Problem with total heat transfer rate
 

I am doing a 2D simulation of catalytic combustion of propane-air in a straight channel microreactor. Catalyst is coated on the wall. Rate equation is supplied using a UDF. Flow is laminar.
My problem is that, the net term of the total heat transfer rate (Reports/Fluxes/Total heat transfer) doesn’t approach 0. In certain simulations it is even more than 100! The net term of sensible heat transfer rate, however, approaches 0. The monitors remain constant for several thousand iterations and residuals are very low, while the total heat transfer term continue to remain high.
Please help me understand how heat transfer rate is calculated by fluent. The flow rate of total enthalpy at outlet matches the value of total heat transfer rate at outlet but the flow rate of total enthalpy at inlet is not even close to the total heat transfer rate at inlet.
By the way, when I disable the species inlet diffusion (in Models), the net term of total heat transfer is approaching 0. But then, the residuals have a tendency to oscillate.
It would be a great help if you could help me figure this out.