Let's Design a Custom HVAC System

Discussion in 'Boatbuilding' started by CatBuilder, Feb 19, 2012.

  1. CatBuilder

    CatBuilder Previous Member

    Whoops! Embarrassed! :( Sorry. I thought you were joking.

    But the reasons above are important. If I were traveling myself and cruising, I wouldn't even install either heat or air conditioning. A spray bottle and a rag by the bed is fine when too hot (as is sailing North) and you just sail South when too cold.

    However, I am responsible for the comfort of some people spending well... I won't even say how much it costs to charter this kind of boat. Suffice to say they get everything they want. :D

    Thank you very *very* much for contacting your son and friend the key points for them are:

    1) Needs to heat and cool the area described above a couple posts back.

    2) Needs to operate off grid

    3) Total systems (from power generation to heating and cooling) needs to be as light weight as possible.

    4) System needs to be efficient on fuel to keep me from going broke running it 24/7 for guests.

    My dream would be to have a diesel GAHP unit (your son will know these, I'd think) that does 50,000 BTU and weighs a coupe hundred lbs. I've never seen anything like that. If I weren't building a boat, I'd take a year and create one.

    The water temperature coming in the intake for the condenser will run from 40F to 90F on average. Though, my intake is less than 2' down, not 20'.
  2. mydauphin
    Joined: Apr 2007
    Posts: 2,164
    Likes: 53, Points: 48, Legacy Rep: 575
    Location: Florida

    mydauphin Senior Member

    Dude, 6 tons of AC is not going to be easy to power on any boat. As far as engine hot water, I personally use solar 90% of the year, I have some pipes running in a box, works fine. Other part of year have electric 110 water heater but it is setup for shorepower. But generator will make all the hot water I could image on anchor, and I just run it a little to charge batteries, and to make sure it works anyway. It is a very efficient way to use that extra heat anyway. Look into Diesel heaters, they work really really good. When you convert fuel to electricity and then back to heat you lose half the efficiency no matter what you do. As far as AC, I find a well ventilated(designed) boat will be comfortable in everything but the hottest, windless environments. You can always take a cold shower. The idea of living on a boat to try to make it like a penthouse with all that AC takes all the fun out of it. But I said well designed and ventilate boat, not that many windows that let heat in, and good ventilation flow fore to aft.
  3. rasorinc
    Joined: Nov 2007
    Posts: 1,854
    Likes: 71, Points: 48, Legacy Rep: 896
    Location: OREGON

    rasorinc Senior Member

    In the summer of 09, I was living in Oregon at the foothills of the Cascade Mts. and we had a heat wave for 1 week of 106* heat. People in Portland almost died as no air in commercial building or schools. I got tired of hosing myself down every 20 minutes so I bought some dry ice, put some if the fridge to save and a block of it on a plate on my desk with a fan behind it. It worked..............While I did mean it as a joke, it did work and cooled off my living room. Oregon cools off at night, thank God. Also, low humidity
  4. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design


    I have modeled your boat according to dimensional data you have given in the post #12. I've done the calculations with all the worst-case numbers and hypothesis I could think of - in terms of outside / inside temperatures, wall insulating properties, windows area at the bridge salon, all walls facing north in the winter, south in the summer, 25% power margin etc. I am including a pdf with a summary of results and can give you a detailed printout of the calculation (done with a commercial HVAC-sizing software) if necessary. It is not based on some arbitrary per-volume factors but on detailed analysis of heat losses through transmission, radiation and ventilation of your boats' shell, plus the heat developed inside the boat, by occupants and by electric appliances.

    Conclusion: your AC plant of 2x36000 btu/h (22 kW) is an overkill by a factor of 2 in this worst-case scenario. It means that it will be an overkill by a factor of 3-4 for the rest of the year (95% of the time), when temperatures are much less extreme.

    By choosing to go that way, you will cancel any advantage of reducing the energy conversion cascade which seems to bother you so much. For example, when an AC plant works at 50% of the maximum load, it's efficiency (measured through overall COP) drops on average by 10-12% when compared to a plant which is correctly sized. When it works at 30% partial load, the efficiency will be 16-18% lower. These losses are to be added to the losses of your Kubota engine working at partial loads.

    At this point, you'd imho be better off with a commercially available electric heat-pump system. Besides getting the installation job done in just few hours of work, you will get a certified system covered by a warranty, and a reliable and tested electronic control of the system.

    This is a first issue you have to face calmly and rationally before getting to the operative (on-board) side of the job.


    Attached Files:

  5. groper
    Joined: Jun 2011
    Posts: 2,467
    Likes: 123, Points: 73, Legacy Rep: 693
    Location: australia

    groper Senior Member

    before daquiri posted the above, i was also thinking the 2x36000BTU`s sounded a bit extreme... you have a well insulated boat cat, i cant imagine needing that much power...

    In any case, i would still plan on having a bit more than calculated...

    Have you considered a multi evaporator system to eliminate the ducting? - ducting may well be the best method however, simple butterflies on damper motors in the duct to control the climate in each part of the boat... id imagine your would only need 100mm flexi duct...

    How will you handle the different load requirements, ie do these compressors simply clutch in and clutch out or function cyclicly or can you setup a variable speed system for them?

    The problem with commercial systems, is they are so darn expensive!!!
  6. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    When calculations are done under the most extreme assumptions, and then a margin of 25% on top of it is added (like it was), anything more than that is a pure waste of interior space, money and fuel imho. But the last word will be CatBuilder's, of course - he is the owner of the boat, after all.
  7. CatBuilder

    CatBuilder Previous Member

    That was an AMAZING analysis, Daiquiri. Thank you so much for taking the time to put the parameters into the software.

    I see the number you came up with is 14,800 Watts per hour. If you convert 14,800 Watts per hour to BTU per hour, we arrived at nearly the exact same number...

    I got 48,048 BTU ( and you got 50,540 BTU (14,800 watts). The only difference in our calculations was that your enclosed volume number was ever so slightly smaller than mine, but your software suggested more BTUs than I had calculated.

    You accounted for the thermal properties of the Corecell, enclosed volume and windows and came up with that? I'm absolutely amazed. We are within 2500 BTUs (732 Watts) of each other. That's a good thing, I think. :)

    On a side note, I see you put in 34C as an outdoor temperature. I need a system that can handle 40C, which is much more common these days than I remember. I wouldn't want to be short on those days, which would be the worst time to be short. -7C is actually even colder than I would plan to operate in, but you never know, so that would be a good assumption for worst case winter heating. Chartering in -7C will never happen, but we may move the boat down to different latitudes in those conditions at times.

    But, I do understand how to size AC units so as they run well. I do know what you mean, that an oversize unit is terrible. It doesn't run often - and because of that, it ends up not taking enough humidity out of the air. You feel damp and gross in that situation. Also, with that case, the diesel will run even when the compressor is not doing anything during the frequent off cycle of an oversize system. I understand what you are saying and I understand the COP. Makes perfect sense.

    This is why I started the thread. To go over the numbers and let the numbers guide a solution.

    Now, I am perfectly wiling to entertain the idea of a commercially available heat pump system, but I have not found one that works for my weight requirements. Specifically, I have not found one that is:

    *Sea water cooled
    *Requires energy efficient and light weight generator (performance catamaran)
    *Has some kind of evaporator I can use to get cooling (and heating) to the entire boat in the mid day sun , and/or individual staterooms in the evenings.

    I am more than willing to discuss changes in the system, I just need to make sure we don't end up with a thread that is not focused on solving the problem. Also, I want to make sure that it is understood that weight and efficiency are of paramount importance, just after being able to cool and heat the boat.

    Lastly, I was planning to have the Kubota pull triple duty - run the AC compressor(s), run a high pressure pump for a watermaker and run an alternator to keep up with dwindling batteries during charters. The boat normally will run 100% from solar. So, there is a different type of efficiency cascade at work... one where a single, very fuel efficient machine will do everything for house requirements.

    PS: In general, remember that a 36,000 BTU automotive style compressor will only deliver that amount of cooling power when it is spun up at full RPMs. See this chart:

    Attached Files:

  8. Boston

    Boston Previous Member

    yup I'm guilty, I saw that you were thinking radiant heat and cooling but was just trying to suggest a simple system based of existing and redilly available stuff.

    if you want to go heat exchanger rout then I'd suggest a soda machine type system with an insulated piping to run the cooled fluid around, same as the box truck systems but designed for restaurants soda machines and very compact.

  9. CatBuilder

    CatBuilder Previous Member

    This is exactly the idea... simple butterflies on dampers with an "always pressurized" duct. It worked well in my last catamaran and they use this on cruise ships (so I have heard). Everyone will get heat or AC, but you can just close the vent if you are not interested in it. or... you can partially close the vent. It is up to the individual in the stateroom.

    We have 4 staterooms, a galley and a large bridgedeck area. Pumping anything but air to all of these places would be quite problematic, especially when it comes to individually regulating each stateroom.

    Very good question about the compressor pumps. They do clutch in and out, just like a car. That could mean I need a governor on the Kubota to maintain RPMs despite changing loads. That's another good thing to think about that I had not been looking closely at. Thank you.
  10. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Yes, Corecell properties were accounted for, and a 360° glass windows at the bridge. I have redone calculations with 40 °C outside temp. and 70% humidity. Please find it enclosed below.

    Please take note one more time that the assumptions behind these calcs are imho very unfavorable and will lead you to an oversized system. If I were you I wouldn't add anything else on top of these numbers. Consider them as absolute maximums which will be very unlikely to be encountered and choose your system accordingly.

    If you have a GA of your boat, with rooms to be air-conditioned, perhaps you could post it here or send it to me via e-mail. I am pretty sure an off-the shelf solution can be found. I am still concerned about the reliability and safety of a DIY AC plant, and the issues related to the electronic control of the whole system.

    Time to go to bed now. :)


    Attached Files:

  11. CatBuilder

    CatBuilder Previous Member

    You are not even listening. I will absolutely not make someone paying for a vacation take a shower to cool off. That's a great way to lose customers. I assure you, an uncomfortable charter guest will take a lot more fun out of boats than any amount of aggravation installing a system can create.

    I think we did misunderstand each other about the heat thing though. I thought you were talking about taking the coolant from the engine and piping it through the boat as heat. That's great for making hot water, and I do plan to do that with this mini Kubota, but the same type of thing is not efficient at all for heating a sailing boat.

    Now, I do agree with you... just a little on the diesel heater, but I am not quite sold. I have not run the numbers, but I think a heat pump is far more efficient than even a diesel heater.
  12. CatBuilder

    CatBuilder Previous Member

    Yes you are up late!! I'm getting ready to go to bed and I'm probably 6 or 7 hours behind you. :)

    Thank you again for taking the time to model the boat. It is very much appreciated.

    The simple thing to do would be to just size the compressors (and system) appropriately. I will scale down the larger compressors to a pair of smaller ones. No problems there. They come in all sorts of sizes for car air conditioners.

    Now, are there any off the shelf solutions anyone can see? This is what I do not see, given efficiency and weight parameters.
  13. CatBuilder

    CatBuilder Previous Member

    Ok, I will make the GA tomorrow or at worst case, Wednesday (huge lamination job coming up tomorrow).

    I would be happy with an off the shelf solution, actually. I have just not found one that fits with everything desired.

    There are some things I can share to help allay your fears about the reliability and safety:

    1) Control systems will be dead simple. Temperature control kicks the compressor on and off. Automobile compressors use an electronic clutch to engage or disengage from the belt. Flow through the evaporator is controlled 100% perfect via an expansion valve that measures the superheat coming out the other side of the evaporator and adjusts flow rates accordingly. Use of a suction line accumulator keeps any bit of flow out of the compressor. Of course a filter/dryer, etc.. I have built marine refrigeration systems, so I am quite familiar with how to do this stuff. It is in the reversing of the cycle that I am having some trouble understanding how to build it.

    2) Safety. Not sure where the issue would be there. All lines would be copper flare fitting, as standard on AC systems. Engine is marinized, compressors are standard issue and are used in millions of cars as well as engine driven refrigeration systems on boats. What safety concerns are there? Am I missing one?
  14. michael pierzga
    Joined: Dec 2008
    Posts: 4,862
    Likes: 115, Points: 0, Legacy Rep: 1180
    Location: spain

    michael pierzga Senior Member

    What control system will you use to regulate the temp in different regions of the boat ? Manuel...its too cold in here, close the vent or automatic user programed ?

    How will you recycle air ?

  15. CatBuilder

    CatBuilder Previous Member

    Butterfly in the vent in each cabin and elsewhere. Close the vent if you want less heat or ac. Just like my last catamaran. Works great. Returns are always on the bridgedeck, near the evaporator. Pretty common and straightforward installation.

    The vents look like this, but have a small knob that allows you to adjust the flow rate. Worked well on my last catamaran.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.