A/C working, but not dehumidifying well

dualrefAugust 4, 2011

The condensation line on our a/c unit became plugged and the condensation went into the overflow pan, then outside the house. I cleaned the drain line and it is working again. But the humidity in the house has gone up to 70% during the day and 60% at night. The air coming out of the vents is cold.

Granted it has been horribly hot and humid outside (Houston)

but the unit has worked better than this in weather just as hot before the drain line plugged up.

The a/c unit is a Trane and was installed in 1994.

Any ideas what the problem could be?

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Is the fan setting on the thermostat set to auto? If not try it on auto and see if you humidity level drops. Running the fan continuously has a tendency to raise the humidity level.

    Bookmark   August 4, 2011 at 9:20AM
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I have the fan set to auto. I tried it on "on" last night and just as you said, the humidity went up.
The unit IS cycling on and off. I thought that since the drain line plugged there may be a lot of humidity/condensation in the unit and may have froze on the A frame. So during the middle of the night I turned the unit off for 2 hours. Then I turned it back on and the humidity levels went down to 57%(with an outside humidity of 71%) Much better. But is still isn't the 48-50% is used to get. But as the day gets warmer the humidity in the house is going up, right now it's at 59%.

Even though it was 85F outside last night, the temp in the house never rose about 78F for those two hours when the unit was off.

What do you think is the problem or possible causes?

    Bookmark   August 4, 2011 at 11:31AM
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Tell us about the system you have. Size home, size system?

You had good humidity control before this problem? Have you made inspection of your evap coil? At the high temperature of the day, tell us about a typical runtime cycle.

Right now if your condensate line is draining properly, I don't see how one problem is related to the other.

You could lower the airflow CFMs on your blower. Slower blower equals better dehumidification.

When was unit/system last serviced?


    Bookmark   August 4, 2011 at 12:45PM
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The unit is a Trane XL 1000 3 ton installed in 1994. The upstairs air handler is a Airpro (Consolidated) unit mounted horizontally. The house is a 1 floor home with 1345 sq feet. I normally do my own service (cleaning outside unit,vacuum out the inside yearly, replace filter (pleated) monthly). I did have a a/c/heating inspection done about two years ago to see if the heat exchanger was in good shape and they guy said it was clean with no cracks. He said the system looked good.
Since there is no access point to the A frame inside the unit, I don't think it has ever been cleaned.

Yes, up until two days ago the house humidity was always between 48-52% during the summer. It was only after the drain plugged and the unit started to drip to the pan underneath the unit that this started.

The way the cycles work is that they cycle on for about 10-12 minutes, and then off for about the same amount of time as the thermostat calls for cooling. But once the outside temps hit 95F, (around 3pm) the unit goes into constant run mode with the interior of the house reaching a tops of 81F. After the sun moves off the house (7pm ish) the house cools down to 76 within an hour or so.

But today, the house wouldn't cool as good as yesterday. I went outside to the unit and checked the pipes (that are insultated) going into the house. The cold pipe is usually so cold you can't comfortably touch it, but today you can. So the pipe is not as cold as it used to be. The outside temp is 100F with a humidity of 40% and indoors it's 81F with a humidity of 75%, which I know is way too high.

I suspect it's low on freon.

Thanks for your response.

    Bookmark   August 4, 2011 at 4:16PM
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The wife was complaining that it got too hot, so I called an a/c company out. They examined the unit and got a head pressure of 140/425 on a 101F day. They diagnosed it as a restricted evaporator coil. I asked them if if could be cleaned, and they said no, you can't clean the freon galleries inside an evaporator, you replace it. My BIL says that they can be cleaned out.
I think somehow dirt got inside the sealed system, maybe from deteriorating parts in the compressor?

So, for an American Standard 16SEER outside unit, new evaporator coil, new plumbing between the two and a new drain line system the price came to $5K installed. And there was no charge for the service call today. They will start the install at 8am tomorrow. It's a 3.5 ton unit.
My wonderful BIL told my wife that we got ripped off on the price.
What do you all think?

    Bookmark   August 4, 2011 at 5:49PM
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paired with what brand, model, age furnace?

var speed blower or conventional blower?

post mdl number of condenser and evap coil to be provided.

if you don't have var speed blower, then you can't put in a 15 or 16 SEER AC model.

new refrigerant lineset to be installed?

post back.


    Bookmark   August 4, 2011 at 6:41PM
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The furnace is a 1994 Airpro(Consolidated) unit. It's 65K btu. The model number is: SGH 050 ND 35R and the serial number is 931006030.

They didn't give me a model number of the evaporator, it's an American Standard brand. I don't know the model number, I handed the brochure back to the service tech.

The unit does not have a variable speed blower on it, just a single speed. I asked him about how well will that higher SEER unit dehyrate the air and he said it would maintain 45-50% RH easily. Both the evaporator and the outside unit are designed to run on R-410.

If you can't use a fixed speed blower with a high SEER unit, what problems will it cause?

    Bookmark   August 4, 2011 at 8:14PM
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You should go with a basic 13 SEER condenser like the Allegiance 13 model.

You cannot achieve the performance/eff numbers on the high eff models without a var speed blower. It means you will pay extra for a capability that can't be achieved.

New line set should be installed. If existing is correct size and in good condition, then it can be reused if flushed according to procedure.


    Bookmark   August 5, 2011 at 11:08AM
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They installed the new unit today. I downgraded to a 14 SEER as they didn't have any 13 SEER's available. I'd have to wait a week-ten days for one to come in.
After they installed the new unit, it cooled the house down from 84F to 75F within an hour. But the humidity level stabilized at 59%. Shouldn't the humidity be lower than that or is it because the house was very humid for a few days and it may take some time to wring all of it out of the air?

    Bookmark   August 5, 2011 at 3:15PM
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BTW, the new unit they installed was the 4A7B4036 outside unit. 14 SEER. I think this model was formerly part of the Silver Allegiance Series. And they did install an AS evaporator.

    Bookmark   August 6, 2011 at 5:51PM
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good stuff clogged evap coil boy they seen you coming. thatsw just a tech who wanted a quick sale and he got it . anyways you got some new equipment. congrats

    Bookmark   August 17, 2011 at 10:54AM
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See, that's the problem. Most a/c companies know that homeowners usually know little to nothing about HVAC systems and some take advantage of this.

The way I reconciled this with myself is that the old unit was 18 years old and I did see that the evaporator coil had tons of rust on it. So I thought replace it this year or wait a few more and pay even more at replacement time. Hopefully, I won't have to worry about it.

But the new system is doing something strange. With the old system, the house humidity stayed around 42-44%. With the new system it starts at 51% in the morning and ends up at 60% by the time we go to bed. What's up with that?

    Bookmark   August 17, 2011 at 3:26PM
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Did someone leave a shower running for more than a week? I couldn't resist so I hope that you have a sense of humor.

Seriously, too much air flow vs. too warm coil temp, maybe. Maybe cycle times are short. Maybe you are sucking in outside air due to leaky ducts? At any rate, the installer said 45-50% easy so call them back!

    Bookmark   August 17, 2011 at 8:50PM
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That's ok. No, we checked for leaks, the house is pretty new so it's sealed tightly. There is no excessive water use in the home. Most houses I know have a stable humidity level, this one changes from morning to night by 10% or so. And even if nobody is at home during the day the humidity level changes like this too.
I called the company that installed the unit and they said that "They would get back to me soon, when they had the time".
They prioritize people who have no a/c before people with malfunctioning a/c in this 100F+ heat.

    Bookmark   August 18, 2011 at 3:17AM
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