High Humidity in Air Duct air after Compressor goes off


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Old 08-27-10, 11:53 AM
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High Humidity in Air Duct air after Compressor goes off

Hello Forum!

I just had a new Carrier AC & Downflow Gas Furnace installed. It's doing a great job keeping my house cool.

When the AC compressor is on ... the air coming out of the vents is around 66F with a relative humidity (RH) of around 45%, which is great.

As soon as the compressor goes off, in about 1 minute the RH of the air vent air jumps to around 75% ... and stays high until the compressor comes on again knocking the RH down in the air vent air to about 45% again.

Is this rapid rise in the RH of the air vent air right after the compressor goes off normal?

I wouldnt have even thought to measure the RH except that I noticed whenever my AC compressor would go off ... the air coming out of the air vents began to smell "musty" ... like an old castle. As soon as the compressor comes back on, knocking the RH down again, the air vent air smells great.

Note that I never noticed this smell problem in my old 20 year old carrier air conditioner -- although I never measured the RH in the vents before.

My AC company told me this was normal. Is it? They said it was caused when the house air started flowing over the wet coils putting the moisture back into the house. One thing that is different is my old Carrier AC had a very high FAN ONLY speed, while the new Carrier AC has a very slow FAN ONLY speed.

I am thinking about having my AC company increase the FAN ONLY speed to very high (my thinking is that the fast air would tend to dry out the coils and drip pan faster)

Note that I run 3 dehumifiers in my 1800 sq ft house ... So my house RH is kept at a beautiful 45% all summer long ... so the air running into the AC intakes (recirculating through the coil) is also a nice 75F and 45% RH ... the only moist air in my house drifts out of my air vents just after the compressor goes off.

Question1: Is this rapid rise humidity of air vent air from 45% to 75% after the compressor cycles offf normal? (Even when my house air is kept at a constant 75F and 45%)

Question2: If this is normal, would it be better for me to:

a.) have the FAN set to OFF in the summer season (would that cause the water on the coils and drip pan to mold and mildew) ...

dark + wet + no air movement = mold

or

b.) would it be better to set the FAN speed higher so the coils and drip pan dries out faster blowing the moisture in my coil and drip pan through my air vents so my dehumidifiers could wring it out.

By the way I have a UV light above the coil ... however it's an N coil and I think they should have installed 2 lights to cover the entire N shape.

Thanks for any advise.
 
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Old 08-27-10, 12:30 PM
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If you have the fan set to "on" so the fan runs all the time instead of "auto" the unit will indeed put humidity back into the house from the wet evaporator coil. And your system has to start all over again to take that humidity out. It's best to leave the fan on "auto" so it shuts off with the compressor in the cooling season to prevent humidity rise. After the system and fan shut off the water on the coil will continue to run off the coil and out of the house through the condensate drain pipe. This will not cause a mold problem because the water is constantly moving from the A/C coil to the outside of the house every time the A/C comes on. If you turn the fan to "auto" and let the system get rid of the humidity as it was designed through the condensate drain pipe instead of putting the humidity back into the house from the fan running all the time you probably won't need the extra dehumidifiers.

The "on" setting for the fan is generally only used when you don't need the A/C and just want to circulate air throughout the home. Kind of a breeze effect.
 
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Old 08-27-10, 12:43 PM
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QUOTE "the air coming out of the air vents began to smell "musty" ... like an old castle" It sounds like you may have some ductwork issues if the air smells musty as you say. Have the duct cleaning company run their cameras in there to find the low spot where the sewer leak under your house has been leaking for years and is getting inside your ductwork. Yuck get it fixed quickly.
 
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Old 08-27-10, 05:12 PM
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Thank you jeggs ... that's great advise ... i just tried turning my fan off ... and no more musty smell ... I do wish the darn fan would SHUT OFF exactly when the compressor shuts down ... my system runs the fan for about 2 minutes after compressor shuts down before shutting off the fan (FAN on AUTO) ... just enough time to get the air in my ducts up to a humid 72% before shutting down the air flow ... is there a way to bypass the AC circuitry so the FAN shuts off exactly when the compressor shuts off ... if i could do that ... the air sitting in my ducts would be a pleasant 45% (a mold growth inhibiting RH)

Why dont the engineers at Carrier have all this stuff figured out? Are they idiots?


- - -

Thank you former_member ... i will have a duct cleaning company check my ducts ... I had them video'd and cleaned 4 years ago ... but it's prob time to check them again ... i am no expert, but if there was a problem wouldn't they smell bad all the time ?... and not just when the RH jumps to 75%? ... my duct work in my ground floor is cement ducts in the slab (now thats a bad design) ... but my top floor has metal ducts ... both smell sort of musty when the RH jumps to peak values when fan only is blowing ... but smell great when the RH is 45% when the compressor is on
 
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Old 08-27-10, 06:27 PM
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The theory behind the time delay for the blower motor is that after the compressor shuts down the evaporator coil is still cold and the fan runs a minute or two to continue removing heat from the house while the coil is still cold. It works the same way with the furnace in the winter removing the last bit of heat from the heat exchanger and cooling it down.

Those two minutes of run time shouldn't raise the humidity in the duct work enough to matter. I think you have other issues. If the musty smell started right after the new unit was installed you need to have the company that installed the unit come out and investigate it. Have them check and make sure the condensate drain is working properly. Have them open up the unit and make sure the area around the coil and the plenum isn't flooded causing a mold issue. If the inside is dry and the condensate drain is working properly you need to do what Former Member said and have the duct work looked at. Musty can mean mold and if it is mold it is airborne and getting blown out into the living space. Not a good scenario.
 

Last edited by jeggs; 08-27-10 at 06:54 PM.
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Old 08-27-10, 06:47 PM
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thank you jeggs ... great advise ... i appreciate your help with this. I called my installer and they are checking into some things ... i think they will come out next week

the problem definately started with the installation of the new units about a month ago ... my 20 year old carrier AC and furnace didnt have this problem --- never smelled any mold or mustiness

(in hindsight i never should have replaced my 20 year old system ... but was lured in by the prospect of about $3000 in rebates and tax breaks -- and the age of my old system)

got a new 90% carrier gas furnace and 14 seer AC (2.5 ton) (performance series) installed for $4600.

my old ac and furnace was bought in 1990
 
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Old 08-27-10, 06:51 PM
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my gut feeling is my old coil did not do as good of a job removing moisture ... and was designed to drip water better ... therefore there was not as much residual moisture on the coils when the compressor shut down ... then with the fan on ... the RH in the ducts never exploded upwards ... as happens with the new unit
 
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Old 08-27-10, 07:18 PM
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What's the model of the furnace and t-stat?

My system(Trane) runs for a short time after it shuts off, and not been an issue with the "musty" smell.

let us know what the dealer finds.
 
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Old 08-27-10, 08:12 PM
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also one last thing ... the moisture is not being introduced by the ducts ... when the AC off ... and the fan on ... after about 4 hours ... the Air coming through the ducts is dry (45% RH) just like my house air ... if the ducts were flooded with water, the air in the ducts wouldnt dry out ... it's clear to me the moisture is being introduced into the duct work by the coil or the drip pan ... or a leaky drip pan
 
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Old 08-27-10, 08:17 PM
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Hey, Thanks for the interest ... Here is what was installed:

AC = Carrier Perfomance Series 2.5 ton

Heater = Carrier Performance Boost 90 (58 MEC) Gas Furnace (Downflow)

Thermostat = Carrier Edge Programmable Thermostat
 
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Old 08-27-10, 08:35 PM
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Do any of you guys have a relative humidity gauge? (or a combination temperature and RH gauge)

If anyone does can you measure the RH of the air coming out of your air vents while the compressor is on towards the end of a long cooling cycle/run (say on a hot humid day).

Then turn the compressor off and leave the fan on ... and measure the RH of the air coming out of your air vents say 3 to 5 to 7 minutes after the compressor shuts off and you are running fan only.

The RH of my air vent air goes from 45% to 70% in 2 minutes after compressor shut down ... then maxes out around 76% about 7 minutes after compressor shut down ... then slowly drops back after that (down to about 60% after 1 hour ... then down to 55% after 3 hours)

I am just curious if these are typical numbers for modern air conditioners ... or if my system is producing an unusual amt of humid air after shut down.

Note that the Temperature of my air vent air is usually between 66F and 68F with the compressor on ... on only slowly rises to around 73F with the compressor off after an hour) ... In other words, the temperture of the air vent air is not an issue at all.

Also note that the ambient air in my house (and therefore the intake air) is a beautiful 75F and 45% RH almost all the time (due to my running of 3 dehumidfiers in my 1800 sq ft house)
So i have a fairly pristine starting point.

If you do run this test and post your numbers, can you give some info about your system (how old it is and the brand/model)
 
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Old 08-27-10, 11:19 PM
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If the air coming out of the vents when the compressor is running is 67 degrees and the return intake air is 75 degrees that is a red flag right there. That is only 8 degrees difference or delta-t as the techs call it. It should be between 15 and 18 degrees difference. The temperature coming out of the vents should be at least 60 degrees with a 75 degree return air temperature. The A/C system is not performing at it's peak efficiency with a delta-t of 8 degrees and the evaporator coil is not nearly cold enough to be removing the humidity in the house efficiently. The system will also be running longer to hit your thermostat set temp which means more electricity wasted. You need to have the company that installed the unit come out and address that problem. They need to make sure the fan speed settings are correct and check the refrigerant charge superheat/subcooling. If the A/C system is performing correctly you shouldn't need any dehumidifiers to take care of the humidity in the house.
 

Last edited by jeggs; 08-27-10 at 11:47 PM.
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Old 08-28-10, 06:43 AM
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As pointed out by Jeggs, that small delta-t is your problem. don't waste time on measuring the temperatures, need to fix the A/C first.
 
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Old 08-28-10, 07:05 AM
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Thank you for all the great feedback, but i do have a concern

my concern is making the AC coils colder is just going to exaserbate the problem

colder coils means wetter coils (more water sitting on the coils and drip pan)

also colder coils means the compressor runs less ... so higher RH in my ducts for longer periods

the temp of my house is fine ... the unit is keeping my house at a smooth 75F on 95F degree days

and with the compressor running constantly practically all day long (constantly) the RH in my ducts is a nice 45%

The problems occur when the compressor starts shutting off more in the evening and night and the RH in my ducts explodes up to 75%

i am about ready to have my new $5000 system ripped out entirely and start over

i think what i need is to have an airconditioning genius come to my house and fix it ... problem is most of the AC people i've seen so far are not very bright or skilled

Is there such a thing as an air conditioner master craftsman? If so, how can I find them? I have tried emailing Carrier directly ... but they just tell me to talk to my approved installer.
 

Last edited by avalon1957; 08-28-10 at 07:21 AM.
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Old 08-28-10, 07:16 AM
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This cant be a duct work problem ... I shut down the compressor last night and am running fan only

After a good 12 hours of no compressor running ... the condition in my house (both upstairs and downstairs) is a beautiful 75F 45% ... the air vent air blowing out through my downflow unit and coil (not compressing) and ductwork is also a beautiful 75F 45% ...

So this is definately a problem in my coil somewhere

I dont really have a complaint when the compressor is running (ok maybe it's not cold enough with a delta T of -9F as you all say) ... but he RH of the compressed air is around 48% or so

But as soon as the compressor shuts down ... it's like a garden hose (or a steam machine) gets switched on in my ductwork ... and the RH of my ductwork air explodes to 76% in 2 minutes.

I can't believe that much water is getting evaporated in 2 minutes time ... after all it's cold water ... even if i had a big bowl of cold water sitting in there ... how could that much evaporate in 2 minutes ... it doesnt seem possible

Even with the unit's FAN on "AUTO" ... when the compressor shuts down ... the fan STILL RUNS FOR 2 MINUTES (damn) ... long enough to fill the ductwork in my entire house with 76% RH air (a perfect environment to grow mold) ... DARK, COOL, STILL, and VERY HUMID ... Having the fan shut off at this point is probably making a bad situation worse as air movement inhibits the growth of mold

I never should have had my old 20 year old system taken out
 

Last edited by avalon1957; 08-28-10 at 07:32 AM.
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Old 08-28-10, 07:27 AM
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Do you think if i had my AC installer take out the Carrier N coil they used and replace it with another brand ... that might help

Replacing the coil would be cheaper than replacing the whole system ...

If i have a Carrier Gas Funace and a Carrier Central AC unit ... can I match that with a different brand coil?

I wish i had kept my 20 year old Carrier coil as I never had a problem with that

The new N coil looked sort of like a many hundreds of car radiator fins ... I think my old 20 year old coil looked more like tubing ... I would think the larger tubing structure wouldnt hold (hide) as much water as the hundreds of small fins ... maybe I just need to find a coil that cant hold as much water
 
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Old 08-28-10, 07:42 AM
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ive been doing some web searches ... does anyone know the difference between a "condenser" and a "coil"

actually i know what a coil is ... but what is a condenser

does my carrier system have both?

(i always thought it was the cold coil that caused the condensaton of air ... so I always thought they were the same thing ... but it sound like from what i've seen on the web they are different)

do you think from the way I described my problem, the problem is in the condensor or the coil?

thanks for anyones help or input
 
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Old 08-28-10, 08:42 AM
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Condenser is your outside unit (the whole unit), in there you have compressor and condensing coil. there is another coil in your plenum, we call it evaporator coil. your ac has to have all those to run. I really think the problem is your AC itself, Carrier is a good brand, if you installed it right, you should not have any problem. Why don't you call a tech to check the very basic first, like freon level, pressures, etc. may be all you need is a little freon. one step a time, you will get it fixed, don't rip out the $5000 unit - yet.
 
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Old 08-28-10, 12:09 PM
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It sure sounds like you have ducts, coil and evaporator tray that could use a good cleaning to get rid of the smell.
This business of measuring the RH as the compressor starts and stops however is going to drive you crazy!

The readings you are getting are what you would expect with any properly functioning a/c system.
Why your system appears to be adding moisture is because the relative humidity reading is just that........it is relative to the temperature.
As the air is cooled it has less ability to hold moisture so the reading you are getting is the percentage of moisture in the air relative to its current temperature.
There is no more moisture, only the reading has changed.

I am not sure if this is too much information but if you want to measure the actual moisture content you would need to take your relative humidity reading and plot it on a psychometric chart to get the grains of moisture per lb of air.
Not sure what you are using to measure RH but some of the better electronic psycrometers show you the wet and dry bulb readings to plot on the chart.

Click image:

Image courtesy of uconn.edu
 
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Old 08-29-10, 10:06 AM
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During this time that your rh explodes from 45 to 70, how much does the temp change?
 
 

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