Until two weeks ago, my parents house used one water heater ( noted "1" on the picture) to provide two bathrooms and the kitchen with hot water. The bathroom on the first floor consists of a bathtub which is used occasionally as a shower. The bathroom on the second floor has a shower and is used often.
The pressure in the shower dropped when someone opened a tap to take hot water, so they decided to add an additional water heater ( "2" on the picture) to get rid of the problem. The two hot water network are now disconnected from each other, and this solved the problem of hot water pressure dropping.
There is now another problem. The pressure in the shower (2nd floor, coming from water heater 2) is very good for 65 seconds, then drops drastically. The water seems to just fall (vertically) from the shower head.
Can this be fixed ?
I'm not a plumber, and I'm not going to do it myself, but I may be tempted to (have someone) switch the output of the two water heaters; the water heater #1 is known to work perfectly on the second floor (pressure wise..), and the first floor shower doesn't really need a high pressure hot water (if that pressure problem can't be fixed easily).
Edit:
About the picture: These are copper pipes painted in white. The red color has been added afterwards using Photoshop to help you visualize the two hot water networks.
I did a few tests this morning:
Time to fill a 17L sink: (17L is an approximation.)
- with hot water only: 35s
- with cold water only: 55s
Time until hot water pressure drops: 65s
Time to fill a 17L sink with hot water when there pressure is low: 75s
Best Answer
It appears to me you are building pressure using the bladder tanks. Once the pressure reserved in those tanks is depleted your pressure drops because the flow is restricted (possibly ahead of those tanks). I would agree with Justin that you have a restriction that is reducing the flow and after the bladder tanks are depleted you are only getting the flow through the main line.
The fact that it takes longer to fill the sink with pure cold water tells me your problem is a restriction in the line feeding the water heaters and the bladder tanks are compensating for this somewhat when you fill the sink with pure hot water.
Look for a restriction in your main cold water feed line, and then at other points in the system. Check the air pressure in your bladder tanks with the line drained. Lastly, the piping looks too small to me. I would upsize that piping for better flow.