Remember that article I wrote about ducts installed against the roof deck and how I said it was probably the absolute worst single location for installing ducts?  Well, in the comments, Dave Roberts, a senior engineer at the National Renewable Energy Lab (NREL), wrote about a paper he co-authored last year and included a link to it. Up against the deck may be the worst place in the attic to install ducts, but Roberts shows that putting them in the attic at all is the worst place in the house you can install ducts. The report, Ducts in the Attic? What Were They Thinking?, summarizes the research that's been done about putting ductwork in unconditioned attics and basically says it's about the stupidest thing we do in homes that do a lot of air conditioning. I encourage you to download and read this report. If you're building or remodeling a home, make sure the general contractor (if it's not you) and the HVAC contractor get copies. I love the analogy they use to introduce one of the main problems with this location.

"Heat exchangers," they write, "are designed to transfer as much heat as possible from one fluid to another." Comparing this configuration to a solar water heater, they make the case that putting air conditioning ducts in a hot attic is an effective way to heat up the conditioned air as it travels from the air handler to the conditioned space inside the home. If you've studied heat transfer at all, you may recall that the rate at which heat moves from a warmer to a cooler body depends on the temperature difference, which we abbreviate as ΔT. An attic can get up to about 130° F in the summer, and the conditioned air entering the ducts is about 55° F or so. With hundreds of square feet of ductwork surface area in the attic and a ΔT of 75° F, the air coming out of the vents in your home will be significantly higher than 55° F. Throw duct leakage into the mix, and the problems are even worse. What Roberts and his co-author Jon Winkler did, in addition to reviewing the literature about this topic, was to model the savings possible when you relocate the ducts from an unconditioned attic to the conditioned space inside the building envelope.

They chose Houston, Phoenix, and Las Vegas as the locations for their modeled houses. The table below summarizes the main results. In addition to comparing ducts in the attic to ducts inside the building envelope, Roberts and Winkler also looked at electricity savings of other measures, such as adding insulation, installing better windows, and using higher efficiency air conditioners. air handling units marketThe table below shows that moving the ducts inside is the first thing you should do to save the amount of electricity you use.heat pump unit stopped working In addition to saving on air conditioning operating costs, the upfront cost of cooling equipment is lower in efficient homes. best ac units for residential homesRoberts and Winkler looked at moving the ducts inside compared to other building envelope improvements, and again, moving the ducts inside beats all the other methods for achieving this objective, as shown below.

This report, which the authors delivered at the ACEEE (American Council for an Energy Efficient Economy) Summer Study in August 2010, shows definitively that putting ducts in attics in cooling dominated climates is a practice that needs to end. Download the paper here: Ducts in the Attic? What Were They Thinking? Unique modular cabinet design Galvanized Steel Metal Cabinet Up to 14.00 SEER Stud mount or over the hot water heater Up to 13.00 SEERClose to nine years ago, I was involved in a bid for more than 100 preconditioned air units for a low-cost U.S. airline. This airline was one of the first to completely outfit all its boarding bridges with both 400 Hz and preconditioned air. This was also just prior to the last fuel cost spike and the industry’s move to be “more green.” Today fuel costs are back up and green airport initiatives are much more common. The difference today is there are so many more choices, but also so many misunderstandings about what preconditioned air units will best service a particular operation.

I am not sure if it is the industry’s engineers or creative marketers, but there are a lot of acronyms in the preconditioned air industry: With that information we can now discuss PC Air like the pros. PC Air’s main purpose is to save aircraft operators money. Yes, it cools and heats an aircraft, but the aircraft already can do that for itself with an APU. The APU is a very expensive little engine, however. To understand how PC Air saves energy and fuel we should follow an aircraft coming into a boarding bridge or ramp parking position that does not have power and air available. Either on approach or during the taxi in, the captain will turn on the APU. That way it will be ready to take over the aircraft’s power loads and cooling and heating needs before the captain shuts down both main engines. From that point until the aircraft is disembarked, catered, cleaned, boarded and engines started again, the APU would be running the entire time. For years this seemed OK until some smart people started to calculate what the costs were to do this.

The first reaction was to make sure 400 Hz and 28V DC power were at either the boarding bridges or available at the ramp parking spaces. That way the power requirement of the aircraft could be transferred to an external power supply. The problem with that was the captains still did not turn off the APUs. While air and heat are not required for aircraft operations to continue, the captains had a responsibility to make their customers comfortable and would keep the APUs running just to run conditioned air into the aircraft. That would be like having a $1,000 electric bill to cool your house for a few hours. It is a big waste of money and energy. Besides burning a lot of fuel and making a lot of noise, many aircraft operators pay fixed maintenance costs based on cycles or run time. After all, this also defeats the main purpose of having an APU – to provide a backup generator on board just in case of an engine generator problem and to start aircraft engines before flight.

I used to be an airline pilot for a legacy airline and one operational delay that was common would be a failed APU unit. The main reason for failure? We would be limited to a small quantity of available external air-start units to get the first engine started, which inevitably caused a delay on push back. More importantly, if an APU is “MEL’d” (minimum equipment listed), you can still fly with that broken APU because the FAA has preapproved that failure to be acceptable for a short period of time – but not in bad weather. Having a MEL’d APU at the airline I flew for meant that flying in icy conditions, expected turbulence or CAT III approaches – basically very low visibility – was prohibited. This FAA requirement would often cause delays or cancellations because the weather criteria could not be met. PC Air units are a part of the solution to help lower these APU outages, save energy and keep the APU ready for use when it’s really needed.By using the APU less and using external PC Air with power instead, the APU can remain off until 5 minutes before departure to be ready to start a main engine.

The low-cost airline I mentioned was one airline that figured this out early. And its solution was to put power and air in all its parking positions and standardize operations with the flight and ground crews to use these external power and air units to save significant money. While I do not have specific numbers on what it saved each year with this solution, I have seen many studies done for airports where millions of dollars can be saved annually at hub airports and hundreds of thousands of dollars at medium-sized airports with the same improvements to power and air. The difficulty, however, comes with different climates, different aircraft in one place and different restraints to infrastructure. Having the exact same POU DX unit at each parking spot in the entire system is not normally going to work for most operators. Let’s discuss some of the basic designs and the differences in technology to help us better understand the options. DX units are the most common PC Air designs used today followed by AHUs.

POU DX units range typically from 20 tons to 150 tons and can be facility-powered or diesel-driven. Smaller, electric 20-ton sizes can be side-mounted on bridges and the largest 150-ton units stand-mounted. Almost anything in between is possible. All of these DX units come now with the following refrigerant: 410A, 407C and 134A. All three are used at JBT AeroTech because each refrigerant has pros and cons depending on what you are trying to accomplish. In sizing the units for application, several factors have to be kept in mind. To name just a few: We use an excellent calculation program that takes all of these variables and adds them up with the aircraft curves and provides recommended sizing. Even with these results, however, we still have to look harder at other factors. With all of this information, basic performance requirements needed to do the job can be narrowed down. Common salient characteristics are a good thing to keep in mind at this point. One quick note on tonnage: Many PC Air manufacturers are starting to give names to different sizes instead of tonnage descriptions because manufacturers know best what their units are capable of cooling.

Tonnage is a difficult measurement anyway because compressor tonnage and actual tonnage performance make a big difference. In some cases, when using 134A powered by a 50 Hz power supply, the compressor tonnage can be 90 tons, but the actual tonnage can be 50 tons. Derates for refrigeration and input power can be that significant with some manufacturers. This is why you really have to know what you are buying. Extreme Conditions: For extreme hot locations, JBT AeroTech successfully developed a DX boost unit by taking a DX unit and adding a glycol coil – like a typical AHU unit would have – to supplement cooling capacity. Until this hybrid design between a DX and an AHU, most operators had to run the aircraft’s APU to keep the aircraft cool. But extreme heat and humidity are two things a good designed PC Air system can overcome. All DX and AHU use 100 percent outside air that is dehumidified in the process of cooling the air. If you’ve ever stood under a PC Air unit on a boarding bridge that doesn’t have the condensate hose connected, you stand a very good chance of getting soaked.

Central Systems: Central air is the other very popular option to DX POU units. Central systems are a good option for some since they offer a lower cost of operation and a better return on investment on projects with more than five parking positions. A central system typically will have a greater initial capital investment than a POU, but has less overall compressor circuits, refrigeration components and has a diversity factor to help lower electrical requirements to provide as good or better cooling and heating temperatures than DX units. A central system uses AHUs placed close to the aircraft parking positions and feeds a glycol/water mixture pumped through the AHU coils to cool or heat the air that is blown through. While a central system may require more time and money up front, the long-term benefits may make it a better option. Hangar Installations: PC Air applications in hangars are starting to pick up again. Since APUs can’t be used indoors, crews typically work on aircraft in unconditioned hangers.

The need to provide comfort, however, is becoming an important enough human factor to consider these systems. Usually a few DX units are placed outside the hanger and then ducted into the hanger underground in what is normally called a trunk line. These systems can range from very basic to fully automated. No Power: Mobile, diesel-driven PC Air units will provide the power when there’s no power. While these units burn diesel fuel and are noise producers, they are far less noisy and fuel hungry compared to an APU. Many of these mobile units can also provide the required aircraft power. This will help conserve ramp space and use one engine generator. In summary, PC Air is not required to cool or heat an aircraft, but it is required if you plan to take advantage of less APU usage. Over the last nine years most operators have quantified the savings upgrading their parking positions with PC Air. Today, there are more options to consider. PC Air systems have a normal useful life of 10 to 15 years.