# VAV-water cooling coil, cooling load calculation

Hi all,

I have a question regarding to VAV-coil, cooling load computed by program and by hand. For the VAV-cooling component "VAV_CoolC" has 4 nodes, water in, water out, air in, air out. For water side, I manually calculated with the equation:

```
Q_calculate = (h_water_in - h_water_out)*m_flow_water
where:
h_water is obtained from "System Node Enthalpy [J/kg]"
m_flow is obtained from "System Node Mass Flow Rate [kg/s]"
```

I compared the Q_calculate with the "VAV_CoolC, Cooling Coil Total Cooling Rate" Q , which is different.

For example,

```
Q = 68119 [W], Q_calculate= 67805[W]
Q = 120,976[W], Q_calculate= 120,406 [W]
Q = 182,859[W], Q_calculate = 185,316[W]
```

My question is, why the Q_calculate is so different from Q that the relative error is 0.46%, 0.47%, 1.34%.

How does the E+ calculate the Cooling load of coil water side and air side?

What output:variable should I use for calculate the cooling load?

PS: My E+ model is typical large office, with HVAC systme: VAV-Cooling coil, VAV-heating coil, reheat coil, VAV-fan; plant system: water-cooled chillers, cooling tower and boiler.