# 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, set point, 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’s 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 (cools 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 sub-bullets here give some detail on the sensible load line (which
will probably have been discussed to some degree in previous slides).
This bullet describes the two outdoor conditions that establish the
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.