Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – An Overview

So here we go;  the first post in the first RCx Findings Series.  The subject of this finding – simultaneous heating and cooling – is a very common opportunity in our systems.  In this post, I will look at both the general case and as well as the specifics that led the team I am working with to focus on the system that will be the subject of this string. 

So, in a way, some of the content of this post will serve to provide general guidance regarding how to identify opportunities in a coping process in addition to a more focused discussion regarding opportunities in make up air systems.

Make-up Air Systems and Simultaneous Heating and Cooling

Make-up air systems, or systems that bring in fresh air to offset exhaust that is removed to control contaminants are fairly common in our facilities because one of the functions of HVAC is to provide a safe built environment.

Make up air systems (and many other HVAC systems) are perfect set-ups for simultaneous heating and cooling by design, for reasons I will discuss in a separate post titled Why are HVAC Systems Designed to Simultaneously Heat and Cool?.  This picture is of a corridor make-up air system for a hotel and is a “text book” design.

Star Pass MAU 01

It’s also a textbook Retrocommissioning (RCx) opportunity as evidenced by the clues from the thermometers in the piping serving the various heat transfer elements.

Simultaneous Heating and Cooling

Note the temperature changes across the elements, indicating energy is being transferred. Specifically, going from right to left (the intake of the system to the discharge)

  • Energy is added to the outdoor air stream, which shows up as a temperature drop across the water side of the preheat coil; then
  • Energy is removed from the air stream, which shows up as a temperature rise across the chilled water cooling coil; and finally
  • Energy is added to the air stream, which shows up as a temperature drop across the hot water reheat coil.

And then, there was this …

Tony Full Screen

… and this.


As can be seen from the second picture, the make-up system was clearly using both heating and cooling energy on the day I took the picture.  But, you might wonder if that is really necessary if the everyone is walking around in short sleeves (third picture) and there are cactus scattered across the landscape (fourth picture).

So, lets take a look at a make up air handling unit  from one of the in-building classes that I am helping to lead along with the facility it is located in, and see if there are similar clues regarding its functionality or lack there-of.

Introducing Our Target System

The system we will be discussing in this string of posts is a make-up air handling system serving some of the guest room corridors in a hotel near Golden Colorado, which is a very pretty place to be working.

Moon at Sunrise 03

This is a picture that I took one morning from my room;  that’s the National Renewable Energy Labs (NREL) there in the background.

To keep things from getting too long, I am going to use a string of posts to discuss the “clues” that led us to target this system.  They include the following items, which will show up in the title of the related posts if you want to search for them.  I will also hyperlink from the list below to the related post as I develop them.

Each clue is will be discussed in the general case and also in the specific context of our target project.  Bear in mind that this is not a comprehensive list of clues, just a list of the clues that led the team on this project to the system in question.

David Sellers
Senior Engineer – Facility Dynamics Engineering

This entry was posted in Air Handling Systems, Retrocommissioning Findings, Weather and Climate Interactions with Buildings and Systems. Bookmark the permalink.

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