8 min read

Musty Air, Poor Indoor Air Quality?  It's your RTU.

Oct 16, 2015 12:04:54 PM

CES_Energy_Plus_HVAC_IAQ_Ask_UsAfter eighteen years performing Indoor Air Quality (IAQ) assessments in commercial buildings, I've seen more than a few repeat air quality offenders.  The overwhelming leader in degrading perceived IAQ is mold and associated "musty odors."

Unfortunately, those of us in Florida or other hot and humid climates, have come to accept a "level of mustiness."   It's the musty odor you sense as you enter into the retail or department store or the movie theater.  Maybe because we rarely frequent these indoor environments or when we do our occupancy is transient, we give the establishment a pass.

Would you give a surgical center or hospital a pass on "musty odors?"   Fortunately, the Department of Health and Human Services oversees and shuts down places with mold problems as " ... an immediate and serious threat to the health and safety of patients" requiring, "... removal of immediate jeopardy."  Let's not go there!

In Today's hyper-connected world, respectable establishments understand the power of social media like Facebook, Yelp and Google reviews.  The following are typical of the reviews I see reflecting poor IAQ:

"...the theater smelled.  It was humid in the theater and it reeked of a musty, moldy type smell that bothered me throughout the whole film.  After sitting through  that stinkiness for over 2 hours I decided I didn't ever want to come back to this theater again and I probably never will."

"This cinema finds the balance between musty and freezing-- usually if a theater isn't ridiculously cold it is musty and nasty, but this one is a perfect medium between those extremes."

"Downside, it can be musty inside and sometimes the theater smells like cat pee."

The cycle goes something like this:  

  1. HVAC "out of Sight - Out of Mind"
  2. Poor Indoor Air Quality
  3. Unhappy Guests
  4. Bad Reviews
  5. Lost Revenue
  6. Increased Cost 
  7. Increased Risk & Liability

The first step in breaking this cycle ... Energy Plus.  

The great thing about Energy Plus is how simple it is to benefit from this valuable service.  Let's increase occupant satisfaction, increase revenues, create a positive feedback loop with good reviews, and reduce costs, risk and liability. 


So where is that infamous indoor air quality problem indicator most likely to be found.  Experience dictates to start with the "out of sight, out of mind" rooftop air conditioning unit; the RTU.  Nearly 40% off all commercial buildings are air conditioning by this prolific source of IAQ problems.  

The conventional RTU has many HVAC deficiencies that are especially exposed in hot and humid climates.  Unfortunately, RTUs are everywhere and not going anywhere.  While conventional RTU system designs can be used, they are not designed to produce the optimum humidity conditions needed in many applications such as theaters, as outdoor air is mixed with return air and then cooled and dehumidified.

Newsflash, every occupied commercial facility requires that outdoor air is mixed with return air which is then cooled and dehumidified.

In no particular order of significance, here you go ...

The six RTU factors that increase the risk of moisture accumulation and mold growth.  

CES's first line of defense in controlling RTU moisture and mold is CES's Energy Plus program.  Click here and a CES representative will explain how this service can provide such benefit.

  1. Outside air ventilation without dedicated dehumidification has been responsible for major mold growth problems in hot and humid climates.  Due to increased latent load (moisture load) on HVAC and increased indoor dew point temperatures.    Improve IAQ with Energy Plus by CES
  2. Negative building pressure created by unbalanced exhaust and make-up air systems.  When the outdoor dew point is higher than indoor surface temperatures condensation occurs.  Surface RH can be over 80% and conducive to microbial growth.             Improve IAQ with Energy Plus by CES
  3. Over-chilling a buildings interior surfaces during humid weather.  Many indoor environments have lower sensible heat factors; special care must be taken to ensure proper humidity control without over-cooling the space.  Improve IAQ with Energy Plus by CES Improve IAQ with Energy Plus by CES
  4. Dirt and dust accumulation on cooling coils and duct surfaces and sound lining downstream of cooling coils can lead to microbial growth in the damp layer of dust that collects inside the cooling system.  Dirt and dust accumulation in the supply air ductwork is a symptom of inadequate filtration or filter fit upstream in the system. The two images below illustrate what can happen in the supply air ductwork.  Improve IAQ with Energy Plus by CES Improve IAQ with Energy Plus by CES
  5. Cooling coil water droplet carryover.  Although the vast majority of systems are designed at face velocities that will prevent this action (500 feet per minute (fpm) or less), moisture carryover can often result when uneven air distribution across the coil face allow areas of higher velocities to exist.  Generally, velocities over 600 fpm may produce moisture carryover down into the duct system.Improve IAQ with Energy Plus by CES
  6. System operation during unoccupied periods.  The dry bulb temperature in occupied spaces should not be set to below 70 degrees Fahrenheit with the relative humidity maintained below 50%.  When the space is unoccupied for a short period of time, thermostats shall be reset to 85 degrees F with the relative humidity maintained below 50%.  If the space is unoccupied for an extended period of time, temperature should not be controlled but the building air relative humidity shall be maintained below 50%.  Improve IAQ with Energy Plus by CES Improve IAQ with Energy Plus by CES

HVAC Psychometric Basics and Recommendations for Controlling Moisture & Mold

  1. Keeping the indoor air dew point below 55°F nearly always ensures that surface RH will stay below 80% even on cool surfaces.  
  2. In contrast, if the indoor air RH is 65% at 70°F, any surface cooled below 66°F will have an RH above 80%.
  3. Note that the relative humidity of air measured in the occupied space or return air does not indicate the RH in the thin boundary layer of air in contact with cool surfaces.  
  4. Monitoring and controlling indoor dew point compared to indoor surface temperatures is the more useful metric for humidity control decisions.
  5. Ensuring that indoor surfaces of both occupied and unoccupied spaces are not cooled to temperatures so low as to create an average surface RH of over 80% that lasts for more than 30 days or surfaces cold enough to allow visible condensation (ASHRAE 2009a).
  6. Must keep the indoor dew point low enough to prevent a 30-day average surface RH above 80% on cool surfaces, 100% RH for 24 consecutive hours, or visible condensation.

CES Energy Plus HVAC Incentives

When forced to control humidity, RTU energy performance is usually poor, as RTUs are typically run to cool all the air to a lower temperature to remove moisture.  Most conventional systems only provide temperature control for the building.

CES Energy Plus program helps businesses save energy and money on their electric bills, improve HVAC system performance and enhance indoor air quality and occupant comfort.  The Energy Plus basics include an energy benchmarking, a thorough HVAC Hygiene inspection, adjustment of the HVAC unit’s thermostat, ventilation settings, refrigerant charge, coils and belts for optimal performance.

CES Energy Plus offers comprehensive, low-cost HVAC commissioning that addresses all major components of your HVAC system.  CES will bring you quality service at a discounted price utilizing utility based rebates and incentives.

The result is a higher-performing HVAC system, which can:

  • Lower energy costs
  • Increase performance and reliability
  • Improve comfort and air quality
  • Reduce impact on the environment

Utility's offer incentives to help customers upgrade HVAC system components to more energy-efficient options, including water- and air-cooled chillers, variable speed drives, energy management systems, demand-controlled ventilation, and air-side economizers.


The first step is easy.  Contact CES to see if your facility qualifies for CES Energy Plus.

Ask about Energy Plus


Additional CES & Energy Plus Resources 

Comprehensive Energy Services

Written by Comprehensive Energy Services

Comprehensive Energy Services, Inc. is an award-winning, self performing provider and recognized leader in Design Build and Maintain Mechanical Contracting and Plumbing; providing a full range of advanced HVAC commercial, healthcare, and industrial services, including mechanical engineering design, mechanical construction, plumbing, IAQ, preventative maintenance and system maintenance throughout Florida.

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