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  1. #1

    Marine HVAC question...

    Does anyone know a source for marine HVAC questions? My googling had got me no where and general boat forums are full of non-pros who argue over the data I've presented...I'm hoping someone can simply explain water cooling expectations as it pertains to my need with a new Webasto 16,000 BTU self contained AC. These are simple drop in and go units...

    Hopefully this doesn't violate 10 different rules of this forum, but I'm having trouble define how out-of-bounds this post is...

    130 sq foot interior of boat. (31ft Tiara Flybridge, fiberglass for those who care)
    Water temp in lake erie marine 80-82F
    Outside Ambient 88F
    Humidity 58%
    AC intake temp 78F

    My boat previous had a 30 year old 12,000 BTU R22 system that worked but couldn't cool the boat (plus it was noisy) so we upgraded to a new 16,000 BTU unit, again for approx. 130 sq ft.

    My first unit from the manufacture was defective and I was lucky enough to have the company (Webasto) in the area for some training so they came down to figure out what was wrong after I spent 12 hours troubleshooting it (it was blowing 30 amp breaker). They installed the 2nd unit.

    The 2nd unit was installed and bench tested prior to install and was running when they left and it is still running. The company tech gave me a list of suggestions including upgrade my water pump as the old unit was only putting out 200GPH but they recommend 400 GPH. He said I could run it with no damage but on a hottest days it was thrown the high side sensor and log a code and shut down...which is never did while I waited for the new high capacity pump to arrive. This included durin 101F weather last week.

    I ordered my new water pump that was rated for 600GPH at 12ft of head. We'll it's only putting out 200GPH like my old pump...before I upgrade the water inlet and outlet to the pump manufactures specs although their specs exceed the inlet/outlet side of the AC unit...here is my question, finally:

    Will I see a dramatic cooling effect from having an increase (hopefully double) flow in cooling water considering the specs I gave at the top of this email? I've got one guy HVAC buddy telling me "no" and others who are not professionals telling me yes. What do the pros here think?

    I was told by the install tech that this unit may be to big for my boat and may short cycle...but the fact is that it's never shut off once in two weeks and my boat at 78F interior ambient on a moderate day (again specs above) is not acceptable to me.

    So I really don't know who to trust and before I start calling in pros I need to get input that water cooling efficiency (temp and flow) can have a dramatic improvement on cooling from the unit.

    Thanks for your patience.

  2. #2
    Join Date
    Jul 2006
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    On any water source unit you have to flow the correct amount of water through the coax for proper operation

  3. #3
    Join Date
    Mar 2005
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    Suppy NC
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    water flo is important
    if the new pump is 600 gph you may need to up size the intake piping to it and upsize the outlet piping
    you realy need to find the exact water flo you need to run the unit to proper levels

    00 gph may be to much for your unit and water flo is to high to pick up enough heat

    also make sure you tubing is clear and the through to hule port is clear

  4. #4
    Quote Originally Posted by tinknocker service tech View Post
    water flo is important
    if the new pump is 600 gph you may need to up size the intake piping to it and upsize the outlet piping
    you realy need to find the exact water flo you need to run the unit to proper levels

    00 gph may be to much for your unit and water flo is to high to pick up enough heat

    also make sure you tubing is clear and the through to hule port is clear
    I'm thinking the larger in/out piping is the next step, but if the inlet/outlet on the AC unit is only 5/8 and the pump calls for 1" in and 3/4" out...it's going to be restricted by the diameter where the hose attaches to the AC, correct?

    As stated the AC unit requires 400 GPH, but I've also caught the engineers of the unit over spec'ing several other aspects of their requirements. In particular while troubleshooting the first unit (blowing 30amp breaker) they told me that I needed to put a 40amp on a 30amp circuit! (yep that's what they said, their new requirement!). Although the tech who installed said they test them on the bench on 20amps all day, but I digress.

    So you are stating that increasing my water flow will have a major impact on getting this unit to cool?

    I do have to reset the through hull fitting as the previous genius install put the shut off vale in a position where it hits the string and doesn't close all the way!

  5. #5
    Quote Originally Posted by PatSea View Post
    As a fellow boater the only marine related AC forum that I have come across is marine-ac.com. It is moderated by a marine ac professional.
    I saw that in my travels but the owner/moderator has died and nobody is maintaining it. Hard to believe there are few recourses for such needs.

    Get straight answers from techs, engineers, installers, ect has been a nightmare. One calculation says my unit is big enough, another says not...one pro says my unit is big enough...another says no.

  6. #6
    Join Date
    Jan 2009
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    Keokuk, IA
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    5,520
    Are you sure you're even getting 200GPH out of the old pump? Do you know what the temprature rise is supposed to be on the cooling water? At 200GPH and 1600BTU's you're at a 10F delta T.

    Fomr a quick google search it looks like they only call for 186GPH for the 16,000 BTU unit. But I don't know that specific model you have.

    It might even show minimum flow rate in l/min on the namplate on the unit.

    It shoudn't draw anywhere near 30 amps when running even with high head pressure. Only when starting... unless your voltage is low for some reason. I assuem your on shore power so it should be fine.

    But I'm not an expert on these, just looking through data on the internet.

  7. #7
    The manufacture (webasto) says they spec a requirement of 400GPH for the 16K unit. We are now testing the 3rd pump going up in GPH each time. The current one is rated at 1200GPH. I estimate a 12-14" head and the graph shows it should be putting out 13GPM but in fact it's only putting out 5GPM!!! So we are looking at intake/outake restrictions for the pump to get the spec up.

    March wants a 3/4" inlet and 1/2" outlet. We are fine on the outlet requirements but have a 5/8" inlet. I can change the pump/inlet (boat thru hull) until the off season but I doubt that increasing the intake side 1/8 of an inch will reduce flow by a factor of three!! So we are also considering swapping out the filter/strainer system as it may be reducing the flow to much too. Very frustrating because 99% of the guys I've talked to said this should have been a simple drop in swap...and that worrying about all this pump flow stuff is a bit of a goose chase. But again I have todo it to get it up to specs before Webasto will help any more.

    But to answer you question our old (OEM) pump was putting out 200GPH with a 5gallon bucket/time test. The new march pump rated for 600GPH was only putting out 225GPH and now the 1200GPH pump is putting out 360GPH. Other then increasing the hose size and filter on the intake side this offseason I'm done with the pump project!

    An HVAC buddy (commercial and good with geo, ice machines, ect) says if I'm getting 360GPH out of the pump then getting it to 400GPH (required) is going to have no serious improvement over cooling efficiency in 75-85F lake waters of getting the unit to cool more. He said engineers always over spec the requirements and he doubts anyone is moving that kind of water thru a 16,000 BTU unit. He feels the 5/8" inlet/outlet on AC unit almost ensures I won't get much over 400GPH even knowing March only requires 1/2" on the outlet side of the pump.

    As far as the delta T on the water side I have no clue, yet.

    It runs at 13amps continuous and the start load spikes about 47amps. The tech who installed says they bench test them on a 15 or 20amp circuit so he had no idea why the engineers insist (now) on a 40amp breaker when 90% of the marinas and boats only have 30amp service! The reason they told me I needed a 40amp breaker was because that is their new spec and the first unit I had was blowing the 30amp "main". They found out the unit was mechanically defective and thus no breaker was going to fix the issue. So I put the original 20 amp breaker back in and it's running as expected.

    Most of the formulas I've found for computing BTU for my sq ft and boat size/type say I need approx. a 12-14K BTU unit...but one formula in particular says 18K btu because they say to add 20% to the calculation due to excessive humidity area (Ohio) and another 20% when water temps are above 75F...also in Ohio. Therefor based on this crazy summer our unit simply cannot keep up with the heat gain is my thought. The only other option is a 24K BTU unit (115v) that would not fit in the space and would pull to my current. I know boats only a couple feet longer then mine often have multiple units but I also figured that was so they could dial in comfort levels for different zones.

    So I thinking it cannot get much better then this. I think the unit is in fact working to spec and I'm simply a victim of a hot summer and boat heat gain. We are considering rerouting some ducting as some testing (of a tempoary duct) showed some improvements in the main cabin.

    I brought my inferred gun and was getting 47F outlet temp (which is located 3 feet from the blower) with a 76F inlet temp on a 60% humidity day. So I think that type of drop is more then reasonable, would you agree?

    Thanks for reading through this...it's been a long couple of weeks trying to fine tune and figure out my variables.
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  8. #8
    Join Date
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    It's not just the inlet and outlet sizes at the heat exchanger that are of concern. If the pipe sizes are too small for the pump, the water flow is restricted along the entire path from intake to overboard. You won't move the right GPM.

    Somewhere that unit, as new as it is, should specify what design GPM is, and then all discussion on how to get there should revolve around that spec.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
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    That temprature split looks pretty good... although I don't know what CFM it runs at to know for sure. But even if only 450CFM, it seems pretty good with that humidity level. But it still might be running higher head pressures than designed and causing high RLA

  10. #10
    Join Date
    Aug 2003
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    One thing not well understood is the importance of mass flow of a liquid through a heat exchanger for proper transfer of heat. Lake water temps notwithstanding, you need the OEM spec of GPM flow through that heat exchanger to assure the system will operate as it is designed to do under its given design conditions.

    If the condenser had the proper flow the system probably would not have tripped on high head pressure, nor would it have tripped its breaker. Sounds to me from several angles this machine is running too high of a head pressure due to inadequate condenser water flow.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #11
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    PatSea, this is the Ask Our Pro's forum, and only Pro members that have been vetted by the AOPC may post advise here. Please apply to the AOPC today, thank you.

    You can find the rules for posting and qualifications here.

    Your post has been deleted.
    Further infractions may result in loss of posting privileges.
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  12. #12
    Quote Originally Posted by Shophound View Post
    One thing not well understood is the importance of mass flow of a liquid through a heat exchanger for proper transfer of heat. Lake water temps notwithstanding, you need the OEM spec of GPM flow through that heat exchanger to assure the system will operate as it is designed to do under its given design conditions.

    If the condenser had the proper flow the system probably would not have tripped on high head pressure, nor would it have tripped its breaker. Sounds to me from several angles this machine is running too high of a head pressure due to inadequate condenser water flow.
    ShopHound,

    Thanks for your input and supporting the thought that flow should not be under estimated, I'm sure it was a concern in the 97F weather we had a few weeks ago. At this point the flow is at 75% of what the spec requirements are (per the tech) and although this data is not covered in their install manual it's hard to define if this is a minimum requirement or just a requirement. Either way we'll continue to tweak the system to figure out why the March pump is struggling to get the advertised flow.

    As far as you comments on head pressure I must have not been clear of your connected some dots that are not there...

    The breaker was determined (by the Webasto) tech to be due to a mechanic failure of the new unit...it never started once. He stated that if flow was low it would trip the high side sensor and throw a code and shut off the system...which is never did. I would not argue that the system needed at the time more flow but he was sure it would trip the high side switch and it never had.

    This weekend we will remove the inline flow strainer/filter to increase flow and take measurements again to get the system to 400GPH...which would still be 1/3 to 1/2 March claims their pump should do!

  13. #13
    Quote Originally Posted by motoguy128 View Post
    That temprature split looks pretty good... although I don't know what CFM it runs at to know for sure. But even if only 450CFM, it seems pretty good with that humidity level. But it still might be running higher head pressures than designed and causing high RLA
    I did find the spec and it's rated at 505CFM.

    So we'll continue to increase flow and accept that the recent high temps and sun are causing a lot of heat gain.

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