I've been getting some warnings in a model I'm currently working on, and
I haven't been able to find a way to fix them.

The warnings are as follows:

**WARNING***************************************************************
*******
PREHEAT/HEATING-CAPACITY in SYSTEM M AHU-26 (VAV)

is too low to provide the requested supply temperature for
the calculated mixed air temperature using total ZONE loads
(on LS-A/B) and SYSTEM design parameters, plus outside air
conditions (on LS-C), and specified capacities & air flows.
Requested temperature is 55.00 calculated is 10.37
Check HEATING-CAPACITY, HEAT-SET-T, PRE-HEAT-T,
MAX-SUPPLY-T,
DESIGN-HEAT-T, and ZONE loads for consistency.

AHU-26 is a PIU system (series boxes). Zone outside airflow is set up
at 20 CFM / person, the # of people is accurate, and the system is set
up to size the minimum OA based on the sum of the zone OA.

I can't figure out what's going on....it almost seems like the system is
not returning any air, and is at 100% OA. But my OA is set to a fixed
fraction, and inputing an arbitrary supply CFM doesn't change the mixed
air temp warning at all (still 10.37).

I know it's hard to diagnose a problem without being able to open the
file (I'm happy to send someone the .inp and .pd2 files), but has anyone
run into this problem before?

Thanks!

James Hansen, PE, LEED AP