HVAC Calculations

Units

British Thermal Unit

Unit of heat energy

There are several definitions of the Btu, HVAC uses the IT definition, abbreviated Btu_IT


\begin{align*}
1~\text{Btu}_\text{IT} &\equiv \left( 4.1868 \times 453.59237 \times{\frac{5}{9}} \right)~\text{J} \\
&\approx 1055.056~\text{J}
\end{align*}

More common than Btu itself is thousand British thermal units per hour (MBH), a unit of power.


\begin{align*}
1~\text{MBH} &\equiv 1000~\text{Btu/hr} \\
&\approx 3.412~\text{W}
\end{align*}

Ton of Refrigeration (TR or TOR)

Unit of power

[!info] Also Known As

refrigeration ton (RT)

ton (refrigeration)


\begin{align*}
1~\text{TR} &\equiv 12000~\text{Btu}_\text{IT}/\text{h} \\
&\approx 3516.853~\text{W}
\end{align*}

Cubic Feet per Minute (CFM)

Unit of volumetric flow.

Measures

Airflow

External static pressure (ESP) measured in inches of water (IN WC) "water column inches"

Heating/Cooling Capacity

Tons (refrigeration)
MBH
Entering air temperature (EAT)
Leaving air temperature (LAT)
Mixed air temperature (MAT)
Latent heat
Sensible heat
Sensible -> dry bulb
Latent -> wet bulb

???

OSA EAT (F)
EXH EAT (F)
IN S.L.
OUT ALT.

Ratings

MERV
SEER2
HSPF2

Efficiency

η (eta)
ISMRE

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