parallel channels;reference density

ario

hi my case includs two parallel vartical channel inwhich hot air is flowing in one and the heat is transfered to the second channel which has water in stand.the water starts moving due to natural convection.the boundary conditionss for air is velocity inlet and pressure outlet(zero gauge presure)and pressure inlet and pressure outlet for buttom and top of the water channel.my problem is in pressure reported by Fluent.the area average pressure at the air inlet is negative(after summing with the hydrostatic presure).i know the reason but i don't know what to do!the problem arise from the reference density set in the operating pannel.there is only on choice for the reference density and i don't know if i set it for air or water or turn it off.ive tried them all but no good result...looking for your help... thanks