Heat transfer through pipe
 

I am designing a gas water heater. i have modelled and meshed the whole structure. unfortunately the flue gases which are at a temprature of 750K do not seem to heat up the water through the wall. the temperature of the water remain at 303K at 2.5sec to 1min. Please help me in this regard kindly. what options in the wall must i select "on" so that heat is transfered through it to the water
thanks