Oh, the humidity!

humidity meter

When the seasons change from Spring to Summer, there is always a certain amount of reluctance to fire up the air conditioning in our house until it's absolutely necessary. After being closed up for the Winter, having fresh air blowing through the house is so refreshing that no one is in any rush to seal off the outside world again until things get too uncomfortable.


The comfort issue is more a matter of excessive humidity rather than temperature, and when it gets to the point that you can feel the curtains and furnishings getting heavy and limp with moisture, you know that it's time for the air conditioner to come to the rescue.


This year, before turning the air conditioning on, I placed a gallon jug under the condensate drain to see just how much moisture would be removed during the first hour. Our 30,000 Btu system that cools our ground floor removed 1 gallon in the first 45 minutes, which equates to 1.25 gallons per hour.


Now, condensing the moisture out of the air is not a free additional benefit of cooling the air, it comes at a price. Our science books tell us that it takes 960 Btu's to change the state of 1 pound of water to vapor without any change in temperature, and the same is true in reverse when we condense vapor into a liquid. A gallon of water weighs 8.3 pounds, so our air conditioner removed 10.4 pounds of water in that first hour, and it took 10.4 x 960 = 9,984 Btu's to do so.


So in round numbers, only 2/3 of the cooling power of the air conditioner was being used to cool the air, and 1/3 of the power was harnessed to remove moisture. After an hour or so of the air conditioner running, as the air in the house becomes less humid, and most of the moisture has been drawn out of the furnishings, more cooling power is available to lower the air temperature.


So remember this next time you go to your boat to open it up and dive down below to turn on the air conditioning. Boats are surrounded by water and can become unbearably hot and humid down below when left closed up between weekends or for long periods.


Be prepared for the air conditioning to struggle initially to cool the cabin until the worst of the humidity is removed, and keep doors, hatches, vents, windows, etc. closed during this period.


If you have a digital air conditioning control like the FX-1 from Coastal Climate Control, utilize the Dehumidification Mode to keep humidity at low levels while the boat is closed up and no one is on board. This feature uses only minimal electrical power, but the results are quite dramatic.


Your boat may still be unbearably hot when you arrive, but the humidity will be far less oppressive and the air conditioning will cool the boat down a lot faster.

What people are saying:

about Solara solar panels:

"Greetings from Tahiti. The solar panels are fabulous. They give us so much power"
   - John W., Lagoon 440 on passage from BVI to Australia, with 6x 120w Solara PowerM panels and Genasun GV-10 solar controllers

 "We're currently in Guadeloupe. Boat is doing well, I wish everything else was as maintenance free as our Solar panels."
    - Leopard 46 with 6x 120w Solara PowerM panels and Genasun GV-10 solar controllers

Marine Cooling, Refrigeration, Monitoring, And Solar Solutions

Are you a boat owner, or soon-to-be boat owner, with questions about your vessel's systems? Let us help you find those answers. For example:

    • Want to know how many amp/hours your solar panel is putting into your battery bank? Philippi's PSM System Monitor can help you there.
    • Water-cooled vs air-cooled refrigeration, see "Which Condensing unit should I use" in Refrigeration > Planning.
    • Where does one begin when planning for solar panels on a boat? With "Marine Solar Panels Planning and Installation Guide in Solar > Planning.
    • Can you have air-conditioning on your boat without running a generator, or the engine, or being plugged into 115v power? (Hint: Yes!)

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