Ventilation schedule for mechanically operable skylights/Sizing heating peak loads/heating energy results discrepancy

Hi Guys,

I would like to lay here two very (probably) basic questions regarding skylights and to ask for clarifications about some possibly strange results I’ve been obtaining by simulating the energy performance for a warehouse project located in Korea (approx 20’000 sqm - I use the EPW of Incheon downloaded through the LB component). I start by saying that I searched for some similar questions in both forums, but unfortunately I have not been able to find any related example.

The client asked to compare system loads required for heating extension zones of a warehouse complex according to two sets of parameters:

  • Insulation degree (specifications of materials and constructions to follow the design of the existing building nearby OR optimized design with better insulation values);
  • Heating system (radiant panels OR floor heating) - the client required for the radiant panels system to run only during the operations schedule (08-18), whereas the floor heating should be running continuously during the weekdays and be eventually shut down during the weekend.

At the end I need to calculate 4 different variations (As Per Design/Radiant Panels; Optimized Design/Rad-Pan; APD/Floor Heating, OD/Floor Heating), and use them to produce an LCCA report for the different variations.

Premise is, I used an assembly method for the zones using HBSrfs and defining each separate zone by each singular face.

Question 1: The warehouse has a 7% skylight ratio for the roof surface. These skylights are almost the only ventilation openings for the warehouse, which has just one additional row of ribbon windows on the South side, although the glazing rate on façade surface is negligible in proportion to the skylights. My question is: how can I define natural ventilation schedules for the skylights? Since they are associated to surfaces defined as roofs I suppose (and it appears so running a comparison simulation by setting the roofs as wall surfaces) the openings of the skylights will obey neither to the ventilation schedule I set nor to the Set EP Air Flow component. As such, I had to run the simulation defining the roof surfaces as walls, but certainly this would impact the simulation, as for example walls have different correction values associated to the CTF function (or at least so I learned) and maybe E+ specifies roof geometries in another way as wall… am I wrong assuming that skylights do not follow both ventilation schedules or EP Air Flow settings?

Question 2: As I run the OpenStudio component to calculate the ZonePeakLoads through the HVAC component (I know the two systems are not HVAC, but I just need the heating loads, and I assume there would be no problem using this component for other heating systems), the peak values during Design Day for the radiant panels are higher than the ones calculated for the floor heating, whereas (and logically), heating loads results decrease in the following order: Floor Heating/As Per Design->FH/Optimized Design->Radiant Panels/APD->RP/OD. The heating loads results are understandable, due to optimization and the above mentioned schedules. I suppose (but I am not sure), that heating peaks are higher for the radiant panels because these have to be turned on at 07:30 AM, whereas the floor heating systems would run for the entire week and therefore zones do not experience the dropout in temperature as for the radiant panels solution.

Could someone tell me if I am on the right track on these two topics? Many thanks for reading and for any help you could give me!



Hi @fabrizio.m.amoruso,

I won’t be much of a help here I think. It looks like you should look into Stack Ventilation in Energyplus for what you’re looking for.

Sorry for the late response here. I’ll try to answer your questions as best I can but I’ll admit that I’ll likely need more information to be of help:

  1. It makes a huge difference if you have both the ribbon windows and the skylights operable or if only the skylights open. If only the skylights open and the ribbon windows remain shut, you will get very low airflow which you can probably approximate by boosting up your infiltration by a factor of 3 or 5 and adjusting the infiltration schedule to account for when the skylights are open. If both the windows and skylights open, you can model the airflow using the Set EP Airflow component with a custom stack object like you see here:,-1655.5990670472813
    In this case, the _opeableEffectiveArea is be roughly the average of the ribbon window operable area and the skylight operable area and the _inletOutletHeight is the height difference between the ribbon windows and the skylights. If you really want to be exact about your _operableEffectiveArea, you should calculate it with this equation:
    Were A1 and A2 are the operable areas of ribbon windows and skylights respectively.
    You can use the oepningAreaFractionalSchedule to define a schedule for when the windows are open or use the different TempForNatVent inputs to set min/max indooor/outdoor temperatures for natural ventilation. I don’t know what your control strategy for opening the windows is so that is going to determine what you use here.

  2. Your explanation sounds correct. If you have a heating system that is operating continuously, your peak heating condition is pretty much always going to occur sometime in the early morning around 5-7 AM. So, if your radiant panels are going to be set back during this time of the day, it should have a big impact on your HVAC size.


Hi Devang and Chris,

Many thanks for the support and the insightful advises on the matter!
As a matter of fact, I wanted to ask about tweaking infiltration values/schedules to achieve the same results for ventilation, but it appears Chris has already prevented that question from surfacing. I will adopt your comments in the file, as it appears to be the best way to handle the situation.

I would like just to ask then for a final comment on the way I used to simulate windows/skylights openings in the code, which maybe was a little bit too concealed in my first question but could help me for future reference:

  • What is the main difference between setting cover surfaces as “Wall”, i.e. SrfType would be “0 - Wall” instead of “1 - Roof” in the HBSrf component, and have the skylights be displayed (and simulated) incorrectly as windows?
    In my case, these surfaces are really the upper bound of the building and are not in contact with other objects or context shapes on every side, so I could have the natural ventilation set to “1 - Window natural ventilation” in the Set EP Air Flow component and calculate heating loads treating skylights as windows.

Of course if I try to display them through “Decompose based on Type” their respective representations would be wrong, and maybe (but quite sure) air infiltration simulation would be wrong too, since I assume windows are considered as object where air impacts with a vertical angle between ca. 0° and ca. 180° and not with an horizontal gradient, as it should be better the case for skylights (if of course the light slope of the roof is ignored), but maybe infiltration is not calculated according to object attributes, rather through their real geometries.
I ask because one indication I received upon commencing the simulation is that the building has been designed to prevent any cross ventilation/chimney effect, (this is due to the particular nature of stored items) and therefore I would like to assume a more cautious approach on the matter, although I could also set cross ventilation factors to 0 or tweak ventilation schedules or settings for both windows and skylights separately to avoid any unwanted effect.

Thanks again for any insight you could give me on the subject!



@fabrizio.m.amoruso ,

Thanks for the clarification. It seems odd that the design you describe includes operable windows and yet you also say that the building is being designed to not have any airflow (no cross ventilation or stack effect). If this is the case, I question why there are operable windows are in the project at all since it seems as if they are purposefully being designed to be ineffective.

To give you my honest recommendation, I’d say that you can exclude the operability of the skylights entirely from the simulation if they are purposefully designed to be ineffective as you have described. Alternatively, you can make an effort to convince the others on the project that they should actually use natural ventilation as a strategy and that both the ribbon windows and the skylights are operable, creating a controlled stack effect that can have a benign impact on energy use.

If people demand that you show them the impact of the ineffective operable skylights, I would model them just by increasing the infiltration slightly to show the skylights having a really small effect. Note that infiltration in E+ is just modeled as a fixed flow rate and not anything that responds to geometry. Just set an airflow on the Set EnergyPlus Son Loads component and assign a schedule using the Set EnergyPlus Zone Schedules component.

Yo answer the question about skylights, they are treated the same as other windows on wall surfaces by the Set EP Airflow component. But I would not use the 1- window based operation for your case here since it’s a bit more complex. Using the 2- Custom Stack/Wind Ventilation will give you a lot more control and allow you to model a natural ventilation strategy properly.

Hi Chris,

Sorry if my answers appear to deliver more doubts than clarifications, it has mostly to do with certain buildings specifications and wishes from (and for the) intended use of the building, and that’s why everything seems a little confusing.

The presence of ventilation through both roof and windows may be explained by a number of facts:

  • The need of ventilation during summer is foreseen to be a completely different affair as in winter. In fact, and I don’t dispute critical opinions on such matter, the LCCA covers just the heating system, but not cooling loads (…). Window and skylight openings supply a different function for each season, as they are manoeuvred in separate ways following different times of the year;

  • The request for stack ventilation is sometime necessary due to the presence of hangar truck doors, and the need - only in special occasions, though, which unfortunately are not yet scheduled - to ventilate fumes as soon as required from regulations;

  • As funny as it would sound, the request of obtaining a ventilation process which could be described as “quasistatic”. This means to effectively slowly introduce air inside the building (the ventilation schedules are tailored to suit such need with very small opening time windows each time, and reflect such wish) from different positions at a very slow rate, and use the placing of windows and skylights in a strategic way, (ribbon windows are usually never to be opened together with skylights) as to allow air infiltration through different combinations. All of this with the intended purpose of trying not to alter consistently the internal temperature or create internal forces due to currents, as task which might appear of course taunting in nature and realization;

  • The main cause is then to avoid any current, but to have multiple options to control ventilation. The ribbon windows are also, in relation to the surface of the exterior facade of the building, very few in number, and serve mostly other purposes such as, very basically, daylight source and control of incoming/outcoming trucks.

I hope this much could clarify in any way your doubts, many thanks again for the very helpful explanation.