How do I fix my Unmet Load Hours (UMLH)?

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.


Categories: 8. Nonresidential Title 24 Performance

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