Is 1/2" CPVC and 1/2" PVC the same size?

No. CPVC is measured by O.D., which makes its sizing similar to hard copper. PVC is measured by I.D., which makes its sizing similar to iron pipe.I am not getting enough hot water from my gas water heater.The thermostat may be set too low, otherwise the burner is probably not burning correctly. If thats the case, it's best to call a service technician.