Why my hot water goes out in less than 2 mins?
Check the dip tubes on hot water heater.
Isolate the hot water heater to make sure the heater was running properly. 
Turn off the radiant heat systems and bypass them with the built-in valves.
Open up each shower fixture and close off the water valves to take those out of the equation. 
If the hot water still run cold do next:
Remove the recirculating pump and check the system.
Remove the check valves beginning with the one near the recirculating pump and discover the broken valve. If this still didn't work, they would've kept going to check every check valve in the system(three total).


Maybe it helps