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The SPACE loads are calculated at a CONSTANT
temperature all year, and should normally be specified as the average
temperature for the year (such as 73F). The ZONE loads take into account
the actual thermostat setting, changes in the setting (night setbacks/setups,
etc.), and deviations from setpoint.
Changes in setpoint cause mass effects to come into
play. For example, if a zone is held at 55F all night, and is then raised
to 70F the next morning, the HVAC system will have to warm the internal mass as
well as offset the envelope losses. As a result, the heating load after
the temperature setback is terminated will be greater than it would be if the
space were held at 70F all night. (Note that mass effects last for
multiple hours, and gradually taper off as the mass comes into equilibrium with
the new temperature.)
Deviations in setpoint (and loads) occur because of
the system response to the load. For example, if the thermostat
setpoint is 75F during cooling, but the HVAC system is too small to
handle the load, then the actual zone temperature will float higher
than 75F; the higher temperature then acts to reduce the load. The actual
temperature will be the balance point between system capacity, envelop and
internal loads of the space, and mass effects within the space.
Air handler loads are a function of the zone loads,
latent cooling, duct heat gain/losses, fan heat, outside air introduced for
ventilation and/or economizer savings, reheat energy in VAV systems, mixing
losses in dual-duct systems, etc.
Loads on a boiler or chiller are a function of the
coil loads in airhandlers, pump heat, and piping heat gains.
So the loads summarized in the LS-A, etc. reports
are space loads that occur at a constant temperature. They exclude all of
the mechanical effects of temperature swings, ventilation air, etc. that
are accounted for in the SS-A, etc. reports.
Given that the LS-** reports do not actually report
the true loads, are they useful? YES! You should always review
these reports when creating a new model. In addition to reporting lighting,
daylighting, and equipment energy, they are excellent reality checks
on the origin of the space loads. The reports give breakdowns on
loads arising from lights, equipment, people, infiltration, walls, windows,
roofs, etc. An abnormally large or small load in a given category is a clue that
there is a mistake in your model.
The SS-* reports summarize the response of the HVAC
system to the space loads. It is wise to acquaint yourself with all of
these reports. One of my favorite reality checks is the SS-F report
which reports the maximum/minimum temperatures within a given zone.
Temperatures too far beyond the thermostat settings are a sure sign that your
model is not working correctly (or if modeled correctly, a sure sign that the
HVAC system was not designed properly!)
The BEPU report is another good reality check, as
it breaks down gas and electricity consumption into the major end-use categories
(lighting, misc equipment, heating, cooling, fans, etc.). Again, once you
have become practiced in knowing what buildings typically consume, abnormal
numbers can jump out at you.
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