Heat Balance Technical Note

It is important to understand when looking at Zone heat balance results that the values do not necessarily add to exactly zero. Factors which have a bearing on this are: 

1. Generally the most significant factor, especially for results accumulated over time intervals of a day or shorter, is that zone heat transfer with walls, floors, roofs, etc involves storage of heat in thermal mass. Any such heat storage in thermal mass will prevent a perfect heat balance. This is especially true in thermally massive buildings. When reviewing heat balances it is generally best to view results over a reasonably long period (> week) to reduce the impact of any such thermal storage issues.
2. Solar radiation values reported by EnergyPlus are gross values and do not account for the radiation re-reflected back out of internal and external windows. EnergyPlus tracks solar radiation in full detail but does not report all of the calculated values.
3. Solar and some internal gains are mostly radiative in nature and find their way directly into the fabric whereas the cooling is typically to the air. In thermal modelling terms, it is not possible to make a simple heat balance of heat that is added to different parts of the thermal network. For example, heat transfer gains between the zone and the surfaces cannot be added directly to ventilation gains because they are to different 'points' in the zone heat balance (surface heat transfer is to the 'rad-air' point and ventilation is to the 'air' point).
4. Minor EnergyPlus inaccuracy in reporting conduction through window frames causes a minor error (~1%) in Glazing heat gain reported values. This does not affect the rest of the simulation, it is just in the Glazing heat gain report.

See also https://designbuilder.co.uk/helpv7.3/#Simulation_Detailed_Results.htm