I live in an 8 year old apt complex with a tankless water heater which is connected to a Viega water distribution manifold. I can't find them on the web anymore so they may have gone out of business.
This problem started about two years ago and has progressed very slowly. The hot water turns cold after about two minutes unless hot water is running elsewhere. For example, if I run the hot water in the bathroom sink, my shower water stays hot the entire time. If I do not run the water in the sink, the shower turns cold in two minutes, I have to turn the water off and wait about two minutes to turn it on and it will be hot again.
Happens every day. Kitchen sink does the same thing. Now the dishwasher is doing it because it used to work perfectly and now the dishes are still dirty. I did an experiment by leaving the hot water running in the sink the entire time the dishwasher was on (and that is a LONG time) and the dishes came out great.
I am very uncomfortable wasting so much water. I do not pay for the water but I believe it is precious. Anyway, my super has been working with the mfr of the tankless heater and has cleaned filters, removed slow flow devices, run white vinegar through the heater, nothing changes. I am wondering if the manifold has filters which need to be cleaned or if there is some other maintenance that needs to be performed.