Mechanical Ventilation

HVAC tab in model data - Simple HVAC

Check the mechanical ventilation checkbox to indicate that outside air and/or re-circulated air is delivered to the zone. You can use this setting to define mechanical ventilation and air delivered through centrally ducted air conditioning systems or local fresh air systems.

The specification of outside air delivery rates is the same for both options as described in the Outside air section.

The way that mechanical ventilation is modelled depends on the Mechanical ventilation method model option:

• 1-Room ventilation
• 2-Ideal loads

1-Room ventilation

With the Mechanical ventilation method model option the introduction of outside air through fans is achieved using the EnergyPlus ZoneVentilation:DesignFlowRate data separate from the main HVAC system. Energy consumption and heat pick up from fans is included as described below.

Fan Type

When using Simple HVAC, enter the type of fan. Select from:

• 1-Supply - fan blows outside air into the zone and the zone receives heat from the fan if Fan in air (below) > 0
• 2-Extract - fan exhausts air from the zone. In this case there is no fan heat pickup. It is assumed that makeup air comes from outside.

Note that in Simple HVAC the Auxiliary energy data accounts for all electric fan and pump distribution energy plus controls and any other electrical energy use associated with HVAC that is not already accounted for elsewhere.

Fan pressure rise

Enter the pressure rise at full flow and standard conditions. Standard conditions are considered 20°C at sea level, 101325 Pa.

You can calculate the approximate fan pressure rise from Specific Fan Power (SFP) data using:

Delta P = 1000 * SFP * Fan total efficiency

Annex E of ISO 5801 shows that by rearranging the formula it can be derived that the SFP is a function of fan pressure divided by the efficiency of the fan system. Therefore the SFP will increase or decrease with a respective increase or decrease in the system pressure.

The SFP is a function of the volume flow of the fan and the electrical power input and is quoted for a particular flow rate;

SFP = Pe/ V

Where:

V is volume flow (l/s)

Peis electrical power input (W) to the fan system or complete air movement installation

[Reference FMA, UK, 2006]

Typical values for various system types are shown in the table below.

Source ESTA: http://www.esta.org.uk/

Fan total efficiency (%)

Enter the product of the fan motor and impeller efficiency of the supply fan. This is the ratio of the power delivered to the air to the electrical input power at maximum flow expressed as a percentage. The motor efficiency is the power delivered to the shaft divided by the electrical power input to the motor. The fan efficiency is power delivered to the air divided by the shaft power. The power delivered to the fluid is the mass flow rate of the air multiplied by the pressure rise divided by the air density. Must be greater than 0 and less than or equal to 100.

Fan motor in air (%)

Enter the percentage of the motor heat that is added to the air stream. A value of 0 means that the motor is completely outside the air stream. A value of 100 means that all of the motor heat will go into the air stream and act to cause a temperature rise. Must be between 0 and 100.

Fan placement

Enter the supply fan placement type. There are two choices:

• 1-Draw through models a system where the supply air fan is after the cooling and heating coils.
• 2-Blow through where the supply air fan is before the cooling and heating coils.

The default is 1-Draw through.

Fan power and temperature pick up calculations

The calculations for fan power and airflow temperature pick up are detailed in the EnergyPlus Engineering Document. A summary is provided below:

Total Fan Power = Mass flow rate. DeltaP / (Total fan efficiency . Air density)

Shaft Fan Power = Motor efficiency . Total Fan Power

Heat to air = Shaft Fan Power + (Total Fan Power - Shaft Fan Power) . Motor in air fraction

2-Ideal Loads

With the ideal loads option, Mechanical ventilation is modelled with heating and cooling using the EnergyPlus ZoneHVAC:IdealLoadsAirSystem data. In this case there are options to include the effects of heat recovery, economiser, humidification and dehumidification. Fan energy and pickup are not included.

Economiser

Note from the developers: The Economiser option is implemented in the DesignBuilder interface but the calculations are not working correctly in the EnergyPlus v7 ZoneHVAC:IdealLoadsAirSystem. Once the problem has been fixed in EnergyPlus v8 this option will be re-enabled. In the meantime, to model economisers you should use Detailed or Compact HVAC.

Economisers are used to provide cooling when the outdoor temperature is lower than the indoor temperature. An economiser 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.

Choose from 3 options:

• 1-None where no economiser operation will be simulated and outside fresh air is based purely on the Mechanical ventilation Outside air rate and associated operation schedule.
  
• 2-Differential dry bulb the economiser increases the outdoor air flow rate above the minimum outdoor air flow when there is a cooling load and the outdoor air temperature or enthalpy is below the zone exhaust air temperature.
  
• 3-Differential enthalpy the economiser increases the outdoor air flow rate above the minimum outdoor air flow when there is a cooling load and the outdoor air enthalpy is below that of the zone exhaust air.
  
  

The upper limit of outside airflow when the economiser is working is set to 15 ac/h.

Heat recovery

When heat recovery is active you can choose the type of heat recovery. Select from:

• 1-Sensible which provides sensible heat recovery whenever the zone exhaust air temperature is more favourable than the outdoor air temperature.
• 2-Enthalpy where latent and sensible heat recovery are provided whenever the zone exhaust air enthalpy is more favourable than the outdoor air enthalpy.

Humidification control type

When humidification is selected there are 2 further options:

• 1-Constant supply humidity ratio means that during heating the supply air will always be at the Maximum heating supply humidity ratio.
• 2-Humidistat means that the humidity in the zone is controlled using the Humidification setpoint defined on the Activity tab. The ideal loads system will attempt to meet the humidistat request, i.e. it will humidify according to the setpoint.

Dehumidification control type

When dehumidification is selected there are 3 further options:

• 1-Constant supply humidity ratio means that during cooling the supply air will always be at the Minimum cooling supply humidity ratio.
• 2-Humidistat means that the humidity in the zone is controlled using the Dehumidification setpoint defined on the Activity tab. The ideal loads system will attempt to meet the humidistat request, i.e. it will dehumidify according to the setpoint.
• 3-Constant sensible heat ratio. means that the ideal loads system will be controlled to meet the sensible cooling load, and the latent cooling rate will be computed using the constant sensible heat ratio (SHR) defined below.

For 3-Constant sensible heat ratio and 2-Humidistat options, if the mixed air humidity ratio is less than the target humidity ratio, then the mixed air humidity ratio will be used. For all options, the supply air humidity ratio will never be allowed to exceed saturation at the supply dry bulb temperature. The selected dehumidification control type is always applied when the unit is in cooling mode. If the unit is in deadband mode (not actively heating the supply air) control type Humidistat will be active. If the unit is in heating mode, control type Humidistat will be active if the Humidification Control Type field below is set to Humidistat or None. This allows the ideal loads system to heat and dehumidify at the same time.

Cooling sensible heat ratio

When the Dehumidification control type above is set to 3-Constant sensible heat ratio the ideal loads system will be controlled to meet the sensible cooling load, and the latent cooling rate will be computed using the value of Cooling Sensible Heat Ratio (SHR), where SHR = Sensible Cooling divided by Total Cooling (sensible plus latent). The default is 0.7.