# RE: [EnergyPlus_Support] system sizing

```Hey

When evaluating the losses by exterior convection of my house during my design
days, I am getting values for my external convective coefficients (h).  My
building is a three zone house, the first zone is only a foot high and
represents the crawl space under the house.  The Second zone on top of it is
the conditioned space that is 8 ft high.  The final zone represents the attic
which a triangular cross section that is also 8 feet high.  Using the
Engineering Doc I calculated the forced and natural convective coefficients
and got the same values as the program.  Unfortunately the forced convection
coefficients for the crawl space exterior are about 2 times greater then the
convective coefficients of the conditioned and attic zone.  How can the forced
convective coefficient be greater for the surface closer to the ground.  I
understand that is it the (p*V)/A term in the forced "h" equation that is
causing this.  It seems that if a surfaces perimeter to area ratio is very
large, its "h" will also be large thereby overshadowing the effect of the
velocity term.  If someone can explaing why, I would appreciate it.

Thanks
Ian

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