Help with high humidity

Yep, oversized or too many units cause short cycling where it will cool a space too fast and not run long enough to dehumidify. Better to have one or two units runs longer than 3 running shorter. Improper sized HVAC's and short cycling is one of the primary causes of mold issues in homes. Especially newer construction where code enforcement is lax in ensuring proper size vs load. Add to that; the AC sits in an unconditioned space and creates a low dew point. (AC and ducts in a really hot space creating really cold air). I am amazed that HVAC systems aren't required to have humidity monitors and run accordingly to arrive at a desired humidity. My guess is the EPA doesn't want that to limit energy consumption. I am also amazed that there aren't building codes to require HVAC units be placed in conditioned spaces. This is not only way more efficient, it lowers humidity generation. Not practical for RV's I get it, but certainly for homes. Anyway - another topic for another day.

You will get better humidity reduction if you operate the units a little short on capacity. That is let it run 24/7 rather than cycle on and off. If you have excess cooling capacity, the units will pull your temperature down and will satisfy the sensible cooling demands of the thermostat. Since you have 3 units, shut one off and let the other handle the load, even if it lets the temperature come up a couple of degrees. With the compressors running, the evaporator is cold and will condense the moisture in the RV.

Ken

cptqueeg wrote:
When sizing residential AC, both window and central air, I was always told that too big of a unit will not run long enough to remove the humidity. Perhaps just running 1 or 2 of the 3 units will solve your issue as it seems you achieve your desired temperature under relatively mild conditions before the air gets dehumidified.

The other suggestions about preventing or eliminating sources of humid, outdoor air and moisture are good ones.

Beat me to it.

If you have all 3 units running at night when there is minimal heat gain, they may only run a small percentage of the time, so they are only removing humidity a small percentage of the time. And on start up, it takes a few minutes before enough accumulates to start draining out onto the roof. If it's just on the fins, it can quickly evaporate when the unit turns off reintroducing the moisture back to the interior.

Run 1 unit and it will run longer and thus start pulling more moisture out and dumping it out onto the roof.

Assuming you have a ducted system, you may want to play with which unit to run to see which both keeps you cool and removes humidity.
- ie: if the bedroom unit isn't ducted and you keep the door closed, it may not cycle much, so won't do much about humidity.

I think using the main AC on auto at night time temps is your best option. I suggest using a different source of white noise.

I have used a natural noise machine for years, that you can set for a half dozen different sounds. My favorite is the continuous bubbling brook, and waves rolling in a close second.

Jerry

The wifelikes the sound of the fan in the bedroom so it stays on. I may try putting the main A/C on Auto or maybe I should turn it off all together to let the bedroom unit run longer.

kkirt1 wrote:
Thanks for the advice. Still curious as why for the first few years it didn't seem to be an issue. Now very noticeable. Wondering if there is something about the settings (temperature, fan speed, fan mode) that has started this humidity spiral.

As to fan mode, I have noticed when fan is continuous on my Dometic 15K, the humidity level will rise after compressor shuts off.

If I set fan mode to auto, and the fan and compressor shut down, the humidity stays where it is at shutdown. I also notice a burst of water drainage from the roof gutters.

It may be possible, on continuous fan, that the fan is recirculating some of the moisture in the bottom of pan??

Do you have/use auto fan setting on your units?

Jerry

Thanks for the advice. Still curious as why for the first few years it didn't seem to be an issue. Now very noticeable. Wondering if there is something about the settings (temperature, fan speed, fan mode) that has started this humidity spiral.

When sizing residential AC, both window and central air, I was always told that too big of a unit will not run long enough to remove the humidity. Perhaps just running 1 or 2 of the 3 units will solve your issue as it seems you achieve your desired temperature under relatively mild conditions before the air gets dehumidified.

The other suggestions about preventing or eliminating sources of humid, outdoor air and moisture are good ones.

The moisture in an RV comes from everyday living. You breathe and each breath puts out moisture in the room. Cooking and showering add moisture to a room.

When it is cool out, the A/C unit does not run as the sensible heat load in the room does not increase enough to make the A/C operate. The A/C will not remove moisture from the air unless the A/C is running, and the evaporator coil is cool.

One suggestion is to run a small space heater to put a heat load in the room which will force the A/C to operate. We frequently do this in cooer damp weather.

You can go out and buy a mechanical or refrigerated dehumidifier, but it is nothing more than a small A/C unit that takes the refrigerated and dried air over its own condenser to drive the air temperature up and away from the evaporator dew point. Thid decrease the relative humidity.

What is relative humidity in simple words?
Relative humidity (RH) is a measure of how much water vapor is in a water-air mixture compared to the maximum amount possible. RH is a ratio of the humidity ratio of a particular water-air mixture compared to the saturation humidity ratio at a given temperature (dry-bulb).

Ken

eHoefler wrote:
Showers, cooking, etc.. all add humidity, particularly showers. Use the bath vent when showering and the exhaust fan when cooking to help reduce the humidity. Running a stand alone dehumidifier would help dramatically.

Great advice! Using a dehumidifier makes a big difference.