# Sensible and Latent Loads

Some of the content on this page is also discussed on the "Equipment Response to Loads" (previous) page. This content is focused on how the RTU Comparison Calculator accounts for latent loads (see discussion below for an explanation of the outline).

**Evaporator Coil Conditions**: Outside air (ventilation) and indoor air mix before passing over the coil. Conditions in the mixed stream depend on weather (outdoor dry-bulb and wet-bulb) and indoor air (dry-bulb, setpoint, and room humidity).

**Latent and Sensible System Capacity:**Components of total capacity are based on mixed air conditions.- Rated system capacities and S/T ratio (at AHRI test
conditions) are projected to coil conditions.
**ASHRAE**psychrometric routines (thermodynamic properties of moist air)**DOE-2**total-capacity curves for evaporator coils**EnergyPlus**Apparatus dew-point and bypass-factor algorithms for calculating latent and sensible components of total capacity

- Rated system capacities and S/T ratio (at AHRI test
conditions) are projected to coil conditions.

**Run times**are determined by the system’s sensible- Building
**Sensible**Loads**:**Load-line established to match (1) the**system**sensible**capacity**at outdoor design conditions and (2) the sensible**internal****loads**when outdoor conditions equal the setpoint. - Building
**Latent**Loads**(condensate):**Calculated using S/T ratio at coil conditions and sensible loads (see “LLdE” and “S/T” columns in the bin tables).

- Building

Discussion:

### Comments on #1:

The analysis of latent and sensible loads starts at the evaporator coil. It is the conditions at the coil that determine how much of the system's total capacity is available for handling sensible loads and how much of that capacity will be used for dehumidification. The coil conditions are determined by mixed-air calculations that consider the contributions from the two air streams (indoor return air and outdoor ventilation air).

### Comment on #2:

Once coil conditions are known, then the rated capacities
can be projected to them. This projection takes the
characteristics of the system at AHRI test conditions
(Outdoor dry-bulb = 95F, Inside dry-bulb=80F, Inside
wet-bulb=67F) and projects to the mixed-air coil conditions
in the simulation (as affected by indoor setpoint and
weather-bin conditions).

The three sub-bullets list the algorithmic sources for the
projection calculations. The result of the projection is
sensible and latent capacities at coil conditions. A
significant portion of the computational code of the RTUCC
is in support of this step.

### Comment on #3:

This bullet starts off with a statement that reflects how a
thermostatically controlled air conditioner works. It simply
tries to keep the dry-bulb temperature from rising. That is,
as sensible loads act to increase the temperature of the
room air (and eventually trigger the thermostat), the air
conditioner runs (cooling the air) until the thermostat says
to stop.

Run times are basically the ratio of the sensible load to
the sensible capacity. So by establishing the sensible
capacity of the system we can determine how long it has to
run (to satisfy the thermostat).

The first sub-bullet describes the two outdoor conditions
that establish the sensible-load line (design and setpoint).

While the unit runs to counteract sensible loads, it also
dehumidifies. To calculate condensate formation
(latent coil load), the split between sensible and latent
capacities (the S/T ratio) and the sensible load (SL), can
be used. S/T is short for the ratio of the sensible to total
capacities.

LL = TL * L/T = (SL/(S/T)) * (1 -
S/T)

This LL (Latent Load) is reported in the RTUCC as the “LLdE”
column in the bin calculations. Another column that gives a
good indication of how the coil conditions are affecting the
sensible and latent capacities is the “S/T” column.

Systems in humid climates will have a lower portion of their
capacity available for sensible cooling (lower S/T ratios).

### Assumptions about Indoor Humidity:

The two plots above show the difference between the outside
and inside humidity ratio as a function of the
outdoor-indoor temperature difference (left is raw hourly;
right is daily average). This data is generated from
EnergyPlus modeling of a medium-sized office building. This
shows a positive difference during the summer cooling season
(delta T greater than -10), and a negative difference in
winter. Summer cooling, and resulting dehumidification,
suppresses the indoor humidity ratio.

The RTUCC has two ways of modeling indoor humidity. The
default is to assume that that indoor humidity ratio tracks
with the outdoor humidity ratio. This assumption produces
mixed-air (coil conditions) more humid than would be seen in
an hourly simulation like EnergyPlus. The corresponding
latent coil loads will be higher in the RTUCC than a
corresponding hourly model like EnergyPlus. The alternate
assumption of fixed indoor humidity assumes that some other
conditioning device is acting to control the indoor humidity
to a fixed level. The latent coil loads of the RTUCC should
be similar to that of an hourly simulation which also uses a
fixed humidity assumption.