Problem with Ideal Loads
I’m trying to verify whether my EnergyPlus/OpenStudio model is behaving correctly, and I would really appreciate some help. I’m comparing my results with another building simulation tool, and the numbers I’m getting are significantly different even though the constructions and general assumptions are the same.
The building is a well-insulated residential house.
Exterior walls have U-values below 0.40 W/m²K,
Windows have U ≈ 1.2 W/m²K,
SHGC in my EnergyPlus model is set to 0.45.
In the other software, with the exact same constructions and the same thermostat setpoints, I consistently get around 22 kW peak heating load and around 18 kW peak cooling load. These values match the expected performance for this location and climate. This programme is without schedules, with constant loads for people, lighting, equipment.
However, in my EnergyPlus model (using Ideal Air Loads), I get much lower loads. Even when I increase infiltration in the living room to 2 ACH to approximate natural ventilation through open windows, my heating load is still much lower than in the other tool. If I reduce infiltration to more realistic values (0.3–0.5 ACH), the heating load drops even more, to the point where it looks unrealistically low.
In the other simulation tool, those 22 kW heating and 18 kW cooling results were obtained with only 0.5 ACH for living spaces, so I’m confused why EnergyPlus gives drastically different results even when I set higher infiltration.
I have checked:
Constructions (same layers and U-values in both tools)
Window SHGC = 0.55
Thermostat setpoints (identical in both models)
Weather files (slightly different outdoor temperatures, but only by a few degrees — not enough to explain such a large deviation)
Still, the total gains and losses in EnergyPlus look very different from the other program, and the distribution of loads across the building is also not similar.
I also included some surrounding trees in the OpenStudio model but they can easily be removed if needed — I can upload the OSM if anyone wants to take a look.
I would be extremely grateful if someone could check what I might be doing wrong or whether there is a fundamental modeling difference that I am missing.
Thank you in advance
Have you tried checking out the EnergyPlus Engineering reference, and comparing the equations in there with the equations in your other modelling tool? I think that would be a good place to start.
I am aware that there will be differences, but twice as big loads? So I need someone experienced to see if that would be correct
The weather file is different by "A few degrees"? That is not a small ammount. If your other tool uses temperatures of 18oC and E+ gets 16oC, for heating of a zone up to 25oC that's 30% more load.
Secondly, check the weather conditions: Is it cloudy or raining vs a sunny day? You need to use the same weather for a proper comparison.
Same with ground temperatures, they need to match or the floor can be an important difference in heat losses calculations.
Hello, thank You for the answer, the thing is that supply air temperatures are the same, the weather file is not, for example here Design Day for winter is 3 degrees higher than in other programe but for theSummer Design Day is within 1.5oC range... For ground heat transfer I have used Kiva, in other model there are constant temperatures of the ground by months, but there is not many conditioned spaces within the bottom floor, also top floor is well insulated so I don't think It can make that much difference. Thanks for the tips anyways, If You have any more ideas please let me know.
Getting two simulation tools to align, even with a simple test cell, tends to be a lot harder than you might expect. There are a lot of differences in how different tools take inputs and set up models that can make it very easy to overlook significant differences between them, e.g. how they model slabs, what the surface reflectances are, how they model windows etc. I would recommend first trying to make a simple single zone box identically in both tools, first without connecting it to the ground.
But also you do need to use the same weather file. What's the other tool anyway?