Instead of getting hot water when turning on the faucet with the handle toward 'Hot', cold water runs for about four/five minutes until the water starts getting warmer and then hot. My guess is that the cause of this problem is the difference in pressure between hot and cold circuits and that the two systems communicate inside the shower faucet. ( 25 years old Delta brand one-knob handle). Could this be possible?
Building is two stories, 16 units condominium.
Tanks.

Is the problem isolated to just that one Delta or all of your faucets. Do you know the distance from your unit to the hot water heater? Is there one central heater or does each unit have a water heater?

Does your neighbor have the same problem? Where is your hot water heater ? Does each unit has its own or do you have one central one ?

If central W/H than it sure looks to me that hot water circulating pump in not working properly or not working at all. Otherwise, the problem is localized in your apartment.

Instead of getting hot water when turning on the faucet with the handle toward 'Hot', cold water runs for about four/five minutes until the water starts getting warmer and then hot. My guess is that the cause of this problem is the difference in pressure between hot and cold circuits and that the two systems communicate inside the shower faucet. ( 25 years old Delta brand one-knob handle). Could this be possible?
Building is two stories, 16 units condominium.
Tanks.

I'm assuming that you have your own water heater and it's not a boiler fed one that services the building, Correct? Do you have this problem on other fixtures in your unit or just your rub/shower valve? Is your Delta a Posi-Temp or Anti Skald valve? Did this happen all at once or come on gradual? Back to you, Tom

How fast do you get hot water to you bathroom sink? If you get it relatively fast and your shower still takes a long time to get hot, your problem is most likely in your mixing valve. Need to know how the other fixtures act before I can help.

Zampano,

All the expert have responded to your question now waiting for your respond to answer their questions, and some one will tell what to do, I had some experience with this issues
Before and have installed an instant hot water Recirculation system and it work fine, I am not sure that it will help you, just respond to the questions, Thanks.

John

Water-source heat pump - I don't think it has anything to do with this problem.
Hot water - boiler, servicing seven small buildings (16-20 units). Boiler is not in my building. Bathroom on the first floor, mine line in the basement. What I don't understand is how can water cool down so fast in the pipe from the main line to the bathroom even during the summer and in less than half hour. Also one time during some repairs, warm water started to run on the cold line. There is some suspicion regarding the circulation you mentioned.


Other people have this problem, they all have the original faucets (25 years old).
Bathroom sing has separate faucets. The hot waters starts to come when I either leave the sink or shower running.

Hot water - boiler, servicing seven small buildings (16-20 units). Boiler is not in my building.
Then you don't have your own water heater. What you have is a common hot water boiler with recirculating pumps assuring a even flow to all units. I would take a real close look at the recirculating system. If it were working correctly then you wouldn't have to exhaust all the cool water out of your hot line before you get any hot water out of your faucet. If the other units are affected also the that makes it a "no brainer". Let bme know what they find. Good luck, Tom

Heat-pump relates to A/C, not to plumbing.

If you have boiler in different location than yes, you will have similar problem with hot water delivery. Follow Tom's advice. Go to your nearest neighbor and ask them if they are experiencing same problem. Ask property manager to check on the Hot Water Circulating Pump ( don't confuse it with "heat pump" )