A fact of modern building construction is that most cooling is produced by rooftop units. Improving the energy efficiency of rooftop units and connecting the sizing and operation of them to actual demands in the space would produce significant energy savings. Instead, most rooftop units are (over) sized by rule of thumb, using a tons per square foot of building space multiplier, and the fans are set to run all the time, which wastes energy and produces wind tunnels in public spaces. It doesn’t have to be like this.
With interest, I saw the headline above this morning in an email newsletter from HPAC Engineering. So what does this rooftop unit have to offer? Some quick facts about rooftop units–cooling is achieved utilizing a compressor and condenser, just like in your house, except it’s all together in one box. There is a supply fan to circulate air and a heating coil, either gas or electric and a filter. For a basic small tonnage commercial rooftop, that’s it for components. In larger tonnage rooftop units, like the Johnson Controls model referred to above, there may be other components–energy recovery wheels, reheat coils. How can you make a rooftop unit more energy efficient? Mainly, making everything variable speed. In a typical rooftop unit, there will be one or two stages of cooling (smaller tonnage) so you get 3.5 or 7 Tons at a time, whether you need it or not. Same with the supply fan–on or off. Making fans or compressors variable speed matches their utilization with space demand, which saves energy. Generally, on smaller tonnage rooftop units, variable speed is not utilized (though it should be).
A single number (not perfect) indication of full load efficiency in a rooftop unit is EER. So just how efficient is this rooftop unit of the future? The 25 Ton standard efficiency is 10.8, high efficiency, 11.4. IEER is another measure and incorporates part load measures. Alleged rooftop of future, 25 Tons, standard efficiency IEER 14.9, high efficiency 16.7. Really??? That’s the best you can do?
I decided to see if I could figure out what the practical limit of efficiency is for a rooftop unit. Is anyone doing better? I would surely hope so. It turns out the industry is doing a much better job with smaller tonnage rooftop units, perhaps because they are more often utilized than the larger tonnage units. In 2012, the Pacific Northwest National Lab (PNNL), hosted the DOE’s high performance rooftop unit challenge, using a 10 Ton unit for comparison. Daikin’s Rebel was the first unit to meet the challenge of an IEER of 18. Currently, five manufacturers meet this standard including the Carrier Weather Expert. They also developed a calculator which compares using a standard rooftop unit with a high performance one. The calculator can be found here: Rooftop Unit Comparison Calculator.
You need to be an engineer to use it, and even then, a lot of values must be looked up. I can tell you from specifying RTUs that there isn’t a lot that goes into it, and if the manufacturer’s values were stored in the program (so that you could compare using a standard Carrier unit to the highest efficiency Carrier unit by using a pull down menu), the calculator would be a lot more usable. I was also a little bothered by the use of EER to compare the units, when most of the savings is in part load performance (IEER). I was able to compare a 3 stage (of compressor, which is what the high performance Carrier is) to a standard 2 stage model. However, the full load EER of the high performance was 13, the standard unit 12. Even the EER, the higher performance rooftop unit saved 25% in annual operating costs (the estimate was $3000/year for the high performance RTU, $3900 for the standard performance.) Depending on the cost difference, you would probably be looking at a payback of 1-2 years. Also, because of the variable speed options, the unit would provide much better comfort from day 1. A win all the way around.
Here is some additional information about the DOE’s high performance rooftop unit project.