100% Outside Air? (part 1)

I was recently asked the question, “Can a packaged roof top unit be used to heat and cool 100% outside air?”  It appears the contractor is trying to turn a packaged unit into a cheap “make-up air unit”.  While I’m sure the salesman would love to sell more equipment, there comes a time that we have to really look at the application and see if it is a good fit.  I have preached for years in my training classes, ” any piece of equipment can only work as well as it is applied”!  When equipment is mis-applied, there are more potential problem, operating issues, etc.  Does not matter who the manufacturer is, mis-applied equipment will have problems.

So, now I have done my “preaching” for the day and it is time to answer the question.  In looking for information on this, I found a paper that was written by one of our application specialist almost 20 years ago (1995) and the answer to this question has not changed since he first wrote this.

The answer is actually in 2 parts — HEATING WITH 100% OUTSIDE AIR  and COOLING WITH 100% OUTSIDE AIR. In this post, I’ll address the first part — Heating with 100% outside air  because it is the middle of winter, the Ground Hog says 6 more week of winter,  and that makes it very appropriate for now.  My next post will address Cooling with 100% outside air, so stay tuned!


First, we need to look at what fuel we will be using to accomplish this — natural gas or electricity.

With Natural Gas as the source for heating the space, we need to look at the APPLICATION minimum air entering the unit. (yes, we are back to that nasty word application). Most manufacturers have a  minimum mixed air entering temperature of 40 ºF. Colder temperatures will cause flue gas to condense inside the heat exchanger tubes.  Since the condensate will be “acidic”, the heat exchanger will corrode and eventually fail.  Since the application exceeded the manufacturer’s published limitations, the warranty, in all reality, should not be covered by the manufacturer.

So, now we need to look at design conditions.  For the northern midwest, we figure design conditions of 70 degrees indoor at ? — could be 0 degrees, -10 degrees, even -20 degrees. Lets take design of 70 degrees indoor with 0 degrees out-door as an example.  With this scenario, can the packaged roof top unit  be allowed to handle 50% outside air? Based on the following calculation using 45 degrees (design conditions say minimum mixed air of 40 degrees) as a safe minimum mixed air temperature across the heat exchanger. Will a 50 / 50 return air to outside air  mix work?

mixed air 2

As you can see, if you tried to bring in  50% outside air, the heat exchanger would not last very long since we are well below our application limit of 40 F mixed air. Because of this, the end-user, your customer, would not be happy because their heat exchanger keeps failing every 5 to 7 years.   All this because someone wanted to use the unit for something it was never designed to do.

For this to work safely, with o F outside, and 60 F return air. you would need to have:

mixed air

So what about electric heat and 100% outside air.  Can that be done?  There is no heat exchanger to corrode.  There is no condensate to be formed because there are no products of combustion. Electric heaters do not have any minimum entering air limitations.  So can you do it with electric heat — YES! BUT– due to their higher cost of operation based on electric heat rates, it makes this application very unattractive for heating 100% outside air not to mention the need to really oversize the heaters increasing the initial cost of the equipment and the power wiring to the unit.

So, in conclusion — packaged roof top units are designed for “comfort” heating and not designed to be make-up air units.  There are manufacturers our there that make specific units just for that type of application.

Have you ever wondered why most make-up air units are “direct fired” into the air stream going into the building?  Because they don’t have heat exchangers that will corrode, fail, and cause other problems down the road.

Hopefully, this has explained the need for proper application of equipment.  In my next post on this subject, I’ll talk about 100% outside air in a COOLING application.  That gets a little bit more complicated.

(Thanks to George Simonson, retired application engineer, who taught me a lot, for the information presented in this and the next post).

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
This entry was posted in HVAC Tech Support and tagged , , , , , , , , , , , , , . Bookmark the permalink.

2 Responses to 100% Outside Air? (part 1)

  1. Whit Perry says:


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s