The following stumper is presented in BPI's March Performance Matters e-Newsletter


A big thank you goes out to Jamie Clark of ARRONCO Comfort Air in Lexington, Kentucky for sending in this stumper! Jamie was called to an 18 year old, 2,500 sf, two-story home with a vented attic in Lexington that had a broken central air system. There were two air systems, one in the attic, one in a closet. ARRONCO decided to replace both systems with two ton Carrier® hybrid systems. After the project was completed, all the registers in the house started sweating. Jamie came back the next day to find the humidity level over 70 percent! The two stage systems were running on low most of the time which should have resulted in lower than average humidity (average in Lexington is 55 percent). ARRONCO inspected all the equipment to make sure it was installed correctly, and tested all the equipment. All air volumes were right, the equipment was perfectly sized, all duct work was sealed (with less than 10 percent leakage).


Question: Why was the humidity so much higher? 


Think you know the answer?


Figure out what is going wrong with the house, write it up along with your prescribed solution, and share your wisdom by posting your answer right here in our comments section. You can also send it to us at If you’re the first person to get the right answer, we’ll feature you, your company and your answer in the next issue of Performance Matters!

Tags: bpi, chump, march, matters, newsletter, performance, stump

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Replies to This Discussion

Not a lot of information here.  Blower door number would be very helpful, duct leakage to outside nice too.  ESP on both stages would tell how/if equipment and duct are matched.  



"Properly sized" in theory should not trump what is actually observed, and without a blower door number "properly sized" has a tremendous margin of error.  If multi-stage equipment is running on low "most of the time" wouldn't that indicate load mis-match, particularly if the other state is off? 



In my neck of the woods, and from what I'm hearing from around the country, it's a pretty crappy house that needs more than 1 ton per 1000 square feet.  Using brute force equipment approach and sizing to "crappy house" worst case conditions instead of addressing envelope issues is bound to be problematic on moderate, meat of the season conditions.



I've only heard of condensation forming on grilles when the equipment is grossly oversized and is removing sensible so fast latent can't keep up, causing surfaces to hit dew point.  Sounds like they went cheap on the Carrier units, as Communicating Infinity logic would attempt to manage latent to sensible.
HVAC Contractors I'm working with are tired of being blamed for these Frankenhouse situations, they're sizing to "windows closed" conditions and telling homeowners that if they don't close windows, don't expect comfort in all conditions.  Homeowners benefit from lower equipment cost up front, more even comfort, and aren't blocked from improving the home.  Oversized equipment not only exhibits problems like these when the house is crappy, when homeowners eventually fix the house these problems get WORSE.  


COOLING DOWN QUICKLY and KEEPING COOL AND DRY are polar opposites. Dehumidification is a slow and gentile process, psychometric curves, dew point and what happens when sensible is driven quickly down took me a long time to get my head around and now my prior lack of understanding is clear. People want "cool quick", so obliging HVAC contractors give the customer what they want. They've been told jerking temperatures around saves energy (as they spend $ on mold remediation)



I suspect a leaky home, definitely over sized equipment. 


I'm not sure there is enough information to positively identify the problem. For instance, what were the outdoor ambient conditions and how long were the existing systems not functioning (were both broken)? What were the thermostat set-points and the current indoor conditions in the living space - both dry bulb and wet bulb temperatures? Where was the humidity reading taken and how long had the systems been running?

Assuming that the systems were sized correctly, were operating properly, that there was no duct leakage or air infiltration, that there were no other issues going on in the home and that the airflow was correct for low stage (and not just high). Obviously from this short description and without actual measurements it's not possible to disagree with the assertion that these things are correct.

I would also have to ask how long the existing system had been down. If this problem was on the next day it is possible that conditions in the home had not yet stabilized. After a space has been without air conditioning (especially if outdoor conditions are high temperatures and humidity) not only does the air have to be cooled (and dehumidified) but the heat being held by the mass of the building and its contents must be cooled. Depending on the conditions this could include a significant level of latent (moisture) heat. Most thermostats only measure, and attempt to compensate for, air temperature (dry bulb) so a system could reach set point and thereby satisfy (temporarily) the thermostat or allow it to operate at first stage (low) while the mass is releasing its heat. It would be a simple matter for the new systems to quickly drop the grill temperatures below dew point and cause the sweating.
Furthermore, if the humidity reading were taken close to the grilles (rather than out in the space) you would expect to have much higher relative readings (discharge air from a coil approaches 100% RH).

Two-stage equipment "should" result in lower humidity assuming proper sizing, application, installation and set-up. Many times very good equipment gets a bad rap due to how it is applied. For instance in an attempt to increase the rated SEER often times the evaporator is oversized because this increases the sensible cooling and thereby the SEER rating (SEER ratings ignore humidity). This has been done a lot over the past few years in attempts to take advantage of Tax Credits - and it has been done with the recommendation of the manufacturer. In many parts of the country this is not a problem - as a matter of fact in many areas it is common practice. However in areas of high humidity this creates problems. Due to the larger coil, the coil temperature does not get low enough for maximum dehumidification. I have seen installations that were “properly sized” and the equipment match was approved by the manufacturer and the equipment was “operating properly” (within specifications) and there was still a humidity issue. Air conditioning equipment must be properly “applied” not just sized and installed

With all that being said I would still contend that there is not enough information to reliably diagnose the issue(s). If I was called on this job, I there is a list of measurements that I would request. My recommendation would be to take further readings – allow more run time, and re-measure.


 March Stumper Revealed!

We are chagrined. We realize we should have given you more clues to last month’s stumper, sent in by Jamie Clark of ARRONCO Comfort Air in Lexington, Kentucky. (But we beg your forgiveness because it's a fine balance! If we tell you too much we give away the easy answer; tell you too little, and you don’t have the information you need to make an educated guess). Readers will recall that Jamie replaced two broken air systems with Carrier® hybrid systems only to find that after the project was completed, all the registers in the house started sweating, with the humidity level over 70 percent! All equipment was installed correctly, air volumes were right, the equipment was perfectly sized, all duct work was sealed (with less than 10 percent leakage).

Despite the gaps in information, we received several creative responses to the puzzle. Dean Smith of Santee Cooper in Moncks Corner, South Carolina came closest, guessing that the customer had the system’s fan in the “on “ position and the blower motor was picking up the condensate from the wet coil, which was being distributed back into the home. Well done Dean!

Jamie reveals what was really going on. When he went back to the house on the second visit, he visually inspected the duct work to make sure it wasn’t pulling in excessive air and humidity from the attic. But when he lifted the scuttle hole hatch it started to float; there were two full size attic exhaust fans in a 6,000 cubic foot attic. They put the entire house on such a negative pressure, they were sucking in moisture from the outside. Says Jamie: “It was like having a blower door on 24/7”. Important note: The first time Jamie visited the house for his initial assessment of the broken central air system, it was a cold and rainy day, so the attic fans weren’t running. To solve the problem, Jamie disconnected one attic fan and turned the other one to 120 degrees so it would only turn on in extreme heat.


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