GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon
Q&A Spotlight

Coping With a Wrong-Sized AC System

A Florida homeowner looks for a solution to a new air-conditioning system that leaves his house with high humidity

This air-conditioning unit is too big. A 4-ton system installed in the Florida home of Chris Marriner doesn't run long enough to dehumidify the house. Now what?
Image Credit: Chris Marriner

Florida is not the kind of place where you’d want to be without air conditioning for very long, so when Chris Marriner’s old system died last spring, he didn’t waste much time in replacing it. But what should have been a ticket to indoor comfort hasn’t exactly worked out that way.

Marriner’s HVAC technician decided to replace the 4-ton system with one of the same capacity, even though Marriner knew that because of improvements to the building envelope the new system probably would be oversized for the 2200-square-foot home. The tech told Marriner the system could be “tuned.”

In a Q&A post at GreenBuildingAdvisor, Marriner complains that indoor relative humidity averages 55% or more, and has ranged up to 66% or 67%.

That’s apparently because the oversized AC equipment doesn’t run long enough to lower indoor humidity. When Marriner spoke to the HVAC installer about the oversized equipment, he got some strange advice.

“I talked to him about it and he wants to bring some attic air into the return so [the AC] will run longer,” Marriner writes. “I think that adding a whole house dehumidifier, like the Williams Air Sponge, or Aprilaire would be the best solution. Any feedback?”

That’s the topic for this Q&A Spotlight.

HVAC tech’s idea is non-starter

Nathaniel G finds the suggestion to bring hot attic air into the system to extend the run time nothing more than an “inefficient Band-aid.” The system should have been sized correctly in the beginning.

“Sounds like your HVAC tech is less interested in doing his job properly than doing it the way grandpa did and upselling you on a bunch of crap to patch up his own mistakes when grandpa’s approach doesn’t cut the mustard,” Nathaniel says.

“Bring some attic air into the return?” writes David Meiland. “You mean deliberately and needlessly increase the load so the unit will run longer in hopes that it will also decrease humidity adequately? That’s nuts; in fact it might be one of the worst ideas I’ve ever heard — it’s about the same as opening a window.”

It’s not clear what the tech might have meant when said the new system could be tuned, Meiland adds, but the real solution is longer run times and an indoor coil that’s not so cold.

Marriner has already explored the possibility of replacing the outside compressor and scaling back the air-handler. The manufacturer told him that won’t work.

“To compound this,” Marriner adds, the HVAC technician is a friend. “If he were not, I’d have raised hell. He got indignant when I asked him about the humidity and said he will not change the unit … He was trying to cut me a deal, so I don’t know if he had an extra one at the warehouse or what. But I’ve learned a valuable lesson.”

Try a stand-alone dehumidifier

GBA senior editor Martin Holladay suggests that Marriner begin with a stand-along dehumidifier. “It may work well enough for you to tolerate your system,” he says. “And it is a cheaper solution than starting from scratch.”

To help the existing AC equipment get more water out of the air, Holladay says, is to lower the blower speed of the air handler from 400 cubic feet per minute (cfm) per ton to 350 cfm per ton. “Of course, lowering the blower speed (especially below 350 cfm per ton) carries its own risk,” he adds, “namely, freezing the coil.”

Holladay’s suggestion for a dehumidifier is a good one, Dana Dorsett says. “If the house is reasonably tight it should do the trick,” he writes. “A 600-watt stand-alone dehumidifier converts the latent load into a sensible load, but it doesn’t increase the total cooling load by more than ~2000 Btu/hr (the 600 watts it’s using while running).

(For more on sensible and latent loads, here’s an explanation from a website called The Engineering ToolBox.)

“I wonder how many other HVAC technicians there are out there who take the approach of increasing the load as the ‘solution’ to an oversized AC system?” he adds. “If it’s a vented attic, it would also increase the latent load. You may end up pretty much in the same place, but be using significantly more electricity.”

Excuse me, but what’s the fuss about humidity?

The indoor humidity levels sound normal, says AJ Builder. “I have no AC and my summer humidity is 50-85% in my home, about 10% less than outside,” AJ writes. “No mold except for bad tile work in one shower. Fifty-five seems right considering Tampa, where outside humidity is high along with the temperature.”

People are healthy and comfortable, Dorsett replies, when the indoor relative humidity (RH) is between 30% and 50% and the temperature between 68° and 75°F.

“When it’s above 50% RH, dust mite populations begin to grow (a problem primarily for those with allergies or asthma),” Dorsett says. “Above 60% RH @ 75°F, humans are more susceptible to skin fungus/yeast infections. Above 70% RH it can be pretty uncomfortable, mold spore counts take off, and the risk of respiratory tract infection (fungal or bacterial) goes up.”

In Tampa, where Marriner lives, high dew points mean that any infiltration of outside air brings in a lot of humidity. “The airtightness of the house and ventilation rates matter quite a bit,” he says.

AJ isn’t buying it: “People who live in a ‘clean room’ are screwing up their immune systems,” he says. “We need to interact with the real world to have healthy immune system understandings IMO and finally in some research published of late.

“And Florida, to reduce humidity to 55% (which he has) is fine,” AJ adds. “Dehumidify maybe just a bit; but in cooler weather, open windows and be part of this great planet.”

Like it or not, Dorsett replies, but at 65% humidity “you won’t have serious mold issues, but if you have dust mite allergies you’re pretty much screwed — they breed faster than flies at that RH.” Moreover, dew points tend to be a lot higher in Tampa than they do in New York (where AJ Builder lives) or in Massachusetts (where Dorsett lives).

“That 70°F/55% RH outdoor air you’re currently experiencing has a dew point of 53°F, whereas in Tampa right this second it’s 92°F, with an outdoor dew point of 72°F (down from the mid-70s from earlier in the day),” Dorsett tells AJ. “Tampa’s average outdoor dew point is about the same as our absolute summertime peak dew point so far this year. (On July 15th the dew point hit 74°F at my house according to data sets — which was the highest reading of the season.)”

Our expert’s opinion

Here’s how GBA technical director Peter Yost sees it:

Higher interior relative humidity can easily feel uncomfortable when you are also dropping the air temperature with air conditioning. Sure, some folks actually do just fine with plenty of air movement at 80°F and 55% RH, but many folks don’t.

There is no excuse for any HVAC technician not to size an AC system based on the real loads, or to fail to specify the appropriate sensible heat ratio (SHR – the portion of the total cooling load handling the sensible load) for the climate. SHR for a Florida climate should be 0.70 or below. As Martin Holladay notes in his blog, Climate-Specific Air Conditioners, most air conditioners have an SHR in the 0.75-0.80 range — much too high for a hot humid climate.

Installing a dehumidifier to drop the interior relative humidity will likely do the trick, but most dehumidifiers are meant to work in just one room or space, unless the unit can be configured into the central forced-air system. (For more information on this approach, see this Building Science Corporation / Building America report). This fix will be more expensive and challenging in a retrofit than it would be in a new home.

The bottom line: get your HVAC good buddy to become “sensible” about loads and how to condition his friends!


  1. Rjp : | | #1

    Two Speed Compressor and Variable Speed Indoor
    I would consider a 16 SEER or better two stage compressor with a variable speed indoor unit that can provide mild weather humidity control and high performance cooling when needed with the right thermostat. A standalone dehumidifier is the second choice although I would not buy a central dehumidifier when the right AC system could be retrofitted at cost for similar dollars. Obviously complete air sealing is essential but difficult in typical Florida houses that were built with the idea that fiberglass is all one needs.

  2. Donald Endsley | | #2

    "Clean Room" comment.
    The hygiene hypothesis that AJ is referring to in his clean room statement is just a correlation and does not show causation. It is not appropriate to suggest a healthy indoor Relative Humidity (RH) based on those studies. The factors that may be causative for the correlation are vast and not understood at this point.

    Moderate Relative Humidity (between 40% and 60%) levels seems to provide the best overall health benefits This is likely due to only moderate survival rates of indoor airborne viruses. Moderate RH also prevents the growth of fungi again resulting in lower fungal infection rates. Airborne bacteria and exposure levels are correlated to dust concentrations , So things that reduce indoor dust (like a well filtered HVAC system) reduces exposure levels resulting in lower infection rates.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |