Rooftop HVAC Economizers - When Are They Worth It?

https://docs.google.com/viewer?url=http%3A%2F%2Fcustomer.honeywell.com%2FNR%2Frdonlyres%2FC457FEB8-10AB-4C5E-B150-4A802C205AA4%2F4564%2F090071.pdf

I haven't got to making the Honeywell program work but will.

There are so many variables, and the variables vary in nature with each installation.

I think eventually most will agree economizers are overrated, except in certain locations, and many if not most locations their use without significant advantage.

This is another opportunity for a complex, time-consuming data logging-analysis project.

Thanks

You are totally correct regarding "new" technologies that monitor CO2 and/or VOC content of the interior air- BUT the action of ANY economizer is still driven by selection of the air stream with the lowest enthalpy to be cooled.

Regarding the need for "power" exhaust, since the intake of the system is directly connected to the outside air and virtually all systems with economizers have dampers that are used to select return air or outside air, those systems also have dampers ahead of the return/outside air damper set that open to some form of exhaust duct. The building becomes slightly pressurized- usually a good thing when trying to maintain an interior conditioning- but ALL of the "excess" return air will be exhausted without the need for an powered exhaust.

Additionally, IF the system is designed as a Variable Air Volume supply, the exhaust fan would also need to be a VAV system, adding further to the installed cost.

My primary discussion was related to the FUNCTION of an economizer system, not on the miscellaneous accessories. By the way, even IF the system was designed with an exhaust fan, the relatively small added wattage used would be dwarfed by the amount of power being saved by shutting down or reducing the load on the cooling system.

you guys are both awesome, and ga to both! You have both managed to teach a course in this, that I couldn't understand that well in my 2 week hvac course. well done. I think you should team up to write hvac technical books and training materials! seriously.

Thanks Chris

Thanks as well. I was away for a while.

Your answer is good for the theory of air-side economizers, and I give you a GA.

However, most if not too much of America - meaning nearly all light commercial has rooftops which in my experience have only rooftop package units with thermostat control and no VOC or CO2 sensors. In time that may change, I know, but not without significant expense, and getting the General Contractors and Architects to go along with the state of the art is difficult.

It is the repair of rooftop package unit economizers that I ask. It seems the best course of action, and sometimes only acceptable with respect to energy conservation is to close the outside air and walk away.

It is not right to charge a customer for repairing something and tell him he will get his money back when he never will.

In addition, cannot think of a time when a commercial space had a chronic air quality problem.

I get what you're saying. But DDC is here to stay. Even smaller buildings (non high or mid rise) are being designed with DDC and economizer controls. Code and the "green fad" are dictating it now.

Re: Current rooftop package equipment. Again, I understand your point. I would add that a great number of techs don't understand how to set one up properly. If it isn't set up correctly the savings wont be realized. I'd also say that if the customer is located in a traditionally humid area they would probably only see a handful of days a year of operation. There are charts available that show by region where you can expect the best results. Desert and mountain areas often have dry air that really fits the bill. Coastal rarely does.

Economizers in good repair save about 10% annually in energy costs.

Finally, a number!

From this I need to determine how many hours a rooftop runs (in Minnesota), and how much it costs per hour to run.

Then compare the cost of repairs against the savings by having an economizer.

Actually, some sort of totalizer data based on daily OA enthalpy should be used.

Finally, I am sure that maintenance and use of a working economizer will not be cost effective for the user, but an argument can be made that it "is the right thing to do", even though few others do.