We've covered HVAC economizers at a high level, so let's dive deeper into what is required in a Title 24 compliant economizer.
Gear Driven Dampers
One of the most common fail points of economizers from years past were the linkage arms. They would fail early and often leaving the maintenance team to either continue fixing them, or, the more common solution, simply close them. Now this meant no further maintenance issues but it also led to no outside air being provided to the building. Occasionally complaints of stuffiness would result. In one case, we've seen a maintenance person use a 2x4 piece of wood to prop open the damper to let a little bit of fresh air in. Ingenuity? To resolve these band-aids, Title 24 requires gear driven dampers. They are exceptionally more reliable with life cycles of 10 or more years but they are heavier, more difficult to fabricate and of course more costly.
Fault Detection and Diagnostics
Fault detection and diagnostic technology identifies problems by continuously testing the sensors and dampers in the system. This lets maintenance personnel know when a problem exists.
This technology is great for resolving economizer problems. In fact, Title 24 requires the controller have, as a baseline, the ability to detect outside air sensor failure, dampers not modulating, excessive outside air, among others. This definitely helps maintain the efficiency of the unit, but it also takes a smarter controller to do this. But smarter controllers = higher cost.
There are other requirements Title 24 puts on economizers and, in our opinion, are good requirements but they do increase the cost.
For most rooftop package units it still makes sense to install them because the only way to get rid of them prescriptively is to provide a higher efficiency unit. In non-coastal climate zones this means a unit that is 30% better than the baseline or the equivalent or 17-18 SEER depending on the unit. It is important to note that the cost to increase the efficiency is typically much greater than that of the Title 24 economizer.
When we start to review the economizer requirements for units like fan coils and VRF the story changes substantially. To install economizer capabilities (i.e. 100% outside air) on a 5-ton fan coil becomes challenging. This is mostly due to the fact that we need to install a full size duct to the unit and provide an actuated damper for the return and outside air paths. All sorts of concerns crop up, such as; do we have enough room, where can we penetrate the roof/wall, the list goes on and very quickly we start considering other options.
One good aspect of fan coils and VRF is that the higher efficiency units are very easy to come by and are relatively close to the same price point. The additional cost of the high efficiency unit is often a fraction of trying to install the duct work, dampers and controls for an economizer.
Avoiding HVAC Economizers
If the energy modeler builds a performance model, the HVAC economizers might be able to be removed from the project entirely. This is contingent on other improvements in the building making up for the penalty of not installing them. This can be a difficult item to model out due to the penalty the CEC calculator attributes to not installing an economizer but it may be an option.
To summarize, in general RTU’s and large custom air handlers should probably go with economizers as their compliance path. Fan coils and VRF are typically better off increasing their efficiency. And if neither of them seems like viable options we can review building a performance model to further review the project’s options.
If you have any questions on any aspect of economizers feel free to get in touch. We’re happy to help!