Equest, fan coils and Ashrae

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This may be a stupid question but it has me stumped. I'm primarily concerned
with fan coil units, but any HVAC system raises similar issues:

ASHRAE Appendix G lets you group zones together into thermal blocks as long
as they have the same HVAC systems and loads and orientation. So, for
example, hotel rooms can be grouped as each is identical to the next. The
question is, how do you model the HVAC of this thermal block?

For example, hotel rooms use fan coil units for cooling. A single FCU uses
X-amount of energy based on fan size and pressure drop. So how do you
accurately model fan energy when grouping several zones, and hence several
FCUs, together?


Nicholas Lander's picture
Joined: 2011-09-30
Reputation: 0


This is a good question and not one that I see discussed much in the
forums. Fan power is directly proportional to flow rate, and directly
proportional to static pressure. For combining multiple FCU zones into
one, I'd combine the flow rates (supply, return, OA etc.) and
heating/cooling capacities if the units are already selected. Use the
total static pressure (or kW/cfm) of a single unit, and the coil head of
a single unit.


William Bishop, PE, BEMP, LEED(r) AP

Bishop, Bill2's picture
Joined: 2011-09-30
Reputation: 0


I have put together a number of hotel models, like you are discussing,
where I modeled all the rooms on like 15 floors with the same exposure
all together. Of course the top floor and the ground floor were split
off and modeled independently, since they have different exposures.
Also, the corner rooms, with four different exposures, for the most
accurate modeling, can be split off and modeled separately. The
internal heat gains should be similar on all these rooms to model
properly. I added up all the CFM's, KW fan loads, outside air, and the
heating and cooling capacity of the equipment for all the rooms all
together. These models modeled accurately compared to actual historical
energy use data of the hotels.

David A. Bastow

David Bastow's picture
Joined: 2011-09-30
Reputation: 0