Economisers

HVAC tab in model data under Mechanical Ventilation header

HVAC Economisers are useful for facilities which require some cooling even when the outdoor temperature is lower than the indoor temperature. An economizer is a damper opening that draws up to 100% outside air when the outside air is cooler than the temperature inside the building, thereby providing free cooling. An outdoor air economy cycle can reduce cooling energy requirements by some 20% to 30%, or around 5% of the air conditioning energy use and are often required by energy codes for larger air conditioning units.

Economisers can be modelled when using Compact HVAC but not Simple HVAC.

See under Heat recovery for information about how economisers interact with heat recovery.

Economiser operation applies to the overall HVAC system and so for Unitary multizone , CAV and VAV systems its data is set at building level. For Unitary single zone systems the data at zone level is used.

There are four options:

1-None where no economizer operation will be simulated and outside fresh air is based on the Mechanical ventilation Outside air rate and associated operation schedule.
2-Return air temperature establishes a limit control on the return air temperature so that when the temperature of the air entering the mixer on the outside air side is greater than the return air temperature the outside airflow rate is set to the minimum.
3-Return air enthalpy establishes a limit control on the return air enthalpy so that when the enthalpy of air entering the mixer on the outside air side is greater than the return air enthalpy the outside airflow rate is set to the minimum.
4-Return air temperature and enthalpy - where both the above limit controls are in operation, i.e. both temperature or enthalpy limits are used so that when either the temperature or the enthalpy of the air entering the mixer on the outside air side is greater than the return air temperature/enthalpy, the outside airflow rate is set to the minimum.

In addition to the above checks, the economiser can also have absolute limits on temperature and enthalpy of the outside air. When the outside air conditions go beyond these limits the EnergyPlus economiser switches to minimum outside air mode, i.e. the outside air damper is closed to its minimum position and heat recovery will be enabled if available.

Economiser upper temperature limit

Enter the outside air temperature high limit for economizer operation. If the outside air temperature is above this limit, the outside air flow rate will be set to the minimum.

Economiser lower temperature limit

Enter the outside air temperature low limit for economizer operation. If the outside air temperature is below this limit, the outside air flow rate will be set to the minimum.

Economiser upper enthalpy limit

Enter the outside air upper enthalpy limit for economizer operation. If the outside air enthalpy is above this value, the outside air flow rate will be set to the minimum.

Lockout

Unitary systems can also have lockout control:

1-No lockout - default option.
2-Lockout with heating - when the packaged unit is in heating mode, the economiser is locked out – i.e., the economiser dampers are closed and there is minimum outside air flow.
3-Lockout with compressor - in addition to locking out the economizer when the unit is in heating mode the economizer is locked out when the DX unit compressor is on. In other words, when the DX coil is off, the economizer must meet the entire cooling load – it isn’t allowed to operate in conjunction with the DX cooling coil. Similarly, when the DX coil is operating, the economizer is not allowed to operate. The Lockout with compressor option is sometimes called a 'non-integrated economizer'.