Frequently Asked Questions

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8. Nonresidential Title 24 Performance

Here are some examples of errors and the causes. 2016-May-12 14:13:45 –  3: Temperature (low) out of bounds for zone=13-PARKING GARAGE -LV 2 for surface=RAISED FLOOR542 In this example, the zone has not been modeled properly.  You may have taken some shortcuts on inputting the surfaces and don’t have a complete description of the zone.  While we ask for 3 surfaces minimum, it is important that those three surfaces comprise a significant area of the zone. 2016-May-12 14:13:45 – 23: Surfaces in Zone=”10-PARKING GARAGE -P2″ do not define an enclosure.  Number of surfaces <= 3, view factors are set to force reciprocity. Similar to the above warning, this space has incomplete geometry.  Possibly adding a 4th surface will satisfy this condition. 2016-May-12 14:13:45 –  15: GetSurfaceData: The total number of floors, walls, roofs and internal mass surfaces in Zone 15-LOBBY / RESTROOM  is < 6. This may cause an inaccurate zone heat balance calculation. While rare, you may have circumstances where you need to define up to 6 surfaces to satisfy the CBECC simulation.

You can review two sample files we provide, the the lowrise example is the MF Example.bld, we also provide a file  Highrise Res Electric.bld Both are modeled at the Plant level of the tree, Domestic Hot Water tab.  The inputs for the CHPWH are at the bottom section of the input screen, you can choose the type of Heat Pump, quantities, etc, as well as tank volumes, and supplemental loop capacities, tank locations and locations of condensers.  The input at the top is not used in this case if you are doing Lowrise projects.  In the case of Highrise projects the water heater input at the top of the screen is used to supply Nonresidential spaces (corridors, lobbies and support spaces).  No you cannot assign those spaces to the CHPWH, CBECC does not allow for that circumstance, so you will have to assign those spaces to a standalone WH.

When molding HIghrise Res projects, at the PV+Batt tab, we also provide for the circumstance where PV assist is being provided to the Central HPWH, only the top boxes need to be checked, the software will size the PV system for you.  It is important when using this feature that you check you compliance results on the PERF-01, since this PV assist is actually reported there, not onscreen, as a “flexibility” credit, along with the PV size.

Assuming the form shows the “Building Complies” statement, this is caused by the fact that you have too many Unmet Load Hours.  You should read the Unmet Load Hours topic on how to correct this issue, then re-issue the report.

If the report shows “Building Does Not Comply” then your energy usage is too high.

Recently, the CEC has started to enforce the ACM requirement that Unmet Load Hours cannot exceed 150 in any given zone.  Start by reviewing the NRCC-PRF-01 report, towards the end is Table R. Unmet Load Hours.  You will see a listing of zones that have this issue.

Unmet Load Hours that are reported basically indicate how many hours the Zone will be outside of the thermostat setpoint.  You cannot adjust that setpoint, Title 24 restricts the input.  There are a number of causes for this message, but the net result is the zone either has too little heating, too little cooling, or inadequate airflow to maintain comfort conditions in the space, as reported by the CBECC engine.  The CEC also offers the following guidance:

In EnergyPro, at the Zone level of the Building Tree, Mechanical Tab, we do offer a checkbox Add Cooling which will add cooling in the background to meet unmet cooling load hours.  Note this checkbox will not impact the reporting, in other words, any extra cooling system will not appear on your reports.

Another cause is you have a Single Zone System  (SplitDX, Packaged DX,  WSHP, etc) that is serving more than one Zone.  The problem with this configuration is that the first Zone will have the Thermostat, and control the System, and other Zones will simply get what ever conditioning is called for by the first Zone.  There are several solutions here, one of which is to combined the Zones into a single Zone.  Another solution is to specify a system capable of having more than one Thermostat, such as a Packaged VAV system.

It may be that the HVAC system is undersized.  That could include too little capacity on the heating side, the cooling side or possibly insufficient airflow.  In some cases, you may not even have heating or cooling as part of the system.

When dealing with more sophisticated systems such as Variable Air Volume, be sure the airflow as the Zone level of the Tree, Mechanical tab are sufficient, and that the VAV Terminal boxes all have sufficient heating coils.

Note that while this requirement must be me to remove any watermarking on the Certificate of Compliance, it does not mean you must re-design your HVAC system.  Only that it be modeled such that the load hours are met.  That may in fact mean modeling your system with “increased” numbers that don’t really exist, so be sure to include a note in the Remarks section to document these numbers.

Please do not send in your file with a request that we diagnose this issue, it is not something that we can provide as a normal support service.  One route you may wish to consider would be submitting Prescriptive Mechanical documentation instead, since that approach does not have this requirement.


The message you are getting comes from CBECC Com and looks something like this:

2019-06-25 14:20:49 – Error:  Space ‘S-1-Retail Zone’ requires occupancy sensor ventilation devices as per Section 120.1(d)5. Please confirm that occupancy sensor controls will be installed as per the requirements listed in Section120.1(d)5 evaluating rule: Set CHECKCODE Spc:OccSensorCtrl  (25:’Space-Ventilation.rule’ line 3777)

At the Zone level of the tree, Mechanical Tab, you must check the box for Occupancy Sensor Ventilation controls.  As outlined in the message, this is a mandatory control now specified in Section 120.1(d)5.  If you do not understand the requirement, please review this code section carefully.  Your alternative is to choose another occupancy type.


You probably have a ductless system and have specified the ventilation at the Zone level of the Tree, Mechanical tab as coming from the HVAC system, but that type of system cannot provide ventilation.  The typical error appears as:

“thermalzone ‘x’ has ‘forced’ ventilation, but the ventilationsystem for the thermalzone ‘x’ is not defined.”

At the Zone level of the tree, Mechanical tab, change the ventilation over to another choice (but not natural ventilation if it is a commercial occupancy)

When modeling cooling towers, it is very important that you input the tower parameters accurately since the CBECC simulation is very sensitive to these inputs.  A message such as this may appear in the LOG file.

2016-Dec-05 19:19:43 –        1: Autosizing of cooling tower UA failed for tower XXXX
2016-Dec-05 19:19:43 –     EnergyPlus Severe Error(s) (1):
2016-Dec-05 19:19:43 –        1: Bad starting values for UA

Review the following parameters on the cooling tower for accuracy as it relates to the load served by the tower.

Tower Tonnage (In the case of a chiller, this generally should be about 20% more than the chiller tonnage)

Tower Setpoint (we suggest 80 degrees)

Tower Fan Airflow (we suggest 200 cfm/ton)

Condenser Water Flow Rate (It is very important this is consistent with the tower tonnage, very low or very high gpm/ton flows here cause problems, so typically 3 gpm/ton)

Careful and accurate input of these parameters will ensure a successful simulation of the tower.

One cause of this is the Visible Transmittance (VT) of the windows in the building, which affects the amount of daylighting provided and this affects how effective the daylighting controls will be in their ability to reduce the electric lighting usage.  The CEC CBECC engine accounts for this by modifying the energy usage of the Standard building in response to the VT of your window selections.  Please review the Prescriptive Envelope criteria for window VT to see what the budget building is using as the reference VT.

A second cause is you have large spaces subject to daylighting provided via skylights and windows as outlined in Section 140.3 of the code.  This section calls for daylighting openings to be provided in a space and your lack of those daylighting openings results in more electric lighting usage than the Standard building.

You are probably seeing a message such as:

 “airsystem ‘xyz” has fan control = ‘cycling’, but this system is defined to provide ‘forced’ ventilation for control zone ‘zone A’ evaluating rule: set checkcode airsys:fanctrl (75:’hvacsecondary-systemcontrols.rule line 330)

This is a message from the CEC CBECC side and basically indicates you have a Zone in which the HVAC system will be providing the ventilation.  However, in the Central System Library, at the Controls Tab, you have indicated the fans do not run continuously.

The fans must run continuously to provide ventilation if the ventilation comes from the HVAC system.  You should change this setting to Continuous.

Note that the ventilation setting is done at the Zone level of the tree, Mechanical tab.  In Hghrise Residential/ Hotel Motel occupancies it is acceptable to use Natural Ventilation and thus this could be set to “Natural”, with the fans set to “Intermittent” but not in commercial occupancies.

In the assembly editor, you will add Exterior Insulation to your wall assembly.  If the project is Nonresidential Performance, this will be done in the Layers tab by clicking on the Yellow Plus sign to add a layer.  Double click on the layer and select an insulation material from the material list, then move it so it is on the exterior of the wall framing.

If the type of project is Prescriptive, then edit the JA4 tab and simply type in the R-Value of the insulation in the Exterior Insulation entry.  If the project is a Residential Title 24 Performance project, enter the Exterior Insulation on the Res T24 Performance tab.

For residential projects starting in the 2016 code, the standard water heater is a gas fired tank-less water heater in both the Prescriptive and Performance compliance approaches so using any unit with a tank, and especially an electric water heater will produce a significant penalty.

For commercial projects, the standard water heater under Performance is also a gas water heater.  However, under Prescriptive, you are allowed to use an electric water heater.  Our recommendation is to submit the DHW using the NRCC-PLB-01 form which is available under the NR T24 Prescriptive forms list.

Under the Calculation Options, bottom left button, click and you will see the NR T24 Performance calculations in the list above.  Select this, then on the right unclick DHW from the scope and then rerun and the penalty will be gone.

In order to guarantee a unique name for each element in the tree (which is a requirement of the CBECC-Com engine), we add a numeric after the name to ensure uniqueness.  You cannot remove the number, it is needed since every item must have a unique name, which can get very burdensome since this would mandate each individual window/wall etc. be given a unique name as you are doing your inputs.

Typically the error will read something like this:

Error:  Construction assembly ‘CONC. WALL7’ may not be assigned to surfaces with more than one Status, i.e. New, Altered or Existing

A limitation of the CEC CBECC engine is that any assemblies or windows in the Building Tree that have a different Status (ie Existing/New/Altered) may not point to the same Assembly or Fenestration entry in the library.

The solution is to create a unique entry for each case, for example create a wall named (e) CONC. WALL that would be assigned to the surfaces that are existing, create another called (n) CONC. WALL for the new instances.


Since a single zone system only has one thermostat, the first zone listed in the tree is the control zone, the other zones just get whatever the control zone calls for.  This should be modeled as one system for each zone.

The calculation engines in EnergyPro that are now used for Title 24 code compliance are developed and maintained by the California Energy Commission. The new Residential and Nonresidential calculation engines they provide are significantly slower than the prior ResSim and DOE-2 engines we provided in older versions of EnergyPro. The new engines incorporate CSE and EnergyPlus, which are more powerful than the older calculations, and simulate the zones much more thoroughly, resulting in longer simulation runtimes.

Try to keep the number of zones in your model to an absolute minimum, but it is not unusual to see run times of over an hour on more complex models.

To speed up run times, you can select the Quick Analysis option, which will run a partial year simulation.  In the bottom left, click on the Calculations.  In the list that appears above, select either Res T24 Performance or NR T24 Performance.  On the right will appear the option for Quick Analysis.  This will cut your runtime in half.  However that option will trigger a report that includes a watermark that states “Not useable for compliance” since you must run the full year for code compliance.

Another option is to check the box for “Do Not Generate PRF01/CF1R”  This will skip the report generation feature that occurs at the end and can save additional time, but you will not get the Title 24 reports needed for permitting.

The CEC has a default chiller operating assumption that is always used for the standard building.  Basically, multiple chillers are sized equally and sequenced on incrementally as the load increases.  The inputs that you use to sequence your chiller operation will not affect the standard building.

For Nonresidential buildings, starting with the 2013 modeling rules, the Standard (Budget) building will typically have a Variable Air Volume (VAV) system anytime the building being modeled is greater than 10,000 sqft or more than one story. Your proposed design is likely a Constant Volume system, and thus is going to have significantly higher fan power usage than the Standard building. We have been recommending you choose prescriptive compliance when you have high fan power.

For High-rise Residential buildings, the Standard building will have fans that run intermittently instead of a continuous running fan for the guest rooms.  Check your input at the Central System, Controls tab to see what setting you have for the fan operation, and consider if using Natural Ventilation (Zone level of the tree, Mechanical Tab) will be possible.

In both cases, review your inputs for the fan BHP to be sure this number is not excessive.  Please also understand that the fan power used in the Performance modeling for the budget does not necessarily align with Prescriptive, so many times your fan will comply under Prescriptive and not Performance.  If so, take that route.  For VAV systems, review the airflows you have input at the Zone level of the tree, Mechanical tab as well as the VAV turndown ratio indicated on the VAV terminal boxes.

EnergyPro offers three possible scenarios here for modeling this type of device as follows:

Nonresidential Calculations that use ducted HVAC systems in EnergyPro that are not Zonal (FPFC, PTAC, WSHP, VRF) have Exhaust Air Heat Recovery ERV or Heat Recovery Ventilator HRV as an option on the HVAC system.  See the Outside Air tab in the Central System Library for these input options.

Nonresidential Calculations can also specify at the Zone level of the Building Tree, Mechanical tab, the use of a Balanced Ventilation system with the option of energy recovery.  This is a new option in EnergyPro 8, and would typically apply to an ERV or HRV in a highrise residential application.

Lowrise residential buildings have the option also.  Start by going to the Top level of the Building Tree, Residential tab and specifying a non-default IAQ fan.  Now at the Zone level of the Building Tree, Dwelling Units tab you can configure the IAQ fan as a balanced ventilation system with the option of specifying the heat recovery effectiveness.

Section 120.1(b) of the code dictates the minimum amount of outside air that must be provided based upon the CBC occupancy type of the space.  This is a known quantity based upon your Zone Occupancy selection from the CBC, and as a result you cannot go below this minimum.  ASHRAE 62.1 is not a consideration in the California Energy Code.  Changing the occupant density and the cfm/occupant at the Room level of the Tree, Occupant tab will have no impact, since the values taken from 120.1(b) are hardwired into the CEC CBECC engine.  The only change you can make is to increase the outside air above the minimum, this is done by going into the Room level of the Building Tree, Occupant tab, and increasing the Occupant Fraction above the default of 0.5 (50%) (taken from 120.1(b)) to a higher number.

Your other choice is to use a fixed number of occupants in the space.

Note that changes to the ventilation numbers will impact your load calculations, since you are free to input any values you wish for these calculations.

This issue has been reported to the CEC and they anticipate addressing this in a future release. For now, our suggestion is that you upsize the DHW system per this message. Note that the size shown in this message is the required output and volume of the DHW system, while EnergyPro has you enter the input rating so you will need to adjust accordingly.

Starting in version 6.5 of EnergyPro, we now have a new layer by layer editor incorporated into the Assembly library. Note that the only way to define an assembly for the NR Performance calculations is using the layered approach. Note also that these layers do not affect any other calculations, just Nonresidential performance modeling. Also, any entries made on the JA4 tab of the editor (rigid insulation, etc) will not be seen in the Nonresidential performance simulation, so If you need to add rigid insulation you will have to add a layer into the Layers tab.

Please be aware that older assemblies from prior versions will have a minimum set of layers built into the assembly, so it is very important that you review these layers before you proceed with your analysis.

The list of materials published in EnergyPro is the only list that the CEC permits in the simulation. While the CEC may decide to add additional materials in a later release, for now you must work with that list. The CEC will permit you to build layers to represent a particular material.

You will need to edit the assembly layers to meet the code minimum u-factors dictated by the Title 24 code. See the topic “How do I create an assembly for the new NR T24 Performance calculation?” for details.

When modeling this type of system with the DOE-2 calculation, we had you put in a dummy zone. That is no longer needed, nor should you do that with the new CBECC Com calculation running under the Nonresidential T24 Performance calculation. Note that currently, this calculation only permits DOAS systems feeding Four Pipe Fan Coil, Packaged Terminal Air Conditioners and Heat Pumps, Hydronic Heat Pumps, SplitDX , PackagedDX, Single Packaged Vertical Units and VRF systems. Simply create that system as you have per prior instructions, and select the “Outside Air From” setting at the system level of the tree. See the DOAS.bld example we include in the sample files section.

Because the PRF-01 and CF-1R reports are created through an online based report generator on the CEC website, you will have to have an Internet connection in order to generate any Certificates of Compliance. Assuming you have Internet, you might check with your IT department to see if your system blocks the report generator from being accessed. You also have the option of turning off the reports, this option is found by clicking on the bottom left “Calculations” and then in the list above selecting the appropriate calculation.  The display on the right will show the option to turn off the reports. You will eventually have to uncheck tis option and rerun the calculations to get the final Certificate of Compliance.

The CEC provides a number of FAQs related to the CBECC Com engine used by EnergyPro at the following link.

If you select the Calculation option on the bottom left side of the screen, then select the T24 Performance calculation (either Res or NR) in the list that appears above this selection, you will see an option to “View CBECC Log”. If you click on this button, you will see the CEC log file reporting the results of the simulation. Note that this file will not exist unless you first uncheck the option for “Delete Temporary Files”, then click on calculate to run the simulation. At the end of the simulation you can view this log file and see what problems you have.

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