Airside economizers are very simple in theory. Their function is to shut off the unit’s compressors and provide “free cooling” when the temperature outside is cold. Done, end of story. But trying to figure out if the local code requires one, and then trying to understand what type is required can leave you dazed and confused. If you’ve done this research, these are the questions you are asking yourself: which local code do I need to follow? (IECC 2006? 2009? 2012?) Does this project need to meet ASHRAE 90.1? (2010? 2013?) What climate zone are we in? (4A, 4B or 5B?) Is this application exempt based on run-time or a special humidity requirement? Should we use dry bulb, single enthalpy or dual enthalpy?
At the end of the day we can make this decision REALLY easy by just asking the question: does adding an economizer make financial sense to the owner?
Let’s first go over when an economizer is REQUIRED by code (and don’t worry, we’re going to keep this simple). We see this missed almost every day, but we think it is good business practice to follow your local codes, even if a permit is not pulled (I know; you pull a permit on every job, right?)
Economizer requirements and exceptions are based on local codes AND climate zone. Kansas City, for example, is in Climate Zone 4A (See figure 1). If you are in cfm’s northern territory, you might be in 5A or 6B. The following is a list of minimum unit sizes that require an economizer, depending on which International Energy Conservation Code (IECC) the local code follows. (Note: this is for Climate Zones 4A, 5A and 6B, which is very similar to the rest of the country, but it is NOT all inclusive)
- 2006 IECC – economizer required on individual DX cooling units greater that 54,000 BTU/H
- 2009 IECC – economizer required on individual DX cooling units greater that 54,000 BTU/H
- 2012 IECC – economizer required on individual DX cooling units greater that 33,000 BTU/H
- 2015 IECC – economizer required on individual DX cooling units greater that 54,000 BTU/H
In addition to the economizer requirement, pay close attention that, since the 2009 IECC, Demand Control Ventilation (DCV) must be provided for each zone with spaces > 500 sq. ft. and the average occupant load > 25 people / 1,000 sq ft of floor area where the HVAC system has:
- An airside economizer
- Automatic modulating control of the outdoor damper
- A design outdoor airflow > 3,000 cfm
Economizers, however are not required in EVERY application; there are some exceptions. For example, in 2015 IECC, economizers are NOT required in these situations:
- Systems expected to operate < 20 hours/week
- Where > 25% of air designed to be supplied by the system is to spaces that are designed to be humidified
- Systems that serve residential spaces where system capacity is < 5 times requirement in Table C403.3.1(1)
- Where use of outdoor air for cooling will affect supermarket open refrigerated casework systems
- Where cooling efficiency meets of exceeds efficiency requirements in Table C403.3(2)
JUST MAKE SURE YOU CHECK WHAT THE LOCAL CODE REQUIRES. If you are in Kansas City, for example, we have a wide range of local codes. Lenexa, for example follows 2006 IECC, but many others, including Overland Park and Kansas City MO, follow the 2012 IECC.
Okay- now that we have the boring details out of the way, let’s find out if an economizer is worth the investment for an owner or tenant. There are lots of ways we can look at this. First we can look at the return on investment (ROI) or payback (in years) for JUST the energy savings when the economizer is operating in “free cooling” mode. We can also look at how an economizer lengthens the life of the compressors and other components in the refrigeration system. Since the latter is a little harder to quantify, the purpose of this study will be to determine if an economizer makes financial sense based SOLELY on energy savings during free cooling mode. In the interest of time, we are going to look at three different applications: a typical office building, a retail space and a process application. And we will look at the most common rooftop sizes: 5, 7.5, 10, 15 and 20 tons.
Full disclosure: this is our BEST, most CONSERVATIVE ESTIMATE. We assumed the economizer only ran when the ambient temperature was below 55 deg F. Most economizers can be programmed to run at a slighter warmer temperature than 55 for a “first stage” of cooling. This will REDUCE the payback length and INCREASE the ROI.
Here are the results:
What about process applications? Since this type of application will require cooling nearly year round, the payback will be less than 6 months with an ROI in the 200% range. In other words, if your process cooling application can handle fresh air, it’s advised to follow Nike’s advice, and Just Do It!
What are your thoughts on airside economizers? Do you like them? Do you think they make financial sense?