For the past three weeks my unit has been experiencing intermittent loss of hot water in both bath and kitchen. The hot water was almost off all last weekend, and it shuts off every two or three days for unknown duration. Most recently, Friday evening, Feb 19th at approximately 7pm, I discovered there was no hot water. This Saturday morning, at approximately 10am, I came home to shower after a walk and there was no hot water. I am alerted to the imminent loss of pressure on the hot water side when I turn on the faucet full hot and feel for warm water starting to come out. Normally, it takes a few seconds for the hot water to reach its highest temperature. If the water stays cold, and actually seems to run colder, I have learned to expect a hammering sound in the ceiling of the bathroom, and the hot water shuts off.
I suspect that the mixing valve located in the bath ceiling is shutting down because of low pressure in the hot water circulating system. The points of low pressure are intermittent because the areas of low pressure are determined by the usage of that particular moment. If there is a rise of hot water usage in a cluster of apartments, pressure goes down, subsequently closing the mixing valve mechanism to only allow cold water to downstream apartments. This may be why it is difficult to pinpoint hot water problems at any one time. A possible solution is to identify relevant sites along the hot water circulating system and install one or two small booster pumps to maintain the proper pressure along the entire line even during times of heavy usage.