Construction tab in model data under Construction header

You can select the inside and outside surface convection algorithms in the Model data on the Construction tab under the Surface Convection header. This allows you to make different selections in different parts of the building. Further details of the algorithms are provided in the EnergyPlus Engineering reference document.

**Tip:** You can override the inside and outside convection algorithm selection described below by setting fixed values on the Surface tab of the Construction component dialog.

You can select from 6 main EnergyPlus inside convection algorithms for calculating the convection between internal zone surfaces and the rest of the zone air in the simulation calculations. Unless you have good reason to do so you are advised to use the default TARP convection algorithm:

- 1-AdaptiveConvectionAlgorithm -This advanced option provides a dynamic selection of convection models based on conditions. Beausoleil-Morrison (2000, 2002) developed a methodology for dynamically managing the selection of hc equations called adaptive convection algorithm. The algorithm is used to select among the available hcequations for the one that is most appropriate for a given surface at a given time. As Beausoleil-Morrison notes, the adaptive convection algorithm is intended to be expanded and altered to reflect different classification schemes and/or new hcequations. The adaptive convection algorithm implemented in EnergyPlus for the inside face has a total of 45 different categories for surfaces and 29 different options for hc equation selections. The tables provided in the Engineering document summarise the categories and the default assignments for hc equations.
- 2-Simple - The simple convection model uses constant coefficients for different heat transfer
configurations, using the criteria to determine reduced and
enhanced convection. The coefficients are taken directly from Walton (1983). Walton
derived his coefficients from the surface conductances for ε=0.90 found in the ASHRAE
Handbook (1985) in Table 1 on p. 23.2. The radiative heat transfer component was estimated at 1.02 * 0.9 = 0.918 BTU/h-ft2-F and then subtracted off. Finally the coefficients
were converted to SI units to yield the values below.
For a vertical surface:
hc = 3.076
For a horizontal surface with reduced convection:
hc = 0.948
For a horizontal surface with enhanced convection:
hc = 4.040
For a tilted surface with reduced convection:
hc = 2.281
For a tilted surface with enhanced convection:
hc = 3.870.
- 3-CIBSE - applies
constant heat transfer coefficient derived from traditional CIBSE values.
- 4-Ceiling diffuser
- a mixed and forced convection model for ceiling diffuser configurations.
The model correlates the heat transfer coefficient to the air change rate
for ceilings, walls and floors. The ceiling diffuser algorithm is based on empirical correlations developed by Fisher and
Pedersen (1997). The correlation was reformulated to use the room outlet temperature as
the reference temperature. The correlations are shown below.
For Floors:
hc = 3.873 + 0.082 x ACH ^ 0.98, For ceilings: hc = 2.234 + 4.099 x ACH ^ 0.503 and for Walls:
hc = 1.208 + 1.012∗ACH ^ 0.604.
- 5-Cavity - This algorithm was developed to model convection in a "Trombe wall zone", i.e. the air space between the storage wall surface and the exterior glazing. (See the later sections on Passive and Active Trombe Walls below for more information about Trombe walls.) The algorithm is identical to the convection model (based on ISO 15099) used in Window5 for convection between glazing layers in multi-pane window systems. The use of the algorithm for modelling an unvented Trombe wall has been validated against experimental data by Ellis (2003).This algorithm gives the convection coefficients for air in a narrow vertical cavity that is sealed and not ventilated. This applies both to the air gap in between panes of a window or to the air gap between the Trombe wall glazing and the inner surface (often a selective surface). These convection coefficients are really the only difference between a normal zone and a Trombe zone. See also note below.

- 6-TARP - based on variable natural convection based on temperature difference from ASHRAE algorithms. This is the same as the old "Detailed" Inside convection algorithm provided in earlier versions of DesignBuilder.
**It is the default option for new models in v3.0.0.085 and later.**

The **5-Cavity** Inside convection
algorithm is not available at the surface level.

The following inside convection options are also available for advanced users:

- 7-ASHRAEVerticalWall
- 8-WaltonUnstableHorizontalOrTilt
- 9-WaltonStableHorizontalOrTilt
- 10-FisherPedersenCeilingDiffuserWalls
- 11-FisherPedersenCeilingDiffuserCeiling
- 12-FisherPedersenCeilingDiffuserFloor
- 13-AlamdariHammondStableHorizontal
- 14-AlamdariHammondUnstableHorizontal
- 15-AlamdariHammondVerticalWall
- 16-KhalifaEq3WallAwayFromHeat
- 17-KhalifaEq4CeilingAwayFromHeat
- 18-KhalifaEq5WallNearHeat
- 19-KhalifaEq6NonHeatedWalls
- 20-KhalifaEq7Ceiling
- 21-AwbiHattonHeatedFloor
- 22-AwbiHattonHeatedWall
- 23-BeausoleilMorrisonMixedAssistedWall
- 24-BeausoleilMorrisonMixedOpposingWall
- 25-BeausoleilMorrisonMixedStableFloor
- 26-BeausoleilMorrisonMixedUnstableFloor
- 27-BeausoleilMorrisonMixedStableCeiling
- 28-BeausoleilMorrisonMixedUnstableCeiling
- 29-FohannoPolidoriVerticalWall
- 30-KaradagChilledCeiling
- 31-ISO15099Windows
- 32-GoldsteinNovoselacCeilingDiffuserWindow
- 33-GoldsteinNovoselacCeilingDiffuserWalls
- 34-GoldsteinNovoselacCeilingDiffuserFloor

To avoid discontinuities in surface heat transfer rate calculations, all correlations are extrapolated beyond the lower limit of the data set (3 ACH) to a natural convection limit which is applied during the hours when the system is off. These models are explained in greater detail in the EnergyPlus Engineering Reference Document.

Substantial research has gone into the formulation of models for estimating the exterior convection coefficient. Since the 1930's there have been many different methods published for calculating this coefficient, with much disparity between them (Cole and Sturrock 1977; Yazdanian and Klems 1994).Unless you have good reason to do so you are advised to use the default DOE-2 convection algorithm You can select from 7 main outside convection algorithms:

- 1-AdaptiveConvectionAlgorithm - This advanced convection algorithm provides a dynamic selection of convection models based on conditions. This algorithm has a structure that allows for finer control over the models used for particular surfaces. The algorithm for the outside face was developed for EnergyPlus but it borrows concepts and its name from the research done by Beausoleil-Morrison (2000, 2002) for convection at the inside face (see above).
The adaptive convection algorithm implemented in EnergyPlus for the outside face is much simpler than that for the inside face. The surface classification system has a total of 4 different categories for surfaces that depend on current wind direction and heat flow directions. However in other ways it is more complex in that the hcequation is split into two parts and there are separate model equation selections for forced and natural convection.
- 2-SimpleCombined - The simple algorithm uses surface roughness and local surface wind speed to calculate the exterior heat transfer coefficient. This
is a combined heat transfer coefficient that
*includes radiation to sky, ground, and air*. The correlation is based on Figure 1, Page 25.1 (Thermal and Water Vapor Transmission Data), 2001 ASHRAE Handbook of Fundamentals.

Note: selecting the SimpleCombined option atbuilding levelon the Constructions tab under Surface Convection or on the Calculation options or Model options dialogs has a special meaning. In this case the outside convective selection covers both convection and radiation combined. This means for example that any hard-set outside convective heat transfer coefficients set on the Surface properties tab of the Construction dialog will actually be used as acombinedoutside convective plus radiation coefficient.

- 3-CIBSE - applies constant
heat transfer coefficients depending on orientation, derived from traditional
CIBSE values.
- 4-ASHRAEVerticalWall - Identical to the DOE-2 option.
- 5-TARP - TARP, or Thermal Analysis Research Program, is an important predecessor of EnergyPlus (Walton 1983). Walton developed a comprehensive model for exterior convection by blending correlations from ASHRAE and flat plate experiments by Sparrow et. al. In older versions of EnergyPlus, prior to version 6, the “TARP” model was called “Detailed.” The model was re-implemented in version 6 to use Area and Perimeter values for the group of surfaces that make up a facade or roof, rather than the single surface being modelled.
- 6-DOE-2 - The DOE-2 convection model is a combination of the MoWiTT and BLAST Detailed
convection models (LBL 1994).
**It is the default option for new models.** - 7-MoWiTT - The MoWiTT model is based on measurements taken at the Mobile Window Thermal Test
(MoWiTT) facility (Yazdanian and Klems 1994). The correlation applies to very smooth,
vertical surfaces (e.g. window glass) in low-rise buildings. The MoWiTT algorithm may not be appropriate for rough surfaces, high-rise surfaces, or surfaces
that employ movable insulation.

The following outside convection options are also available for more advanced users:

- 8-WaltonUnstableHorizontalOrTilt
- 9-WaltonStableHorizontalOrTilt
- 10-AlamdariHammondStableHorizontal
- 11-AlamdariHammondUnstableHorizontal
- 12-FohannoPolidoriVerticalWall
- 13-NusseltJurges
- 14-McAdams
- 15-Mitchell
- 16-BlockenWindard
- 17-Emmel
- 18-ClearRoof

**Note:** When the surface is wet (i.e. it is raining and the surface is exposed
to wind) then the convection coefficient appears in results as a very
large number (1000) and the surface is exposed to the Outdoor Wet Bulb
Temperature rather than the Outdoor Dry Bulb Temperature.

As well as being able to define convection options on the Model data on Construction tab under the Surface Convection header as described above, building level inside and outside convection algorithm settings can be made on:

You cannot make changes to individual surface convection settings where the surface is an internal partition.

All of the above algorithms are described fully in the EnergyPlus Engineering Reference.

Note: the equivalent inside and outside surface convection options in the Calculation and Model options dialogs control building default values (equivalent of making a setting in Model data at building level) and any changes made at block, zone or surface levels in the Model data will override these default settings.