Erik,
I'm not sure why the cooling would turn on when you manually size the fan, but another way you could turn the cooling off is to put in an availability schedule that never turns on. That might fix your problem. Also, you can turn on the AHU heating coil by setting the heat source to hot water in the system.
Jeremy
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Jeremy McClanathan, P.E., BEMP, HFDP, LEED(r) AP BD+C
