HVAC and A/C sizing - Is it accurate?

Discussion in 'OnBoard Electronics & Controls' started by chrisyk, May 28, 2013.

  1. chrisyk
    Joined: Dec 2012
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    chrisyk Junior Member

    Today's air conditioning market is focusing on efficiency as a means to reduce energy consumption.

    • But how do you size your system?
    • What is the best type of HVAC systems for my vessel?
    • Should I design for the worst case scenario or the most frequent scenario?
    • What impact does this have on my Gen sets?

    These are the questions that builders and designers are faced... but can be sometimes ill advised by the suppliers, who appart from wanting to sell the device to you, may want to be absolutely sure that all scenarios are catered for.

    I am asking this forum therefore, to consider best practice for A/C system sizing & what would be the best approach to selecting and installing the system?
     
  2. Murat124
    Joined: Oct 2013
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    Murat124 Junior Member

    it is necessary to calculate volumes to be cooled or heated for heat excanging capacity and to ask enquiry from a marine climatization systems sales specialist for example to you ( chris )
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Sizing an A/C or HAVC system, is the same in a condo or in a cabin on a boat. After an assumed thermal efficiency of the structure (how tight and how well insulated) it's really just a matter of the size of the air mass, that needs to be heated or cooled.

    Undersize or oversizing a system has obvious drawbacks and isn't recommended by anyone. The math is simple, do you have any specific questions about why this math is used or maybe it's effectiveness and a sizing route? What part of adding up the BTU variables are you having difficulty with? A typical approach would be a base BTU requirement for the space, with variables for the number of people, windows and other thermal sources/loses, etc., which gives you the BTU requirement.
     
  4. jonr
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    jonr Senior Member

    Solar gain will have a big effect and generally you design for something just short of worse case loads. Consider an inverter driven compressor - this will greatly reduce the startup load and the size of the generator needed - and it will improve efficiency.
     
  5. El_Guero

    El_Guero Previous Member

    You need to know your insulation (R-value), you need to know your volume.

    And you need two things seldom discussed. What humidity & air cleaning will your AC unit carry. If you need either of those, usually for allergy reasons, you will need to upsize your system a little.

    You need to size your system, and your R-value of your insulation so that, on a very hot day in your area of operation, your system runs about 90% to 95% of the time.

    That is how you get the most efficient unit, and that effects longevity of your unit. The more often a unit cycles on and off, the shorter the life expectancy (MTBF).

    If you desire more cooling, instead of using a bigger main system, add a smaller unit to add additional cooling, maybe to the main cabin, or 'master suite.'

    And get a higher SEER rating, I think Europe uses a different rating. The higher the SEER, the more electrically efficient your unit is.

    wayne
     
  6. FAST FRED
    Joined: Oct 2002
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    The amount of insulation (if any) and the usual huge size of windows compared to a dirt house make calculations very unreliable.

    IF you can wait , use an electric heater or two on a really cold day , and measure the outside and internal temperatures after a cold night.

    This will give a great look at how the boat really behaves when the insulation is working.

    We have used RV rooftop ( on a lobster boat) air cond . 13,000 BTU or so of cooling.Feeds thru std RV 14x14 hatch.

    Most reverse cycle units stop when the air or water is about 40F.

    The new air Mitsubishi mini splits work well down to -30F so should be fine on any fun boat. 400% to 500% more heat per KW than a resistance wire..

    Weather they will last with boat motion and sea air is still unknown.

    But they are the current future of HVAC
     
  7. michael pierzga
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    michael pierzga Senior Member

    Boats are unique in that they must conform to the available power supply on the dock.

    Consider this when you conceive an efficient system
    Several small ac units rather than a central unit a prefered.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A central unit will have a lower total draw then multiple smaller units. As an example two 5,000 BTU units will have a minimum of 4 - 5 amp load (each), once up to temperature with startup load over twice this figure. A 10,000 BTU central unit will have a lower startup load and about 7 - 8 amps once up to temperature. Also stand alone, bulkhead or roof mounts aren't well insulated, so there's a lot more thermal lose, then with a central unit, plus a central unit makes much less noise and is much more efficient, because it doesn't have to be a "package" setup, like a self contained unit is.

    Yes, to a degree you have to work within the capacity of what's available at a dock, but small units (10 k or less) will be able to run on a 15 - 20 amp 120 VAC, which every dock will have. Larger units will require 240 VAC and most docks usually have a 30 amp 240 available, which is more then enough for a substantial BTU count on this modest sized cabin.
     
  9. jonr
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    jonr Senior Member

    When it comes to A/C, humidity control is critical. And humidity is driven primarily by your air leakage rate. So it's worth spending some time with a fan pressurizing the cabin and a smoke pen to find and plug the air leaks.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    As I mentioned in my first post, you do need to perform the preliminary stuff and determining how "tight" an area is is right at the top of the list.
     
  11. jonr
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    jonr Senior Member

    That's the problem, people "assume", not "determine" and then skip the remediation step and go straight to selecting a system.

     

  12. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Good point, I should have said determined, which is what I meant, with assumed, in as much as there will be things you have to check. By assumed I simply implied you will have to account for loses or make modifications to reduce them.
     
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