drain the hot water available and so your shower goes cold.

Flexible PVC pipe

Hot water boilers contain a reduced orifice insert (presumably to maintain pressure in the boiler. These can become clogged with deposits that build up over the years. Any recent repairs of your water supply, can aggravate the situation providing contaminated water to add to the deposits. You probably need to clean or replace this orifice.

answerIf the previous answers have not helped solve the problem, try this: What is the source of energy that lifts and drives the hot water from the tank to your fixtures? It's the cold water, under pressure, pushing INTO the tank, that pushes the hot water OUT. If EVERY cold water outlet runs at full pressure, then start at the cold water feed at the heater. The union at the top of the heater will tell you if the water is getting there. The pressure pushing out of the water heater drain at the bottom will show you if the dip tube is o.k. (IF the drain itself is not plugged). The union on the hot line at the top will tell you the energy from the cold water push in is doing the job pushing the hot water out and if the problem is further downstream, and so on......by the way, how old is the heater? The first four digits of the serial number usually tell you. i.e. - A098745362 would be a heater built in September of 1987. Good luck.

Answer 2

One possible reason is a clog or blockage in the hot water line or water heater. This blockage would specifically affect the flow of hot water, leading to a loss of pressure, while leaving the cold water line unaffected. Another possibility could be an issue with the hot water valve or a malfunction in the water heater itself, causing a decrease in hot water pressure.