# Verifying energy balance of Energy+ simulations

I am trying to verify the energy balance of Energy Plus models. As a reference I took the very simple ‘1ZoneUncontrolled.idf’ file for location CO and looked at the ‘Zone Air Heat Balance Deviation Rate [W]’ and also monitored Output:Diagnostics-> DisplayZoneAirHeatBalanceOffBalance. One segment from the output from the .errr file was

```
************* ** Warning ** Zone Air Heat Balance is out of balance ... zone named ZONE ONE
************* ** ~~~ ** This error occurred 7254 total times;
************* ** ~~~ ** during Warmup 0 times;
************* ** ~~~ ** during Sizing 0 times.
************* ** ~~~ ** Max=657.858682 {W} Min=1.957503E-003 {W}
```

- Does this mean that even for this very simple model, the energy balance is not being met?
- What do EnergyPlus modelers use for verifying whether the energy balance condition is being fulfilled?

Suggestions on which variables to monitor to track would be appreciated. I have gone through the zone and surface outputs listed in 'input-output-reference' documentation but no luck in finding a balance of the heat transfers in the zone. Thanks.

****Update of question with the next part following the answer from Neal Kruis**

Thanks Neal for the suggestion. Changing the HeatBalance Algorithm does improve the ‘Zone Air Heat Balance Deviation Rate [W](TimeStep)’ (it is basically zero…order of E-13) and there is no more warnings from Energyplus. So, the air heat energy balance is being fulfilled by Energyplus.

But just to be sure I calculated the balance manually. Going back to the basis for the Zone and Air System Integration, I used the following variables as outputs in csv.

```
A) Thermal Zone: Zone Air Heat Balance Air Energy Storage Rate [W](TimeStep)
B) Thermal Zone: Zone Air Heat Balance Surface Convection Rate [W](TimeStep)
C-H) Six Surfaces*: Surface Inside Face Convection Heat Gain Rate [W](TimeStep)
```

Since the idf file has no HVAC or internal loads or infiltration, all other parameters in the balancing equation, like convective internal loads, heat transfer from infiltration, heat transfer due to interzone mixing, air systems output are all zero.

**Check 1:** I looked at the difference between A) and B) and it is basically zero. So this check of energy balance is fulfilled.

**Check 2:** Here I took the summation of C-H) and A) i.e. summation of the outputs from the six surfaces (convective heat transfer from the zone surfaces) and zone air heat balance (energy stored in zone air). From my understanding, this should ideally be zero. But that is not the case as the summation of the variables deviate largely from zero and is still in the order of hundreds of watts.

**Check2b)** By following up on why this difference arises, I realized that if I take the `‘Surface Inside Face Convection Heat Transfer Coefficient [W/m2-K](TimeStep)’`

and then calculate the convection heat gain myself by using `coef*Area*deltaT`

, the energy balance is fulfilled for the heat gains calculated manually. I took the zone temperature and surface inside face temperature for calculating deltaT.

So my questions/comments on this issue are:

- Maybe I have misunderstood the report variable
`‘Surface ...`

Are you also looking at the convection from fenestration surfaces? Subsurface convection is not included in the convection of the base surface.

I also looked into this briefly - that model does not contain any windows. It does seem likely to be a genuine bug in the Surface Inside Face Convection Heat Gain Rate [W] report variable.