Dehumidifier advice

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woody_diy

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I have a problem with musty odor in my 1960's ranch house in Atlanta. Atlanta is very humid, and my house has a lot of tree cover, making it extra damp. The top level of the house is at grade level, but the ground slopes, so the lower level of the house is partly below grade. Below the lower level is a dirt crawlspace which is fairly well sealed with plastic. The foundation vents of the crawlspace are also sealed, so it's pretty air tight.

I have a portable dehumidifier in the crawl space and a second portable dehumidifier in a utility area on the lower level. I'm able to get the spaces with the dehumidifiers down to about 40% humidity, but the upper area is still going up to 58% or so. I would like to try to get the top floor down below 50% to try to get rid of the musty smell.

I had a HVAC guy come in to look at the situation. I thought that a whole-house dehumidifier installed in the crawlspace and ducted into the AC system might do the job. He proposed putting the whole-house dehumidifier in the crawlspace without ducting it into the AC and he thought that would dehumidify the whole house. Based on the fact that the current, portable dehumidifiers are not doing the job, I'm skeptical that a larger unit will make a difference. He said that he could duct it into the AC, but that the only way he could do it would be to make it return-to-return.

So, now, I'm not sure what I should do. It seems to me that a dehumidifier with a dedicated return and ducted to the supply side would be the ideal solution. If I go with a whole-house unit ducted return-to-return, would that be effective? Looking for advice here.
 

oldognewtrick

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Did your HVAC tech check to see if your HVAC system isn't oversized? A unit too big will not run long enough to remove moisture from the living space. Bigger isn't better in cooling a house.
 

woody_diy

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No, he didn't check on the current AC. If it is oversized, that could be a factor. I wonder if I could determine that by looking at the run-time of my current AC? I have an Ecobee thermostat, so I have a lot of data on the HVAC. What would I look for?

Another issue is that during fall and spring seasons, we get a lot of high humidity during times when AC is not needed. Resizing the AC might help in summer, but would not solve the problem completely.
 

woody_diy

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So, I took a look at my ecobee data. Over the last couple of days, I see that the AC occasionally runs at a short cycle (10 minutes), but most of the time, it's running for > 1 hour at a time. I also see that the indoor humidity reported by the ecobee is sometimes still high even after the AC has run for 2 hours. This doesn't suggest that the AC is oversized, I guess. I'm wondering if there's an issue with the AC if the humidity doesn't fall after an extended run like that. Or is it just the case that the humidity is too high for the AC to deal with?
 

Eddie_T

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A dehumidifier is basically a small AC which passes air over the evaporator coil to condense moisture then over the condenser coil to reheat the air. The net result is the air leaving the dehumidifier is warmer but dryer air. That's why it would be on the return side.

One hit I got on google said it would cycle with the AC. I can't see how that would help.
 

woody_diy

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A dehumidifier is basically a small AC which passes air over the evaporator coil to condense moisture then over the condenser coil to reheat the air. The net result is the air leaving the dehumidifier is warmer air. That's why it would be on the return side.

One hit I got on google said it would cycle with the AC. I can't see how that would help.
There are several ways to duct in a whole-house dehumidifier. As I understand it, the preferred way is to provide a dedicated return for the dehumidifier and then duct the output to the supply side of the HVAC unit. This allows the dehumidifier to use the existing HVAC supply, but bypasses the AC coil. Otherwise, the dehumidifier will sometimes be blowing across a wet AC coil and will pull that moisture back into the house. The method proposed by my AC guy is to pull air out of the HVAC return and then put the dehumidified air back into the return. There are probably other issues with return-to-return ducting, but I don't know what all of them are.
 

oldognewtrick

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A whole house humidifier not dehumidifier. I ask because I was called to look at a roof leak one time and the problem was a whole house humidifier was wired to run with the a.c. on and created a lot of moisture that accumlinated on a upstairs ceiling.
 

woody_diy

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A whole house humidifier not dehumidifier. I ask because I was called to look at a roof leak one time and the problem was a whole house humidifier was wired to run with the a.c. on and created a lot of moisture that accumlinated on a upstairs ceiling.
No, sorry, no I don't have a humidifier.
 

68bucks

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Do you have any hygrometers around the house or are you getting your readings from the dehumidifiers and the Ecobee? I have several hygrometers around and they vary from what my Ecobee reads. I had to correct the reading to get it close. The readout on a humidifier is only reading at that spot. I have a whole house dehumidifier I run in the crawl but it is not ducted, just drys the crawl. I don't seem to get any effect in the living space. I have debated adding a second unit and ducting it to lower the living space humidity without getting too cool. I don't have any oder issues though.
 

Eddie_T

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Before I installed my PTHP I used a fairly powerful ozone generator occasionally to freshen rooms. Now I hardly ever need it. Ozone can cause problems though, in my wifes closet it damaged some shoes made with imitation leather. Not so bad since she had transitioned mostly to Minnetonka deerskin moccasins for her go-to shoes.
 

woody_diy

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Do you have any hygrometers around the house or are you getting your readings from the dehumidifiers and the Ecobee? I have several hygrometers around and they vary from what my Ecobee reads. I had to correct the reading to get it close. The readout on a humidifier is only reading at that spot. I have a whole house dehumidifier I run in the crawl but it is not ducted, just drys the crawl. I don't seem to get any effect in the living space. I have debated adding a second unit and ducting it to lower the living space humidity without getting too cool. I don't have any oder issues though.
Yes, I have several different hygrometers. I agree that the Ecobee is not too accurate at reading humidity. Interesting that you also are not able to control humidity using just a dehumidifier in the crawl space. I think my AC guy didn't do enough investigation before he proposed that as a solution.
 

68bucks

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Yes, I have several different hygrometers. I agree that the Ecobee is not too accurate at reading humidity. Interesting that you also are not able to control humidity using just a dehumidifier in the crawl space. I think my AC guy didn't do enough investigation before he proposed that as a solution.
My humidity runs in the low to mid 50's during the warm months. I don't have any odor issues. A round here thr RH doesn't get below 50% often when it's warm. So to keep the house <50% would take some work. Do you have the smell in the winter when it's a lot dryer? I know Atlanta is pretty mild in the winter but you get some longer cold spells.
 

woody_diy

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Yes, I get the smell all year. In the winter, it's sometimes very dry, but can get humid when it's raining. Indoor humidity is highest in fall and spring when I'm not using heat or AC. I have a lot of tree overhang, and part of the lower level of the house is below ground level.
 

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