Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – #1 – The Nature of the System or Equipment Sets Up a Potential Problem

Sometimes, the very nature of a system or piece of equipment sets up a retrocommissioning opportunity.  Make-up Air Units (MAUs) are a common example of this sort of thing.

The General Case

For reasons that will be discussed in a separate post titled Why are HVAC Systems Designed to Simultaneously Heat and Cool?, simultaneous heating and cooling are a necessary evil if we want to make our facilities safe and comfortable (the bottom line goal of HVAC). Our goal as operators, commissioning providers, facility engineers, and others engaged in the day to day operations of HVAC equipment is to deliver comfort and safety as efficiently as possible.

Unfortunately, the system configurations that facilitate this are also system configurations that can waste energy very quickly if things get out of hand. And the processes are insidious; a totally dysfunctional system can deliver perfect occupant comfort and product quality. Meaning that if we only gauge the success of our efforts by those factors, we may be spending much more energy than necessary to deliver them.

The make-up air system pictured below from my Komatsu Silicon days illustrates this.


Note that by design, it could heat, then cool and dehumidify, the heat then humidify the air. There are valid technical reasons for configuring a system to do this. But, if those reasons are not properly implemented or communicated, you end up with something like this.


Note that the system is doing unnecessary preheating, then over cooling (and over dehumidifying), and then compensating by reheating and humidifying. At the time, the conditions in the clean room it served were perfect and product quality was great.

The “silver lining” is that retrocommissioning processes and techniques and the ongoing commissioning processes and techniques that they can lead to will identify issues like the ones illustrated above and resolve them, leaving the system operating as shown below on a similar day in the future.


In the case of the system illustrated in the preceding paragraphs, the fix was worth about $7,000 a month.  Implementing it took about $500 in parts and 40-80 man hours as I recall;  a pretty good return on the investment I would say. If you want to know more, I wrote a paper about it that you can download from the Texas A&M website.

The point here is that a system properly configured to deliver a mission critical result may also be configured to deliver that result in a highly inefficient manner, and there-in lies the opportunity.

Our Target Facility’s Case

In the case of our hotel in Golden, Colorado, we can anticipate that there will be a number of things going on that will require significant amounts of make-up air.

  • By their nature, hotels cater to large numbers of people, especially in their meeting spaces. Good practice and building codes will require that ventilation be provided to ensure a safe and healthy environment.
  • Food services and the hospitality industry go hand in hand.  Most hotels have at least one major kitchen, sometimes several and these areas will have large volumes of exhaust air taken from them at the cooking hoods. And, as my mentor Al Black would always say the goes inta’s gotta equal the goes outa’s, meaning that if you require large exhaust quantities, then you will need to augment them with large make-up quantities.
  • Laundry services and the hospitality industry also go hand in hand and many hotels handle their laundry in-house, as is the case for our hotel in Golden. Laundries require make-up air to support the clothes dryers, both for the purpose of combustion air in gas fired equipment, and for the purpose of removing the moisture that is driven out of the laundry.

The fact that the processes contained in the facility along with good practice and codes will require significant amounts of make-up air means that there will be systems performing that very function, i.e. systems that will handle significant volumes of outdoor air and which will have the ability to heat it or cool it or do both at the same time, whether you want that to happen or not. Taking some time to check out these systems is usually worth the effort.

For the hotel in question, that meant the following air handling systems would merit some attention.

  • The two, 100% outdoor air systems serving the kitchen area.
  • The large, high outdoor air percentage system serving the laundry.
  • The air handling systems serving meeting spaces.
  • The corridor make-up systems.

The meeting room systems are economizer equipped systems but might require higher than normal outdoor air percentages due to the high occupant levels in the areas they serve.  So, they have the potential to have some of the opportunities associated with the 100% outdoor air MAUs.  But the economizer adds another layer of complexity and opportunity.  So, our team broke them out as a separate target.

The corridor MAUs provide neutral air to the corridors serving the guest rooms which is then exhausted via the bathroom exhaust in the individual rooms. Since there were four corridor make up air systems and their design was similar to the design of the kitchen make-up air system, our team targeted one of these systems for further assessment.

These screen captures from the contract documents show the arrangement of AHU12, a typical system and how the coils were to be piped.



This screen shot shows the scheduled performance for the unit; I added the coil performance in a text box to keep everything legible.


The drawings provide a few good insights and clues regarding the design intent behind the system.

Clearly, the System is a 100% Outdoor Air System

This is evident from the drawing, which only shows an outdoor air connection and a discharge air connection, and from the entering conditions for the coils.

Clearly the System was Designed to Deal with Some Fairly Extreme Outdoor Air Temperatures

This is evident from both the performance data for the coils, which reflect the Denver climate (more on that when I discuss Clue#3). But it is also evident from how the coils are piped. Specifically, the coils are set up with pumps, which are intended to maintain a constant, high flow rate while the control valves blend water from the hot and chilled water system into the secondary pumping loops serving the coils to maintain the required discharge temperature.

This is a common freeze protection strategy, one that I have used myself with success as a designer. The idea is that moving water will not freeze. There is a section in the Functional Testing Guide that discusses different configurations used to protect preheat coils from freezing, including this one, if you want to know more.

So far, so good in terms of the system’s ability to perform its intended function, meaning at this point in getting familiar with the project, I was not worried about performance issues.

But clearly, the system was a good target because it was required to condition a lot of outdoor air, a very energy intensive process and a potential energy waster if things were not operating properly.  And there were a number of other systems just like it. So any improvements we identified for this system would likely be compounded as savings in the other similar systems.

Further back in the set of drawings, I came across the control system design information.

Denver West AHU12 Controls

As you can see, the original project had been designed using pneumatic controls;  not a particular surprise given the vintage of the building (1982-1983).  At that point in time, the industry was transitioning from pneumatic controls to DDC controls, so buildings constructed around that time could go either way. 

While it was possible that the pneumatic controls had been updated sometime in the past 28 or so years, if they hadn’t then they represented a potential opportunity to save energy and improve performance, for reasons I will discuss in the next post.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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