Retrocommissioning Findings: Make Up Air Handling System Simultaneous Heating and Cooling – The Clues – #7 – The Obvious Stuff

This post brings us full circle from where we started this string by looking at the obvious clues you might encounter out in the field, just like the MAU with the textbook design and textbook retrocommissioning opportunity I discussed at the beginning of the string.

The General Case

We all carry some pretty useful field instruments around with us virtually all of the time; our senses and our brain. By paying attention to what we see and hear as we walk around a facility and thinking about what it might mean in terms of the efficiency and performance of the systems that we are observing, we can often identify potential opportunities.

For instance, when I walked into the mechanical penthouse that served the dysfunctional make up air system I discussed in the General Case portion of Clue #1, as soon as I walked into the room, I felt heat radiating off of the large steam trap on the preheat coil.


Since it was the middle of July and about 80°F outside, there should have been no need for preheat, so my senses and curiosity were immediately aroused.

At just about the same time, I noticed a lot of flow noise coming from the chilled water valve, which was mounted on the wall across from the steam trap.


This told me that the system was also using a lot of chilled water, probably more than necessary to offset the unnecessary preheat energy.

As I walked further into the mechanical room, I noticed a pretty steady stream of condensate pouring off of the cooling coil and running down the floor drain. This told me that the cooling coil was had a significant condensation load. Oregon is relatively dry in the summer; the July dew points tend to run in the mid 40s to mid 50s°F (compared to someplace like St. Louis, where they might run in the 60’s and 70’s°F.

These graphs from will give you a quick feel for the climate relative to other US cities.


The point is that a heavy condensation load in a relatively dry climate implies a low cooling coil discharge air temperature, maybe lower than it needed to be.

The real shocker was that when I looked into the section of the air handling system where the humidifier was located, I saw that it was injecting steam (moisture) into the air stream.


There I was, standing in a puddle of water rung out of the air by the cooling coil not 4 feet ahead of the humidifier watching the humidifier put water back into the air stream.

So, my point here is that for this make up air handling unit (MAU) a bunch of things added up and pointed to a significant opportunity to save energy and other resources:

Our Target Facility’s Case

When Chuck took me on a tour of the facility he had inherited, I was very interested in taking a look at his make up air systems for a number of reasons including the nature of the process, the nature of the system design, the nature of the local climate, and the facility’s energy consumption signature. These were all “clues” I had assembled in reviewing resources before I even set foot on the site.

In addition, as I mentioned, Chuck had his concerns about the system and was on the verge of implementing some potential improvements.  Chuck had also acknowledged that the controls likely needed attention, a comment that had further “tweaked” my interest. In fact, the gradual drift and loss of calibration over time that pneumatic controls can exhibit if nobody is paying close attention could be one of the drivers behind the rising average daily gas consumption pattern.

When we got to the first MAU mechanical room, I reached out and touched the piping for the preheat and cooling coil and discovered that the preheat piping was hot and the chilled water piping was cold.  The former was expected since it was probably in the low 40’s°F outside at the time.   But the cold chilled water lines suggested that simultaneous heating and cooling was in fact going on.

Given the number of clues that were adding up and the obvious evidence of simultaneous heating and cooling, Chuck and I decided we would go ahead and deploy a logger so I could pick up a day or two of data and make a preliminary assessment of the potential savings.  In opening up the unit to do that, I could more clues became evident.

For one thing, there was evidence of tube freezeups, something that Chuck, and later, another engineer who had worked at the facility confirmed as being a problem at the facility.

Frozen Return Bend

This tended to confirm some of the potential issues I became concerned about as I reviewed the project’s control drawings before coming on site.

In the following picture of the MAU before we put the coil covers back on, you can see that the coils are in fact installed with very little space between them.

MAU Logger Deployment 01

This arrangement makes it very difficult if not impossible to install a temperature sensor after the preheat coil to provide an independent control process for the preheat coil.  It also makes it difficult or impossible to install the freezestat after the preheat coil but ahead of the chilled water coil.  Both of these items were concerns I had after reviewing the project control drawings.

In fact, you can also see in the picture that the freezestat is not installed between the two coils.  It is the little gray box above and to the right of the open coil access.  If you look closely, you can see that its sensing element enters the unit casing after the cooling coil.  Air flow is from left to right in this picture and the freezestat element is the thin, copper colored line wrapped up in a spiral before going into the side of the unit.

The bottom line is that things were adding up and it looked like we had a good candidate for our retrocommissioning project, both in terms of being able to save some energy and also in terms of being able to improve performance and minimize maintenance challenges like frozen coils.   And my guess is that a couple of days of data from the logger that Chuck and I deployed will make the case in terms of economics.  I’ll explore that very topic next,  as I move from a discussion of the clues to a discussion of the initial assessment associated with this particular finding.

David Sellers
Senior Engineer – Facility Dynamics Engineering

This entry was posted in Air Handling Systems, Retrocommissioning Findings. Bookmark the permalink.

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