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Old 09-02-2009, 09:50   #1
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Need help sizing a boat heater

I have a Searunner trimaran, 40 feet long, in which I'd like to install a diesel heater for liveaboard conditions in mild to chilly (above freezing) temperatures for several months a year (San Diego in winter, Puget Sound in late summer). The boat is effectively divided into two cabins by the center cockpit. The engine room, where I would mount the heater/blower unit is located below the center cockpit. I would like to duct the heat aft, into an approximately 500 cubic foot living space, and forward into about 200 cubic feet of cabin/head area. If the unit had a water heater capability, that would be dandy, as well.

Given that a heater needs to be correctly sized for an appropriate application, so as to reduce or avoid carbon buildups and excessive maintenance, I'm looking for some advice on brand and capacity from folks who actually have them and use them. I have previously used a Dickenson solid fuel stove for heating the aft cabin. It was adequate for the heating needs of this space, but the forward cabin and head could not be heated by the air flowing through the crawlspace connecting the two cabins beneath the cockpit. Also, solid fuel is too demanding of time for replenishing and cleaning. I'd like something a bit more self-tending. Since this is a multihull, weight is an issue which will affect my decision regarding size and water heating capacity. I have a variety of alternate installation sites, should other factors (venting, physical volume of the unit) need consideration. Any ideas?
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Old 09-02-2009, 10:08   #2
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One word- eberspacher :-)
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Old 09-02-2009, 11:10   #3
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The computation requires that you know the volume of the space to be heated as well as the heat loss factor of the perimeter.

I have and Espar Airtronic 4 which does a 36 - 3 cabin boat well in the frigid NYC area.

My suggestion would be to get two smaller units, perhaps the airtronic 4 or the next size up and have each heat one hull and a supply air to the center common area.
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Old 09-02-2009, 11:46   #4
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You could go to the 2 air heaters as suggested or a Boiler type heating system with different zones.

Check out the "Hurricane Heaters" and "Kabola Boilers" as well as well as "Espar" and "Ebespacher"(sp?).

Nortec Marine Inc.

ITR Diesel Heating Systems
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Old 09-02-2009, 14:17   #5
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The boilers have an advantage in this installation since it is easier to plumb hot water than hot air. It also lets you adjust the air flow at the point of heating. These sytems cost a bit more up front but require less service and easier installation.
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Old 09-02-2009, 15:34   #6
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see Webasto Marine website, they have a special table for water heater
correct sizing in their catalogue:

http://www.webasto-marine.com/filead...Katalog_08.pdf
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Old 09-02-2009, 17:23   #7
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Put an electric space heater in it for an afternoon.

I'm an engineer and I do heater sizings all the time, and the safest way is a test; put a known heater in it an see how it heats up. Then go up or down from there. You will have to convert from Watts to BTUs by multiplying by 3.4121 (1500 watts = 5118 BTUs). Also, assume the efficiency of a fired heater with be less - perhaps 75% is safe - because some heat goes up the stack. Electric heaters (not counting reverse cycle) are all 100% efficient.
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Old 09-02-2009, 17:37   #8
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ignore the catalogue ratings for the west coast.

twice now I've had a diesel heater installed that's considerably smaller than the unit the installer recommended, and twice now the system has been able to heat the boat quickly and efficiently. (Living aboard in Northern California) The installation guides seem to be thinking that you're going to use the heater in Maine or Wisconsin where all that white stuff falls out of the sky, but what you're going to need around these parts is a whole lot less.
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Old 09-02-2009, 22:24   #9
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Here's another way to get the approximate size to look for.

Boat Mechanical Systems Handbook - Google Book Search

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Old 10-02-2009, 09:25   #10
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Thanks, folks! The electric heater standard was used by a vendor, helping me to confirm the size that I would need for my own boat. My trimaran is rather narrow (about 6 feet) at hip level, then widens immediatly to about twelve feet from cabinside to cabinside. I have been using a 1500 watt heater, with auto thermostat, for some time now, and it has worked very well, so well that I didn't want to leave the dock to go to places without an extension cord.

As of the moment, my best candidate for a unit is the Webasto TLS 17. Its output is exactly in the range of my volume and Btu needs, meaning it won't have to run constantly, yet when it runs, it can operate at near-full efficiency to reduce problems of sooting. After a couple years, I will have to inspect the burner tubes and fan motor for wear, but this isn't rocket science. The unit, itself, is tiny (about 4" wide, 8" long, and 6" high), heats water, sips electricity and diesel fuel, weighs about 6-7 lbs., and can be installed easily in my engine compartment, ducted fore and aft, and uses piezo ignition. The vendor is willing to offer a good price (lower than what I've been able to find elsewere). I will be investigating the alternatives, but so far, this seems the best "fit" I've been able to achieve. I want to spend all my time in temperate waters, but still be comfortable when a cold snap hits (I've been in Hawaii when it dropped into the very low fifties-yuck!). Again, thanks for your suggestions and research. This is the best feature of CF, the range and depth of experience, and the willingness to share information, of its membership.
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Old 10-02-2009, 10:15   #11
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The Dave Gerr book (Boat Mechanical Systems Handbook) to which cal40John linked < Boat Mechanical Systems Handbook - Google Book Search >, is an excellent resource.
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Old 14-02-2009, 09:52   #12
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We also supplemented heat by installing a Heater Craft 5H, which puts out 28,000 BTU. They make larger models too, and it has a 3 speed fan. It works when the engine is on and is plumbed in a series with the hot water heater. Somewhat like a car heater. Not expensive, and it warms up the cabin when motoring into an anchorage.
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