Infiltration heat loss

ondrejg · Posts: 23

Hi,

I am using a scheduled natural ventilation in my model and I understand that this way the calculated heat loss from infiltration in the heating design should be dependent ONLY on the infiltration rate entered in Construction tab. However, I found out that the infiltration heat loss is different for the same room with and without windows. Also, in one particular case the infiltration heat loss is higher in a room with smaller volume than in room with bigger volume, which should not happend.
Can someone explain how the infiltration heat loss is calculated in the heating design? Do the external elements (windows, walls, doors) matter?

Thanks,

Ondrej

Andy Tindale · Posts: 2432 Location: Stroud

Hi Ondrej,

I think that the difference between the 2 cases (with and without windows) probably causes different air temperatures which in turn causes different air density. The density of the air will impact on the infiltration rates reported by E+ (given a fixed input value). I've copied the text from a knowledgebase article that discusses this issue further:

The difference in air changes per hour (ac/h) between the sum of mechanical and natural ventilation and infiltration values you entered on in model data and the values reported in simulation results is caused by the method used to calculate and report ac/h in EnergyPlus v4 and later. The new way of reporting ac/h includes consideration of the air density and the exact air change rates reported will depend on the density (temperature) of the outside air being introduced into the building relative to the density of the zone air.

More specifically, the design volume flow rate entered in ac/h is converted to mass flow during the simulation using the current outdoor air density. The simulated mass flow rate is then reported back out in indoor air density (for infiltration and natural ventilation) or EnergyPlus standard air density for HVAC system outside air, neither of which will ever (or extremely rarely) match the outdoor air density. So, the ac/h outputs will never exactly match the inputs.

ondrejg · Posts: 23

Hi Andy,

Thanks for your quick response. You are right. I tried to simulate the same thing with the temperature control set to "Air Temperature" (previously it was Operative) and the infiltration heat losses are identical for room with or without windows. This confirms your explanation.

Thanks,

Ondrej

Shortcuts

Login