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Excess humidity - HVAC system too large

So, My AC system died last spring, and I was prepping to put new windows (impact resistant, low e) in my house... My house is approx 2200 sq.ft in Florida, south of Tampa... Zone 2A

I had a 4 ton system, and knew that would be too big as a replacement, but my HVAC tech replaced it with what was here and said he could tune it... Now my house averages 55%rh or higher... Up into mid 66-67...

I talked to him about it and he wants to bring some attic air into the return so it will run longer... I think that adding a whole house dehumidifier, like the Williams air sponge, or April Aire would be the best solution... Any feedback??

Asked by Chris Marriner
Posted Aug 18, 2014 9:50 PM ET
Edited Aug 19, 2014 7:01 AM ET


19 Answers

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I think the real solution was to size the system correctly, unfortunately. Anything short of that is likely to be an expensive and inefficient band-aid. 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.

Answered by Nate G
Posted Aug 18, 2014 11:14 PM ET


Bring some attic air into the return? 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.

Not sure what he meant when he said he could "tune it", but you need longer run times and an indoor coil that's not so cold. What is the make and model # of the outdoor unit that's installed?

Answered by David Meiland
Posted Aug 18, 2014 11:59 PM ET
Edited Aug 19, 2014 12:00 AM ET.


The system is a Payne (literally) 4 ton... I've already explored whether they could replace the compressor unit outside and scale back the air handler... But it won't work... Actually called carrier and explained my problem. That is the lowest capability for the tonnage... I can still provide the models for you, but I think I know where you were headed...

To compound this, he 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... I didn't ask him to?.. I just said that I thought the unit was oversized.

So as I see it, I'm left with a couple of options...
1) lawsuit for not running the heat load calc which I asked him to run, which would have dictated a 3 ton system... I've had one done since... (County doesn't require)
2) whole house dehumidifier
3) live with it and the allergies and mold that comes with it - not a real option...

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!

He was actually talking about cutting an opening in the return, in the superheated attic to increase the run time to try and dry it out... It didn't make sense to me, so I came to this forum.

Answered by Chris Marriner
Posted Aug 19, 2014 12:26 AM ET
Edited Aug 19, 2014 12:30 AM ET.


One way to wring a little bit more moisture out of the air is to lower the blower speed from 400 cfm per ton to 350 cfm per ton. Of course, lowering the blower speed (especially below 350 cfm per ton) carries its own risk -- namely freezing the coil.

The main benefit of your question is to serve as a warning to other GBA readers. Oversizing equipment can create all sorts of problems.

If I were you, I would install a $250 stand-alone dehumidifier as a first step. It may work well enough for you to tolerate your system. And it is a cheaper solution than starting from scratch.

Answered by Martin Holladay
Posted Aug 19, 2014 7:06 AM ET


+1 on the standalone dehumidifer approach. If the house is reasonably tight it should do the trick.

A 600 watt standalone 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 600watts it's using while running.).

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? 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.

Answered by Dana Dorsett
Posted Aug 19, 2014 11:48 AM ET


I had thought about the portable and just run a drain tube outside... Is there a rule of thumb on square footage of the house and number of pints per day required... Or is that based on how much I want to drop the rh??

Answered by Chris Marriner
Posted Aug 19, 2014 11:59 AM ET


The humidity sounds normal to me so what would be normal, anyone?

I have no AC and my summer humidity is 50-85% in my home about 10% less than outside. No mold except for bad tile work in one shower.

55% seems right considering Tampa where outside humidity is high along with the temperature.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 19, 2014 12:02 PM ET
Edited Aug 19, 2014 12:11 PM ET.


The humidity load in a house is a function of the number & type of humans & plants & fish that might be living there, how they use the place (are you boiling buckets of pasta every day without a cover, and without running the kitchen exhaust?) and in a FL climate, the infiltration/ventilation rates count. Since you're getting the place down to the ~65% RH range with the AC duty cycling as-is, you should be able to bring it down to a more comfortable & healthy ~50% RH with single Energy Star 70 pint unit. Set it up to drain into a sump or drain. It will probably have more capacity than you really need, but would cover you on days when it's gray and sticky outside but only low sensible loads to keep the AC running.

Humans are healthy & comfortable anywhere between 30-50% RH @ 68-75F. When it's above 50% RH dust mite populations begin to grow (a problem primarily for those with allergies or asthma). Above 60% RH @ 75F 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.

So, maybe 8 out of 10 people are comfortable and healthy if you hold the line at 60% RH in the summer, and 50% RH would be fine for 9 out of 10. But about 5% of people are never going to be comfortable happy or healthy, at any temperature or humidity. ;-)

The mid-summer outdoor dew points in Tampa run about 73F, so any infiltration or ventilation is bringing in substantial humidity. 73F dew-point @ 76F dry bulb temp has a relative humidity of about 90%. During midsummer nights the average dew-point low in Tampa drops to about 70F, which would still be about 82% RH @ 76F. The air tightness of the house and ventilation rates matter quite a bit.

Answered by Dana Dorsett
Posted Aug 19, 2014 3:33 PM ET


Dana, I just don't agree about all the trouble with humidity.

70 degrees out right now here, 55% humidity outside
69 inside and 64% humidity.

No mold or moisture issues;

Comfortable inside and outside, nice.

People who strive to live in a "clean room" are screwing up their imune systems. We need to interact with the real world to have healthy immune system understanding of our surroundings IMO and finally in some research being published of late.

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

I lived in Curacao with open glassless windows, and we used some great strong fans along with mid day siestas... no problemo... (hot hot hot place to build homes!)

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 19, 2014 7:05 PM ET
Edited Aug 20, 2014 9:46 AM ET.


AJ: At 64% 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.

50% is really the threshold for dust mites:



60% is really the threshold for mold, (though it doesn't go ballistic until 70%):



Between 30% & 50% RH is widely accepted in the health trades as the ideal range for humans- drier than that makes us more susceptible to airborne virus infections above that and the mites & mold start creeping in.

It's true that most people living in the tropics in non air conditioned buildings live in higher than optimal humidity levels all the time, but that's not to say they don't suffer the health consequences. (Some fungal infections are all but unheard of outside of tropical areas.)

When the outdoor dew points (not the outdoor temperatures) are in the 50sF, sure, open the windows. But in Tampa (unlike upstate NY where you live, or MA where I live) that's only going to be happening with any regularity between November and April though. That 70F/55% RH outdoor air you're currently experiencing has a dew point of 53F, whereas in Tampa right this second it's 92F, with an outdoor dew point of 72F (down from the mid-70s from earlier in the day.) 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 74F at my house according to weatherspark.com data sets- which was the highest reading of the season.)

Answered by Dana Dorsett
Posted Aug 20, 2014 3:06 PM ET


>>I have no AC and my summer humidity is 50-85% in my home about 10% less than outside.

Seems like it would be higher inside than outside, or at least equal. How is moisture being removed? You have a dehu?

Answered by David Meiland
Posted Aug 20, 2014 6:34 PM ET


David, my home has a roof.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 20, 2014 11:07 PM ET
Edited Aug 21, 2014 2:51 AM ET.


Thanks AJ, generally a roof is a good idea. But, would not having a roof explain high humidity levels?

Answered by flitch plate
Posted Aug 21, 2014 7:48 AM ET


Chris your key note to me is the fact that you're is in FL, likely have AC on constantly in summer and there are periods of 67% humidity. (cloudy days with less cooling?).. so that is much more humid then i would expect in a fully air conditioned home.

Yes 55% for an average isn't bad at all IMO (if he was seeing 50-59% range).

As far as tuning - if you have a newer thermostat (visonpro8000 etc) you can adjust the cooling and heating settings pretty easily to increase cycle times/deadband/pre-action... whatever its called. Changes the cycle times to 1 or twice per hour... Helps a little bit to increase run time length. Temp might fluctuate a hair more, but will increase run times. This will increase the amount of time condensate/dew can collect on coil and drain down out of unit.

Another thing you can do if you wanna get OCD on it, is temporarily increase set-point at stat... for a half hour or hour... then switch it back down a couple degrees for a half hour or hour. So manually controlling the duty cycle on occasion.

Answered by Nick T - 6A (MN)
Posted Aug 21, 2014 9:15 AM ET
Edited Aug 21, 2014 10:02 AM ET.


David- AJ actually measured:

"70 degrees out right now here, 55% humidity outside
69 inside and 64% humidity."

The dry bulb temps were essentially equal, but his interior humidity was higher. The dew point of his indoors 69F/64% RH air was 56F, whereas the dew point of his 70F/55% RH outdoor air was 53F.

That means his indoor air was quite a bit more humid than the outdoor air at that point in time.

That's why statements of the outdoor air humidity as "...50-85%..." is pointless without the temperature to which those measurements are relative.

I see overnight 85% humidity outdoors in summer here too, when it drops to say, 60F outdoors. But moving that 60F/85% RH air indoors where the house is still coasting along at 75F would result in an indoor relative humidity of 50% RH even without air conditioning (which isn't too far off from what's actually been happening at my house this week.) The notion that you'll always be "....in my home about 10% less than outside" is just plain wrong, unless your ventilation rates are so high that your indoor temp (not just the absolute humidity) tracks the outdoor temps within a handful of degrees.

Answered by Dana Dorsett
Posted Aug 21, 2014 5:44 PM ET


Dana, gotcha, guilty of not reading everything he wrote. That is precisely why I like my Protimeter that reads grains per pound.

Answered by David Meiland
Posted Aug 22, 2014 12:54 AM ET


Dana, great math... but the humidity has not been a problem more than a few days a year.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 22, 2014 10:33 AM ET


AJ. Eyeballing Weatherspark.com data sets..

... the mid-summer mean dew point average in Saranac Lake NY runs about 57F, so if you're in the higher elevations in the Adirondacks, sure, humidity is rarely a problem, and you can usually use outdoor ventilation without pre-conditioning/dehumidifying it.

In Plattsburgh the summertime mean is about 61F, definitely more humid, but will have many more high-humidity days than Saranac Lake

In Albany the summertime mean is about 62F, and it creeps higher the closer to the coast you get.

In NYC or Providence RI it's about 64F for a mean, and much higher peaks on the muggy days.

Answered by Dana Dorsett
Posted Aug 22, 2014 4:27 PM ET


62 dew point right now. Basement cardboard boxes are fine off slab, one on slap bottom has slight damage. Basement slab is at 60 degrees and has no under slab insulation. Proof underslab insulation is a good thing along with poly and stone.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 22, 2014 11:02 PM ET

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