Air economizers are required by the International Energy Conservation Code (IECC), yet their impact and value are somewhat abstract at first thought. How much energy and money do they actually save? Do economizers operate enough during the year to be cost effective? Why does the Energy code require them in certain climates and not in others? Which economizer control type is best, dry bulb or enthalpy? The data provided in this blog will provide some insight into all these questions.
The above graphs (A and B) depict the energy (10^6 BTU) and cost ($) savings per year and are calculated using Trane Trace to run an energy model analysis based the below parameters:
- Code minimum envelope building per climate zone
- Single-story, 10,000 SF office building
- Conditioned by constant volume rooftop units using either dry bulb or enthalpy economizer control
By looking at Graphs A and B, you can see that dry bulb economizer control is more effective in virtually every ‘#A’ climate zone, with the ‘A’ designating a moist climate zone (see Graph C above). Additionally, the data tells us that the value of economizers in terms of energy and costs savings increases the further north the climate zone is with the savings generally tapering off beyond climate zone 4A. It is also interesting to point out that the savings economizers provided in climate zone 1A is essentially not existent which is a smart move by the IECC to not require economizers in this climate zone. Enthalpy economizers in 1A generated zero savings and dry bulb economizers only saved $4/year. However, the savings in climate zone 2A are not exactly impressive. Depending on the tonnage of the unit, an economizer accessory can cost on average of $1,000. With the best cost savings of a dry bulb economizer in 2A only being $56/year, that is pushing close to an 18-year payback which happens to be near the end of the average lifespan of a DX packaged rooftop unit.
Interestingly, as seen in Graphs D and E directly above, the tables have turned in the dry climates (#B) as enthalpy economizer controls provide the most energy/cost savings. It is also important to note that all the dry climate zones, aside from 2B, have a similar savings return. Whereas in the moist (#A) climates, demonstrated in Graphs A and B, there are clearly some zones with ideal conditions for economizers and others where economizers were not compatible. Take climate zone 5B enthalpy economizers for example as the highest cost savings zone at $199/year. This would result in a payback period just over 5 years, which is much more reasonable and beneficial than the 18-year payback in climate zone 2A!
Closing
The data above provides evidence of the value that economizers offer, in some climate zones more than others, and selecting the right controls type can have a significant impact on your system’s energy/cost savings.
