Plenums

HVAC tab in model data under Mechanical Ventilation header - Compact HVAC

Note: this topic relates to Compact HVAC. You can find out about plenums in Detailed HVAC in the Add Supply Plenum Add Return Plenum Tool topic.

A plenum is an air compartment or chamber including uninhabited crawl spaces, areas above ceilings or below a floor, including air spaces below raised floors of computer/data processing centers, or attic spaces, to which one or more ducts are connected and which forms part of either the supply air, return air or exhaust air system, other than the occupied space being conditioned.

DesignBuilder Compact HVAC Unitary multizone, Unitary single zone, VAV and CAV systems can have air supplied and returned via plenums. The system supply and return plenums in the diagram below serve each of the 3 zones.

There are 4 types of plenum:

1. System supply plenum - a supply air plenum serving all zones in 3-VAV and 2-Unitary multizone system. If any zone supply plenums have been specified, the outlet from the system supply plenum will flow through the zone supply plenum to the zone terminal unit. The system supply plenum is defined at building level on the HVAC tab under the Plenums header and only one is allowed per simulation.
2. System return plenum - return air plenums serving all zones in 3-VAV and 2-Unitary multizone systems. If any zone return plenums have been specified, the return air from the zone will flow through the zone return plenum and then into the system return plenum. The system return plenum is defined at building level on the HVAC tab under the Plenums header and only one is allowed per simulation.
3. Zone supply plenum - a supply air plenum serving only this zone in 3-VAV, 2-Unitary multizone and 1-Unitary single zone systems. The plenum zone is added to the supply air path after the system splitter or system-level supply plenum and before the zone terminal unit. The zone supply plenum is defined at zone level on the HVAC tab under the Plenums header. A zone supply plenum can serve only one zone.
4. Zone return plenum - return air plenums serving only this zone in 3-VAV, 2-Unitary multizone and 1-Unitary single zone systems.  The plenum zone is added to the return air path immediately after the zone and before the system mixer or system-level return plenum. The zone supply plenum is defined at zone level on the HVAC tab under the Plenums header. A zone return plenum can serve only one zone.

When specifying a system or zone supply or return plenum you should enter a unique zone name. The zone does not need to be in the same block. The zone name you enter is checked just prior to the simulation and not before so you should enter the names with care to avoid mistakes.

Plenum modelling

• Maximum of 1 system supply plenum allowed per simulation
• Maximum of 1 system return plenum allowed per simulation
• Each zone plenum can only serve one zone
• Set the Zone type of any plenum zones to 4-Plenum. This makes the necessary changes (remaining requirements and recommendations in bullet list)
• Plenum zones cannot be controlled.  Use the HVAC template <None> to switch off heating, cooling and mechanical ventilation in all plenum zones.
• Remember to ensure plenum zones have no internal gains, occupancy, lighting etc.
• We recommend using the 4-Ceiling diffuser interior convection heat transfer coefficient for plenum zones to model the effect of fast moving air on surface heat transfer.
• You will probably find it easiest to use the Combined method for Floor/slab/ceiling definition for models using plenums as each surface element in the model has just a single construction and you don't have the confusion of Model data referring to 'raised floors' and 'suspended ceilings'.
• The floor of a ceiling plenum is the ceiling of the zone below so you will often define the plenum floor as 'Ceiling tiles' construction or similar.

If the building you are modelling has more than one plenum zone, each serving multiple zones you should use zone plenums. Use virtual partitions to partition the large plenum zones (which in reality serve multiple zones) into multiple plenum zones, each serving a single zone. See image below.

Red lines show the 5 zones of the occupied space. Each zone has its own return-air ceiling plenum.

This technique is also illustrated in the example file supplied with DesignBuilder Zone vs System Plenums Example.dsb.

Note 1: when modelling plenums in DesignBuilder there is no need to add holes between the plenum and occupied space to represent supply diffusers or extracts. These are included in the model by EnergyPlus albeit as part of the system network.

Note 2: it is not possible in DesignBuilder v2 to use the suspended ceiling/floor (as set on the Constructions tab when using Separate constructions) as a plenum - the plenum must be a completely separate zone.