During periods of high humidity, with low cooling load, a central dehumidifier is the only way to go. Quote: The Bear.

V

Check out the Ultra-aire whole house dehumidifiers. They work well.

If your old house has a high rate of air infiltration (if there are many unintended places that outdoor air enters), you may want to have additional work done to plug some of those holes and reduce the infiltration as well. Too much air infiltration can bring in large amounts of water vapor and increase the humidity substantially.

Here is a link that might be useful: ultra-aire dehumidifiers

I had a DeLonghi dehumidifier in my basement a few years ago. My electric bill was thru the roof. (When I replaced my electric hot water heater with a heat pump hot water heater my basement humidity "issue" went away and my electric bill was reduced dramatically from even the pre dehumidifier levels.

Is there any way to dehumidify the whole house with the OP's existing A/C unit?

"Is there any way to dehumidify the whole house with the OP's existing A/C unit?"

Only if you set the temperature low enough it has to run.

Residential central air dehumidifies along the way, and the humidity is counted in load calculations, but the systems are not set up to control humidity.

Commercial systems often are set up for year round humidity control (the air is always cooled to dehumidify, hten heated and water added back).

It is far more expensive to run.

Residential cooling are not set up in a way that allows varying the system capacity.
This would allow it to run longer when the full cooling capacity is not required, and result in better humidity control.

There are some variable speed compressors starting to appear, and before that condensers with two separate compressors showed up (with valves and controls to stage the compressors).

The smaller compressor is used when the load is not as large, allowing the system to run longer for humidity control.
The larger compressor is used when the system load is closer to maximum.

"Residential cooling are not set up in a way that allows varying the system capacity.
This would allow it to run longer when the full cooling capacity is not required, and result in better humidity control. "

Mt two-stage compressor with variable speed air handler seems to work this way. Or maybe I'm missing something.

In any event, dehumidifiers are expensive to operate - at least in my area.

I had a similar problem - live in the mtns of VA. During the late spring and summer, we often get days when the temps are too cool for the AC to turn on and we had an issue with dampness and moldy smell coming from the basement. I also had an issue in the basement during winter thaw when the humidity in the basement would get too high (above 55%))

The first thing i did was add in a whole house dehumidifer. I used a sante fe model in the basement - and hooked it up to a digital humidistat controller. That fixed the smell issue right away and made the house feel cooler even though the AC wasn't on. I run the HVAC fans to circulate the air between the basement and first floor.

The next thing I did was add IAQ Tstats on the first floor that have a built in humidistat (Honeywell Vision Pro). The tstat has a setting that allows the AC to run up to 3 degrees below the set point if the humidity is above the humidity setting you set. This allows me to use the AC as a dehumidifier too.

I found that the combination works well. When I only had the whole house dehumidifier - it ran almost continuously during the summer. When I added in the IAQ tstat, I cut that DH run time down by using the AC most of the time. In the winter time when the AC isn't running, the DH with HVAC fans handles that case.

A good purchase are standlone humidity and temp guages that you can place around the house. I try to keep my humidity below 50% in the summer. You can really feel the difference with the dryer air.

Goodluck.

sniffdog,

Any sense of electric costs for dehumidifier and also additional run time on A/C?

Mt two-stage compressor with variable speed air handler seems to work this way. Or maybe I'm missing something.
---------
The next thing I did was add IAQ Tstats on the first floor that have a built in humidistat (Honeywell Vision Pro). The tstat has a setting that allows the AC to run up to 3 degrees below the set point if the humidity is above the humidity setting you set. This allows me to use the AC as a dehumidifier too.
--------
There are certainly ways to design and tweak conventional HVAC systems to reduce humidity on humid days when no significant cooling or heat is needed, but in a very humid environment, a dehumidifier is really going to be an advantage to increased comfort and health on many Spring and Fall days. My mini splits have a program to dehumidify (cold coil, low blower speed) or the blower speed can be manually set very low to shift to humidity control with minimal cooling. Like variable-speed ducted systems, they still cool some on those settings. Note that some mini splits (Daiken and Mitsubishi (City Multi systems)) can be set to cool in one room while heating in the next. That should be a very efficient dehumidifier, but is a costly investment. For all I know, they make dehumidifier modules for them as well.

OP, if you seal up your house, your humidity problems will be significantly reduced. See the response to "Insulating an old home bang for the buck" in this forum.

"Mt two-stage compressor with variable speed air handler seems to work this way. Or maybe I'm missing something. "

You should have kept reading.

The spec sheet shows that my DH unit draws about 1 kilowatt at 80 deg F and 60% relative humidity. In my area that costs about 11 cents per hour during the summer. If it ran all day it would cost around 2 dollars. When we first turned it on, it ran for 2 days straight. After we dried out the house, it ran at most for 12 hours a day but usually less (depended on the weather).

I don't know how much longer my AC runs or what the extra cost is for providing the DH function. All i know is that when I looked at the electric bills with both the DH and extra AC activated I did not see a large jump in my bills that would have caused me any concern.