Imagine that you are just walking into the grocery store when your significant other calls and asks that you also pick up an avocado, but is insistent that it must have a small stone and maximum flesh. Normally the biggest challenge with avocados is finding the almost-ripe ones and then getting them home before they whisk through the ripe stage in transit and swiftly progress to the over-ripe stage just as you unpack them. But this stone-size issue poses a different challenge.
Now you're in the produce section and you find a choice of two offerings of avocado; regular and organic. Knowing the variety or origin is no help, (no Siri on your flip-phone and your Mum told you never to talk to strangers) and both are of the same size, color, and weight, with the organic version being higher priced. So, how would one determine which pile of avocados would have the highest probability of having the smallest stone?
If it were me I would take one from each pile, and then once back home secretly use a pin to pierce the skin to see which had the smallest stone. If the stone in that one is declared by your S.O. to be too large and you're accused of being an incompetent shopper, present the other and wager that the stone in that one would be even larger. That should be good for $10! (Warning! Trying the pin trick at the store will probably result in the manager demanding you purchase the entire display of avocados, as there's no way of knowing which ones you'd pricked with your germ-infested pin and then put back.) And because you had the savvy to buy two avocados, you now have more than enough flesh than was requested and the rest can go on the salad.
Choosing a marine air conditioning unit poses a similar conundrum.
How would one know which one is "best". They all blow cold or hot air and reduce humidity, but apart from obvious features of comparison like the display, physical dimensions, etc., how could we tell which one would actually perform better? Even if you take the avocado route and purchase one of each of the top two choices and install them in exactly the same way on a boat and operate them in the exact same conditions, there is no way to measure capacity.
There is no magic hand-held Btu meter, and blowers will almost certainly be of different flow rates, resulting in differing discharge air temperatures even if the cooling capacity were equal. Let's say the two units we are comparing are both rated at 16,000 Btu's. Does this mean that they will both "produce" 16,000 Btu's?
I suppose this is theoretically possible, but highly unlikely, as there is no control over the capacity ratings of marine units like there is for residential or commercial HVAC equipment. Since its inception back in the '50's, manufacturers of air conditioning for pleasure boats have been free to label their equipment with whatever Btu capacity takes their fancy, and that may be more at the whim of the marketing department than the engineering team.
The ABYC recommends that the rating of marine air conditioning units be based on whatever the compressor manufacturer says the Btu output of their compressor would be at certain test conditions. But note the key words; "based on". The compressor is just one component of an air conditioning unit, and the quality of the other components, plus the way the whole package is engineered and constructed can radically affect the final capacity.
Let's say we are considering two units from different manufacturers that have the exact same compressor installed. One is a very well known brand that boasts using only the highest grade components available and has a sticker price to match, and the other is a lower-cost item of a lesser-known brand where a lower standard of construction is evident.
Would the lower-cost manufacturer give their unit a Btu capacity rating that is lower than the published capacity rating of the compressor to compensate for the losses incurred where corners had been cut to reduce the price? Of course not! Would the well-known brand also have a lower capacity rating than the compressor but to lesser degree? Absolutely not, and why should either of them? As long as the published capacities of their marine air conditioning units are "based on" the compressor specs, they are totally in compliance and good to go.
Of course, there will always be some manufacturers that take things to extremes. A few years ago, one manufacturer new to the scene went to the trouble of making available what they said were their compressor manufacturer's data. This looked just too good to be true, and sure enough this data had almost certainly been altered with editing software and the capacity figures pumped up by an even 2,000 Btu's from the original! Now that's innovative thinking, and well worthy of an award.
For those of you in the market for a marine air conditioner there's no easy solution here, and no simple "avocado test". Even if those two little words "based on" were to be omitted from the ABYC recommendations, we would be no closer to knowing which unit would actually perform better, and we'd have only the manufacturer's word to go on.
Maybe the lower-cost unit would actually be the better performer. It's certainly possible.