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Old 02-22-2022, 10:22 PM   #1
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Default Appropriate Sizing for Maxum 3200SCR AC / Heat Pump

My 3200SCR has never had an AC/Heat Pump in it though Maxum provided a hint as to where their customers thought it belonged - in the stairwell which is why they ran flex tubing from there to the underside of the middle upper galley cabinet. Okey dokey - this is where I would like it to live too. I have searched various forums online respecting how to size an AC properly and decided to use cu. ft. for my calculations. As painstaking as it was , I measured every space in the cabin (LxWxH) in the bow's berth, from bow berth (galley) to aft cabin opening, the stairwell space, the head and aft cabin and came up with a total of 800 cu. ft. which includes an additional 10% for error I know is present. According to my number, (800 x 14 BTU per cu. ft.), I arrived at 11,200 BTUs and added 1,000 BTU to account for 4 portholes, 3 bomar hatches (exposed to sun), the aft cabin (and the window there) plus the black plexiglass cabin entry door. So, in my view, an AC/Heat pump of 12,000 BTUs would have been what Maxum designed for give or take a few hundred BTUs. I was surprised to find that when I requested quotes, the first one I received included a 16,000 BTU unit. Okay so now what? Before I reach back to the installer about their choice, a few questions I have for other owners who have an AC on their boat is: 1) can you share what size unit is installed 2) how you or another other person measured to decide on the sizing 3) what manufacturer you went with and 4) in terms of functionality, does the size unit you have meet your expectations in terms of climate control (would you have chosen differently) and finally, how do you find the switching on and off of your current unit? About right, not enough, too frequently? I'm thinking that a happy medium might be a 14,000 BTU but as you know this is not a common size. So I am hoping to make the best decision for my 3200SCR with input from others. Thanks in advance for any insights you can offer.
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Old 02-23-2022, 02:01 PM   #2
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Sounds like a big job, but worthwhile. We use our AC all the time. Especially if you have little kids that you donít want to wake up early, we keep our portholes closed and dark.

Hereís some comments from my experience on our 2900. It is a 12k BTU mermaid unit and had plenty of capacity. Their website mmair.com says it works for boats up to 32í.

Water temp is also a factor in capacity Ö the AC works best when the lake is cold and the heat works best when the lake is warm. If you run the boat in extreme water temps, you may want to compensate. The boat definitely cools off faster in the early season.

A few things to consider Ö if you donít have a genset and plan on running the AC off a portable generator, a Honda 2200 will (just barely) run a 12k unit. Any more you will need a larger generator.

A few features to consider Ö
- variable speed fan is really nice to keep the noise down when high speed is not needed.
- a stainless or plastic condensate tray is a must.
- my mermaid just used a standard household 24v thermostat. The marineair thermostat controllers are really nice and have many useful features such as dehumidifier mode.

Installation notes ..
- plan your ducting. If you sleep with curtain closed, you will find you need a duct behind the curtain to be comfortable. Also, itís really cold in front of the ducts, so try not to route it where itís inconvenient. My 2900 had an big outlet under the dinette and you couldnít sit there without freezing. The variable speed blower would also help there.
- depending on your comfort level with leaving seacocks open, locate it where itís convenient to close if you want to do so when you leave the boat.
- donít drain your condensate to the bilge Ö they produce a surprising amount of water on a humid day. Route it to a shower sump or overboard.

Hope that helps a bit.


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Old 02-23-2022, 09:42 PM   #3
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Thanks for your insights. I like the idea of variable speed fan/blower. I can certainly see where you'd want to turn the output down when you're preparing a meal by the galley stove/sink or sitting down to dinner at the table across from the supply grill. The seacock - yes. I agree it should be located where you can close it off when you're not at the boat. Agree on your point about draining condensate into the shower sump pump or overboard. The generator. Ugh. I've been going back and forth on this question quite a lot lately. There's a 'smart start' unit you can install to most any AC and it's supposed to reduce starting amps. Good. Then, there are some new AC / Heatpumps that draw less running amps too. I have a 3 bank battery charger and have been seriously considering a separate 'uber' bank of batteries through an inverter to primarily support the AC system and some very minor ac usage on the boat. Surely, a couple of light weight 200amp lithiums will be adequate for a 24 hour period to provide reasonable AC though not on every minute of the day or night. And though these batteries are very expensive, a 400amp bank of lithiums will come in far less expensive (and quieter) than a generator installed on the boat (I was quoted 28K for a 5K Westerbeke and just about fell off my chair!). I'm thinking of using my Yamaha 2,000 portable generator to activate the battery charger and pop up the charge on the battery banks. Or, I could install a super alternator to do this and just start up the engines though the latter would not be my first choice. I still have to do the math and if it could work I really don't see myself keeping the AC running constantly when I'm on the hook. Rather, I would program the unit's activation via an app where it kicks on for 30 minutes and then off for 2 hours at night and kick it on only when I find it uncomfortable during the day. I'll have to see how it goes with this kind of cycling or, it may make more sense to keep it running all night but at a lower temp and slightly higher humidity setting. Still haven't decided.
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