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Old 16-03-2010, 15:42   #1
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How to Size HVAC

OK Guys:

Looks like I have to replace my HVAC in the boat...Its hard to tell what size is currently in there..the Pro doesn't even know??

So they are trying to direct me to replace them with 9000btu units which would give me 18,000 btu combined....Is this good enough for the tropics? 41' tri cabin....If it cools at a rating of 9000 btu does it heat at that as well?

Also these are hydronic units...what ratio of heating compared to cooling can I expect out of our 43 to 46 degree water here in the winter? My curent system utilizes heat strips for the heater part.

Here are the units.

Webasto FCF Series ( Feel Cool Fast! ) Click on photos for larger image



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Old 16-03-2010, 16:27   #2
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HVAC for the boat

We have an Irwin 37CC and installed one 12,000btu unit in the main salon (way too big) and a 7,000 btu unit in the aft cabin (about right). They are both powered by a 4 kw Apolo generator under the aft bed (great insulator). By the way most HVAC units mfr are now owned by Dometic Corp so now so I would suggest buying local, during a boat show (boat show price) from a local rep. The have worked well for the 3 years we have had them. Make sure you buy everything they sell for the units as the teak grills are hard to find. Even the heat cycle worked well the winter in Mazxatlan as it got down to the low 50 this season. There is the usual delay for the compressor to get the coolant pressure up, something you wern't used to with a heating element. Enjoy
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Old 16-03-2010, 16:38   #3
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A rough "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 (below decks) to 17 (above decks, glazed) BTU/H for each cubic foot of interior conditioned volume.

A (theoretically) perfectly-sized air conditioner will run continuously during the hottest 2.5% summer design (outdoor) temperatures.

Don't buy an oversized unit. An over-sized unit short-cycles (turns on & off too rapidly), so doesn’t properly de-humidify*, and uses more energy (starting currents).

Make sure any ducting and grillework is sized large enough to allow low-velocity air distribution.
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Old 16-03-2010, 16:55   #4
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Well I really don't know how to accurately calculate that...if I take 41x13x6 then divide by 30% for tapers I get roughly 2240 cf x 14= 31,360.

So the 18,000 btu is going to be way shy...I would need one 16k and one 12k unit to be close and not be over sized....does this sound right?


Hollen:
Thanks for the input...so what do you think of the above calculations? or would you think better of a 12k in our larger saloon and a 9k in the aft stateroom?
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Old 16-03-2010, 17:22   #5
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Well I really don't know how to accurately calculate that...if I take 41x13x6 then divide by 30% for tapers I get roughly 2240 cf x 14= 31,360...
NO.
The "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 BTU/H for each cubic foot of interior conditioned volume. This includes inside living spaces only.

You need to get your "pro" to answer some of your specific questions in detail. If he cannot, he isn't.
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Old 16-03-2010, 17:28   #6
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That was interior cubic footage...or at least a best guesstimation with out actually setting up one foot increments inside the hull and measuring it.
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Old 17-03-2010, 13:04   #7
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OK Everybody... Let me word this another way.

What are you using and are you happy with the size?

Real world applications here...Like Hollen Offered

Thanks!
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Old 18-03-2010, 16:07   #8
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We have a CruiseAir stowaway unit of 18kBTU for our main salon and forward owners hut and it is enough in the tropics. Before that we had a 16kBTU version which was also enough (but same unit is 18 kBTU with lower power draw now).

cheers,
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Old 20-03-2010, 08:40   #9
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We have a 16,000 BTU King-Air (see KING-AIR - The Best in Marine Comfort ) installed in the main salon of our '86 Beneteau First 42 without ducting to the fore-cabin or port quarter cabin. Instead, we leave the cabins doors open and use a couple of small fans to move the air. This arrangement will keep our boat as cold as a meat-locker, even in the dead of summer here in southwest Florida, unless we keep an eye on the thermostat. The only problem we have is the amount of condensate, which can be a pain in the neck!

FWIW...
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Old 20-03-2010, 11:00   #10
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Regarding condensate...is it better to add a topside through hull and let it gravity flow out ( both units will sit next to the hull port and starboard) or is it better to let it run into the bilge where the pump will take care of it?

I can see a possible problem when healed over with the former method.
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Old 20-03-2010, 23:54   #11
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HVAC for a boat

I have almost the same boat and 12,000 was fine for the main salon, though I do have to turn the fan speed down a couple of notches as it blows way to hard. Honestly if I had to do it over I would get a 10,000. As for the aft cabin the 9,000 was as small as Dometic made them so that is what I got and it is fine. By the way, I think I said it before, Dometic bought out every one, even the Cruisair I boughts' manual says Dometic on it. As for the condensate I use an automatic bilge pump in both bilges and even if they fail there isn't that much to lower the water line over night. Just make sure you use check valves or at least vented loops for the pumps.
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Old 21-03-2010, 03:46   #12
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Quote:
Originally Posted by GordMay View Post
NO.
The "rule of thumb" for sizing marine air-conditioning (for use in sub-tropics) is between 14 BTU/H for each cubic foot of interior conditioned volume. This includes inside living spaces only.

You need to get your "pro" to answer some of your specific questions in detail. If he cannot, he isn't.
Sorry Gord, buy that does't do it for me, or most of us.
Reason is, it is pretty hard to size a volume of a typical boat, esp a CC one. There is just to many variables. Sure you can measure a little here, and a little there and add it all up, but I don't think that works well. If you are going with a "professional" to install it, then putting it on them might work, but then again, might not.
The answer is .... I don't know.
Here is what I did. I have a 40' cc endeavour that previously had a 16K unit in the middle of the boat and the PO lied thru his teeth saying it kept the boat cool. It did not. Not even close. Plus the unit tripped so much due to being poorly installed that it wasn't worth keeping. I replaced it with 2 marine turbo air vector units, now made by dometic. 10K for the aft cabin, with 1 4" duct in the salon, 1 6" duct in the aft cabin, and 1 4" duct in the aft head. and 1 12K unit in the vberth area with 1 4" duct to the foreward head, 1 4" and 1 6" duct to the main salon. So far in the 2 times I have used it, it worked very well. But hot days are not here yet.
I figured I could keep the aft cabin cool with minimal drain with a honda generator. Don't have one though. They are fed off one thru hull, and one march pump and 1 strainer, to a relay. The pump is on a seperate breaker.
All wire is over sized. I used the metal foil duct they supplied, but am not happy with it, and substituted the trident poly duct in parts of the install. If you can afford it, go for it, it is great stuff. I used both teak and plastic grills, and teak return grills. It has reverse cycle heat. I would have liked resistance heat, but the amp draw is high. The units are compact, light, relativly quiet and vibration is tolerable. Much better than the flagship unit it replaced.
Putting 6" ducts in a sailboat is a PITA. It takes up a lot of valuable locker space. cutting the holes was dificult. But it will more than make up for the endless hot texas summers. If I was going to be cruising full time not in marinas, I would have used the money on something else. But in the end it was a good investment.
Bob
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Old 21-03-2010, 04:44   #13
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Sorry Gord, buy that does't do it for me, or most of us.
Reason is, it is pretty hard to size a volume of a typical boat, esp a CC one. There is just to many variables. Sure you can measure a little here, and a little there and add it all up, but I don't think that works well. If you are going with a "professional" to install it, then putting it on them might work, but then again, might not.
The answer is .... I don't know...
The interior living space(s), which you’d want to air-condition, is/are a series of nearly rectilinear boxes. Furniture, such as cabinetry & settees provide nearly straight vertical & horizontal faces, and though one end of the compartment may be narrower than the other, you can take your measurement in the middle for an average width. Height is relatively constant, in each area.
Since you will have to install a standard sized A-C unit, which is unlikely to match your load calculation exactly, the Rule of Thumb should suffice in most cases.
It may not be perfect (it’s only a “rough Rule of Thumb”, per my post #3); but IMO it provides a better starting point than “I don’t know”.
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Old 21-03-2010, 06:27   #14
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I would not let the condense run into the bilge. We divert it to a gray water tank. I don't think I'd like another hole in the boat for this either. There are special "vacuum adapters" that you can install in the cooling circuit where you can connect the condensate drain to, which is what I would use when I didn't have a gray water system already.

About everything being one big Dometic company now. While they surely acquired a lot of competitors, this doesn't mean all these products are now same quality. I see some units that are the exact same one with different name labels on them but there are many units that are unique for that name brand.

cheers,
Nick.
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Old 21-03-2010, 18:00   #15
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I would not let the condense run into the bilge.
cheers,
Nick.
Why not Nick?
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