100% Outside Air? (part 2)

In my last post, I “preached” that gas-fired roof top packaged unit should not be used as make-up air units with 100% outside air.  Now the question of attempting to use a unit  for 100% outside air in a cooling application needs to be addressed.


 Before we can determine if a unit can do so on cooling, we need to review some basic facts:

  • As air increases in temperature and humidity, it becomes easier to cool.   A DX coil that has been designed, let’s say, for 20 tons when cooling 78 degree air will have a much greater capacity when cooling 95 degree air.
  • Since the compressor, condenser coil, and TXV (expansion valve) can still only do 20 tons of cooling, however, the excessive capacity of the DX coil will overload the rest of the system. On such an application, the system will shut down when cooling is needed the most.
  • The only way to keep the DX coil from overloading the rest of the system is to offset the high heat content of the outside air by reducing the CFM.   (see the following table for CFM recommendations based on summer design conditions for a location)

   Outside Air Table 3

  • A packaged unit cannot be used to temper 100% outside air from 95 degrees down to 75 degrees. The air leaving the DX coil must always be cooled to at least 62 degrees F or lower.  As the temperature of the air leaving the DX coil increases, both the evaporation temperature of the DX coil and the saturated suction temperature of the compressor will also increase. If the saturated suction pressure of the compressor reaches 87 psig for R-22 or about 152 psig (54 F) for R-410A, the compressor will overload.   At a reduced air flow, the temperature of the air leaving a DX coil may only exceed the saturated suction temperature of the compressor by as little as 10 degrees.
  • And NEVER use a heat pump to heat or cool 100% outside air. the air entering the indoor coil should never be less tha 50 degrees.

Now that we have established some basic guidelines, we can determine how units will perform.

To begin with, what are some typical applications that might require 100% outside air?  Any area where the air can’t or should not be returned to the unit is a candidate for 100% outside air.  These could include operating rooms, hospital wings where contagious illnesses are treated, animal kennels, kitchens, hotel corridors, condo common areas and corridors, some manufacturing areas, locker rooms, just to name a few.

Please note — however — that most manufacturer’s packaged units should not be used on ALL of the above applications. Some of the more critical applications will require special systems that should be specifically designed for the job.

So now we get to the real question — When can a package unit be used to cool 100% outside air?

I’ll try to address this with a “question and answer” format and break it down into various conditions and pieces.

  • When can it be used for 100% outside air?
    • only when the conditioned space can tolerate wide swings in supply air temperature.  If a space requires close control of temperature and/or humidity, a packaged unit can only be used to treat the “make-up air”.  Other equipment will be required to control the COMFORT LEVEL within the conditioned space.
  • What units do the best job of cooling 100% outside air?
    • The more stages of cooling a unit has, the better it will operate. Since its CFM will be low, the temperature drop across the DX coil could be as much as 40 degrees.  As the number of stages increase, the temperature drop per stage will decrease and the supply air will become more stable.
  • What happen if the unit has only 1 stage of cooling?
    • When the compressor is energized or de-energized, its supply air temperature will change approximately 40 degrees F.  As the outside air varies between 85 and 95 degrees F, the supply air temperature will vary between 45 and 55 degrees F.  When the outside air is below 85 F, however, two things may happen:
      • a supply air temperature below 45 F my be too cold for the conditioned space
      • condensate may begin to freeze on the surface of the DX coil.
    • If the compressor is de-energized (and locked out for a few minutes by an anti short cycle timer) the supply air will jump approximately 40 F  and the supply air temperature of 85 F may be too warm for the conditioned space.  Now, if the conditioned space can tolerate 40 F swings in supply air temperature and doesn’t require cooling when the outside air is below 85 F, a single compressor packaged unit could be used  — but the application is limited.
  • So, can we “fudge” and add a RAWAL Valve or HOT GAS BY-PASS Valve to the unit to improve its performance?
    • Yes (see my posts on HGBP and Rawal valves) it can.  By adding capacity control of some kind like HGBP, a 1-stage unit will be able to operate until the ambient temperature is between 75 and 85 F.  The hot gas by-pass will prevent:
      • the supply air from dropping below 45 F
      • the condensate from freezing on the DX coil.
    • Trying to run the unit at a lower ambient temperature, however, will require more than the manufacturer’s recommend limit for HGBP which is usually 30% of the compressor capacity. If too much hot gas is bypassed, too little refrigerant will go to the condenser coil and the unit may experience low head pressure problems.
    • Also remember, HGBP requires a TXV and a lot of single compressor units don’t have them.  So now you need to add a TXV (and tie it into the distributor tubes of the DX coil with a distributor that  has a side tap to accept the HGBP piping), along with the HGBP, run the line through the unit, and attach it to the outlet of the TXV for it to work properly.
  • Then, are units with 2 stages of cooling better for 100% outside air?
    • Yes — if the unit has an “interlaced” DX coil as opposed to a “split faced” coil.
    • Both stages will operate until the outside air temperature drops below 85 F. Since no air will be bypassed on first stage with the interlaced coil, first stage can operate at an ambient temperature down to 75 F without HGBP or down to 65 F with HGBP.
    • The 2 stages of capacity will reduce the 40 F swing in the supply air to 20 F.
  • With all this said above — what’s the best way to control a unit in cooling with 100% outside air?
    • Ambient thermostats are really the only good way to do it.  On a 2 stage unit, energize first stage at 75 F and second stage at 85 F.  De-energize second stage at 82 F and first stage at 72 F.  The 3 degrees F differential should be wide enough to prevent the compressor from short cycling.
    • DO NOT control the unit with a discharge air sensor or any DDC system.  these type of controls will demand too much from a unit that has such a wide swing in supply air temperature.
    • Even controlling the unit with electro-mechanical thermostats mounted in the conditioned space is expecting too much.  the thermostat will never be satisfied. the supply air will almost alway be too warm or too cool and will over shoot the set point whenever a stage of cooling is energized or de-energized.

The secret to a good application of a packaged unit handling 100% outside air is to know its capabilities and its limitations!  If the application is suitable, the unit could do fine. For the most part, it is always better to use equipment that was designed for a specific application instead of trying to “force” a unit to do what it really was not designed to do.

We have to help our customers understand and ACCEPT what a packaged unit can and cannot do.  Remember, most manufacturers make these units for “comfort heating and cooling.  Trying to get a unit to do what is was not designed for, in most cases, will only lead to disappointment and equipment problems that could have been avoided if a properly applied piece of equipment was used in the first place.

I started with a little preaching and I now end with a little preaching.  Again, my thanks go out to our retired applications engineer, George Simonson, who made these last 2 post possible!


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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