Duct size is critical, and specified by the manufacture.
In the case of our PDQ catamaran, we wanted to be able to run the boat on 15 amps (including charging and lighting)-so we underestimated BTU requirements.
#WINDOW AC UNIT SQUARE FOOTAGE CALCULATOR WINDOWS#
A 16,000 BTU AC will cool your boat quickly, and 10,000 BTU will probably do a fine job so long as windows are covered and you can wait a little while. The problems with oversizing is increased power draw at the dock, possible modifications to the power system (you may need a new electrical panel), less efficient dehumidification, more noise, more space, and more expense. It is safer for him to oversize a little. If he downsizes the unit and the cooling does not meet customer expectations, he is in a pickle. It still runs no more than 25 percent of the time on a hot night, once the cabin is cooled.Īn AC installer has to be conservative. Because our unit is smaller, the cabin is dehumidified and thus comfortable without cooling to an excessively low temperature. The air is cooled, but not dried, which is actually more important than cooling in most boating areas. What is the problem with oversizing? In addition to weight, noise, lost storage space, increased power demand, and expense, dehumidification is less efficient when an AC unit is oversized.Īlthough this is counter intuitive, unless the AC unit has a relatively long cycle time (on for at least 70-percent of the time), the compressor does not run long enough for the coils to accumulate condensation and have it drip off and drain away. We are also willing to let the AC take an hour to fully cool the cabin.
#WINDOW AC UNIT SQUARE FOOTAGE CALCULATOR PLUS#
How can we get by with so much less? Most of the cabin is cored with 1.25-inch of foam, plus a foam ceiling liner, and we cover all of the windows to reduce solar gain. In actuality, a 12,000 BTU AC would have suited our boat, but our home marina has dock power limitations, so we skimped and settled for 10,000 BTUs, demonstrating that rules of thumb are often oversimplifications. This yields about 17,000 BTU for the test boat, which is close to our needs if we did not use window covers and awnings to reduce the solar load during the day. Another rule of thumb is to multiply the actual accommodation volume by 14 (height x width x length in feet x 14 = BTU). Houses, on the other hand, tend to have similar insulation values, and the rate at which they cool is not important because people leave the AC on all summer.įor example, the simplest rule of thumb for sizing is about 100 BTU/square foot, which would calculate out to a whopping 33,000 BTU for our test boat (a PDQ catamaran). It also depends on window covers and awnings and how much window space you have. Estimating size has always been tricky, because it depends on the insulation value of the boat, climate, and how quick you want the boat to cool down.
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