Knowledgebase : Building Envelope > Heat Transfer
It is extremely difficult to model the interactions between thermal zones which are connected by a large opening. If the zones are controlled to the same conditions, then there is little to be gained by making them interact, so you could neglect any connections between the zones. In fact, if this is the case, you might consider combining the spaces into a single thermal zone. If you expect the zones to have significantly different temperatures and/or humidities, then use one of the following options. If they are modeled as separate zones, EnergyPlus models only what is explicitly described in the input file, so simply leaving a void (no surfaces) between two zones will accomplish nothing - the two zones will not be connected. The main interactions which occur across the dividing line between two zones which are fully open to each other are:

1. Convection or airflow, which will transfer both sensible heat and moisture. Some modelers use MIXING (one-way flow) or CROSS MIXING (two-way flow) to move air between the zones, but the user must specify airflow rates and schedules for this flow, and it cannot be automatically linked to HVAC system operation. Other modelers use AirFlowNetwork with large vertical openings between the zones as well as other openings and cracks in the exterior envelope to provide the driving forces. It can also be connected with the HVAC system (for limited system types). This requires a much higher level of detailed input and should be used only if the detailed specification data is available. If the two zones are controlled to similar conditions, this effect could be safely neglected.

2. Solar gains and daylighting. The only way to pass solar and daylight from one zone to the next is through a window or glass door described as a subsurface on an interzone wall surface. Note that all solar is diffuse after passing through an interior window.

3. Radiant (long-wave thermal) transfer. There is currently no direct radiant exchange between surfaces in different thermal zones. Windows in EnergyPlus are opaque to direct radiant exchange, so an interzone window will not behave any differently than an opaque interzone surface for this aspect. However, a large interzone surface (opaque or window) would result in some indirect radiant exchange since the interzone surface will exchange directly with surfaces in zone A and in zone B. The surface thermal resistance should be low in order to most closely approximate this effect.

Note that surfaces which transmit long-wave radiation will be available in EnergyPlus v3.0 (to be release in October 2008)

4. Conduction. If an interzone surface is placed between the two zones, it will conduct sensible heat between the two zones. Using a low thermal resistance helps to move radiant exchange between the zones.

5. Visible and thermal radiant output from internal gains. These gains will not cross zone boundaries. But again, they will impact any interzone surfaces, so some of the energy may move across to the next zone."
When two surfaces are linked as interzone surfaces, the "exterior" side of these surfaces does not really exist. EnergyPlus links the two surfaces by using the inside temperature of surface A as the outside temperature of surface B, and the reverse. For example:

Zone1WestWall has an outside boundary of Surface = Zone2EastWall

Zone2EastWall has an outside boundary of Surface = Zone1WestWall

Let's say that at hour 2, the inside surface temperature of Zone1WestWall is 19C, and the inside temperature of Zone2EastWall is 22C. When the heat balance is calculated for Zone1WestWall its outside surface temperature will be set to 22C. Likewise, when the heat balance is calculated for Zone2EastWall its outside surface temperature will be set to 19C. So, for interzone surfaces, h ext does not apply. That is why it is reported as zero.


Posted on: 20 Jan 2010 02:38 AM

I have analized the obtained results and are the following:

P_203_6_0_0 (west interior wall belonging to outer zone):

- h in = 3,447

- h ext = 0

P_203_5_0_0 (South interior wall belonging to outer zone):

- h in = 3,296

- h ext = 0

P_203_6_0_0 (east interior wall belonging to outer zone):

- h in = 2,866

- h ext = 0

On the other hand:

P_214_4_0_10004 (west interior wall belonging to inner zone):

- h in = 2,556 --> This coefficient should be the same as the exterior coefficient of the same surface in the adjacent zone, isn't it?, so equal to h ext of P_203_6_0_0. Which is the explanation to this results?

- h ext = 0

P_214_5_0_10002 (south interior wall belonging to inner zone):

- h in = 2,568

- h ext = 0

P_214_2_0_10000 (east interior wall belonging to inner zone):

- h in = 2,556

- h ext = 0

Question: How to you calculate the enthalpy values for MaterialProperty:PhaseChange?

1. Choose a cold temperature and a small enthalpy value, such as -20C, and h1=0.1 J/kg
2. Calculate the enthalpy at the melting point(tMP) h2 = h1+(tMP - (-20))*SpecificHeat
3. The melting stage needs a finite slope, so add 0.1 to the melting point and add the latent heat
so, the third enthalpy pair is tMP+0.1, h2+LatentHeat
4. Then calculate a fourth point by choosing a high temperature such as 100C and then calculate the final enthalpy h4 = h3+(100 - (tMP+0.1))*SpecificHeat

What is the meaning of 'Surface Ext Convection Coeff' report returned as 1000? I seem to remember that it means rain and this seems to be confirmed by weather data but what is the heat transfer coefficient in this case? or does rain mean some other mechanism is used?

From the Engineering Reference, Outdoor/Exterior Convection section (v3.0.0.023, pdf p. 79):

"Note that when the outside environment indicates that it is raining, the exterior surfaces (exposed to wind) are assumed to be wet. The convection coefficient is set to a very high number (1000) and the outside temperature used for the surface will be the wet-bulb temperature. (If you choose to report this variable, you will see 1000 as its value.)"