HVAC and A/C sizing - Is it accurate?

Jonr,

 

For those so inclined, consider using a home window ac outright, or stripping one down and adapting it to the situation. Putting in something like Cruisair units, especially aftermarket, is a huge pain, with ductwork, wiring, filtration, under and above water throughhulls. You get anemic performance for an absurd marine price and if you ever need a part, the only way to get it is to have a certified technician come diagnose the problem and replace the part.

Of course such a rigged solution won't work for a lot of people, but for some it does.

 

I like the ductless AC systems. They are very flexible regarding installation and no musty ducts to worry about.

Here is an example. https://www.goductless.com/LG/LMU369HV-LMN096HVT-LMN096HVT-LMN186HVT-36000-BTU-17-SEER-Heat-Pump/19385.ac?catId=ductless-best-sellers

If you can find a spot for the condenser that is well protected and with some air flow it would probably last for years before it rusted out.

 

Ductwork issues should not be a reason to consider hydronics over package units or splits. There are several reasons why you might choose to adopt hydronics, but once you have, the system must be just as carefully designed as a ducted system (moreso usually). It should in no way be considered an escape hatch, and there is a bit more involved than just routing pipes though a smaller area than ducts (of which there may be several, and they must be insulated). The most common reason to go with hydronics is to take advantage of a better heat sink that can't be accessed any other way. A river that is 15 degrees colder than midday air can more than make up for the extra losses. Boats aren't likely to find a better heat sink close by than the one they are floating in. In a humid environment, hydronics will often benefit from a separate dehumidifcation system. They are lousy dehumidifiers. And you still require airhandlers for air quality control (air exchange, turbulence and comfort drafts).

 

Just to be clear, the input and outputs of a heat pump are independent in terms of air/water. You can have air in, hydronic (water out), the reverse or any other combination. And on the output side, you have a third transport option - refrigerant.

 

it is clear to see that there are many different methods of establishing the thermal capacity of the system.

However, I am seeing alot of suggestions for thermal loading's of rooms, which are often segregated into below deck, middle deck and upper deck values (http://www.dometic.com/enie/Interna...ct-an-AC-For-Your-Boat/Specifying-Guidelines/)

Although it is unclear as to how this has been derived?
What sort of thermal gains need to be considered and how are these typically calculated?

Secondly, although the use of psychometric charts is useful in deciding the enthalpy of a systems which needs to be removed or added. Who decides what the thermal comfort range is? Isn't this constantly changing?

Would be interested to hear your thoughts.

 

You decide what the comfort range is... what kind of temps will you set the thermostat at? What will the ambient outside air temp be? Size your system based on what you need and the area you will be cruising. Typical comfort levels used in commercial premesis is around 22-23degrees celcius, its a touch cold for my taste, i prefer around 23.5-24deg C. Choose an ambient temp for a worst case scenario of say 35deg C and 90% humidity, anything beyond that is quite rare for boat floating in the ocean.

The additional heat load primarily comes from the sun. So decks and or rooms that are exposed to the sun, have large windows etc will have more additional heat load. Same goes for lower decks that are near machinery spaces, heat load from the engines, gensets etc may effect other nearby living spaces.... Consider insulating the walls and floors etc improving the R value to reduce this added load. Windows can be double glazed and tinted with reflective coatings etc...

Your right its constantly changing, which is why a variable speed drive (inverter) type system is more efficient than a fixed speed system which starts and stops periodiclly via the thermostat in order to maintain target temperature. The system is sized by the peak capacity requirement, but it will run at reduced speed when the demand is lower. The start current draw is much higher than the run current, so constantly starting and stopping is less efficient, upto 30% is the published figure but it really does depend on the demand, if the system runs at 100% all the time, then the savings are much less.

 

Thermal comfort depends on sensible temperature, humidity, radiant heat uptake, air velocity, stratification, clothing, activity level, health, and acclimatization. These are all considered as a continuum. For instance, clothing is measured in units called "clo". Activity has a similar scale used for HVAC. They are generally grouped into two sets. One set measures the first five and formulas evaluate ambient conditions. The second group are evaluated to get your range of acceptable values for the first group.

As a practical matter, conditions are often measured with a thermometer inside a specified metal ball. The reading lumps sensible heat, air velocity, and radiant heat into a single reading. Over the years, formulas have been developed to tease out the the individual contributions so that the designer can build a predictable system. This gives you a much simpler curve - ball temp vs humidity - of isocomfort. Now you can play with how to efficiently hit your comfort zone target over the range of environmental and load conditions you are facing.

 

The dometic.com recommendations are a good example of overly simplistic methods. But it may work well enough (especially with inverter heat pumps) to not justify the expense of someone to determine more accurate numbers.

I'll reiterate the importance of air infiltration and humidity control. Both for comfort and mold reasons.