Unmet Load Hours (aka “The Bane of the Completed Simulation”)
What is an unmet load hour?
Oddly, the definition of an unmet load hour is partially dependent on the software you are using, which may or may not allow you to define the throttling range (the acceptable +/- degrees where a space is deemed to be within its target setpoint).
An unmet load hour occurs when the HVAC system lacks the capacity to keep the design setpoint within the throttling range for the duration of an hour. The other caveats are that the space must be conditioned, and in operating mode (occupied).
Why do unmet load hours affect you?
Unmet hours will stop a “completed model” from reaching completion. Some have said that they can quickly ruin your professional life (seriously).
If you have greater than 300 unmet load hours, your model will not comply with ASHRAE 90.1-2010 Appendix G. This means you can’t get your LEED points or achieve similar compliance unless you have a very good reason for the unmet hours. It's usually easier just to eliminate them.
How can you lower unmet hours?
There are an infinite number of inputs that can cause unmet hours. None of these are surefire fixes (except #5, which is cheating if it's the only one you use!), but here are 6 tips that should help in most simulations:
Tip #1: Make thermal blocks
On any given hour of a simulation, you can have an unmet hour in any given space. If you combine multiple spaces into a single thermal block, simple statistics are in favor of less unmet hours. This tip does work beyond statistics though. Smaller rooms are more likely to have rapid temperature fluctuations and typically yield more unmet hours as a result.
Tip #2: Appropriate Driftpoints
If you allow a room or zone to drift several degrees or more when it is unoccupied, also consider the time it takes to change temperatures. Zones that contain very heavy cement or block walls take more time to change temperatures. In such zones, decreasing the delta T between occupied and unoccupied hours results in decreased unmet hours. Alternatively, you may need to allow adequate time for the zone to change temperature prior to occupancy via “Optimum Start”
Tip #3: Add partitions
Partitions, as opposed to walls, transfer heat between zones. If two adjacent zones have different setpoint temperatures, adding a partition allows one room to indirectly condition the other room. This can reduce unmet hours especially in rooms such as storage closets.
Tip #4: Manually Enter Design Airflows and temperatures
Sometimes an assumption in your energy modeling inputs (such as forgetting the partitions) can lower the calculated peak loads of a room. This can result in unmet hours. In many cases where we are concerned about unmet hours, the building is already designed and oversizing has already been implemented. The actual designed flow rates and temperatures should be implemented. Since equipment generally has a safety factor, the designed flow rate usually encompasses a higher load capacity than what is auto-sized by the model, and thus reduces unmet hours.
Tip #5: Change your throttling range
In some software, such as eQUEST, you can change the throttling range. Now, please don’t cheat. Some spaces are not meant to fluctuate +/- 10 degrees! However, with some spaces, a throttling range of 10 degrees might be reasonable. On the other hand, you should be able to resolve most unmet hours without resorting to this option (see Tip #6). Also, to be clear, for typical office spaces, a throttling range greater than 2 degrees is pushing the limits of thermal comfort.
Tip #6: Due Diligence on your models
I have no math to support this tip, but this notion would likely be supported by many (or all) of my peers. Unmet hours can provide a guage on the quality of an energy model. Now, if you have an occasional model that is troubling, that's different. But, if almost all of your models have large quantities of unmet hours, you may want to reconsider some of your typical energy modeling practices.
Sometimes unmet hours result from errant modeling practice (such as using too many schedules on a single airside system). Improving your modeling skills “magically” diminishes unmet hours.
Remember that these are just general observations. When it comes to unmet load hours, there are onlyl two things I am certain of:
- Virtually anything can cause unmet load hours.
- If you have to resort to tip #5 for most of your rooms, the problem is not your throttling range!