poor air cond performance in guest staterrom

[MikeP]

The guest stateroom ac in our 53 MY has always been poor performance-wise. I had an ac guy on board today to check all the systems/recharge as necessary. But the guest stateroom ac problem was found. And it was Hatteras' fault! When the tech checked pressure, he found it was much too high on heat and too low in cool. So on heat it acted as if it was overcharged and on cool as if it was undercharged.

The ducting in that stateroom runs from the evaporator up the inside of the closet wall, to a box mounted to the backside of the forward wall in the stateroom. A register is mounted to the hole in the wall. Turns out, the register, which protrudes well into the airbox, dramatically obstructs the flow of air through the evaporator. That, in turn drives the pressure up on the compressor, causing the unit to cycle off. The tech showed me the gauge set and how the pressure dropped dramatically to below normal in heating/cooling when the register was removed and that the unit needed some additional coolant (and the register off) to work properly, which it does now.

The solution is a surface mount register and a larger opening in the wall. Interesting stuff, especially knowing that it was this way from the factory. Of course, I've also learned that not all 53's are set up the same way...

 

[Angela]

Very intersting, Mike. That same stateroom is problematic for me, too. Fortunately, it's not used much. That stateroom on Sanctuary cycles too frequently. I will look into the solution you mentioned and see if it does anything to help my situation. I'm sure the same folks who built your chase for that room also built ours with the same design and parts.

On the topic of air conditioning and "it's Hatteras's fault!"... something else you may want to check if you have continued problems - we have discovered that the entire lower level's air conditioning, as designed by Hatteras, has 29,000 BTU worth of fan coils calling on a 24,000 BTU condensing unit. In short, unless we manually manage the load by turning off a stateroom, the system can't work properly because the math doesn't work - the condensing unit is undersized to service the call of all of the fan coils. I use that guest stateroom you discussed as the one I turn off most of the time in order to push the cooling power to the other units. And the real kick in the pants is that I got a quote today to overhaul the entire air conditioning system for the whole boat so that all fan coils can run with properly sized condensing units, and it's just over $30,000 in parts and equipment alone. Granted, that's for a dual chiller system. Hatteras doesn't screw up often, but they screwed this up.

 

[REBrueckner]

Angela,

I'm not an air conditioning pro by any means, but in general I would NOT have thought "....29,000 BTU worth of fan coils calling on a 24,000 BTU condensing unit..." would necessarily be a problem. Sounds more like you need more air conditioning compressor capacity....closer to 29K BTU....in other words, the compressor is a bit small, not the evaporators too large...

It is also possible that your system is now a bit undersized because the original refrigerant has been changed out and the newer stuff is not quite as efficient... older, original refrigerant was the order of 5% to 7% more efficient...but atmospheric ozone worries have phased it out....old stuff was R22, newer 134A....?? maybe they have something newer now???

In any case, if a reliable pro told you that, perhaps it IS a problem...but in refrigeration, I know single size compressor can be used to run different size evaporators......just the refrigerant charge differs.

Perhaps some with design experience or system knolwedge can clarify it....

 

[REBrueckner]

"Turns out, the register, which protrudes well into the airboxdramatically obstructs the flow of air......"

Interesting.....in my 48 YF forward staterrom I noticed somebody had done some cutting in that area....never thought about it..I now wonder if Hatteras modified a standard register on my prior boat and maybe somebody "forgot" to make the modification on yours....

As I understand it, such restricted air flow can also lead to the evaporator freezing...ice build up....especially in very humid conditions...

 

[jim rosenthal]

I think it can. All the parts of the system have to be sized to fit each other, AND they all have to be properly installed. I saw what Mike is talking about today on his boat, and my eyes popped. It's hard to believe it, but the evidence is right in front of you. It's easy to see, looking at it, why the system didn't work; there's no way for the air to get out of the duct. Effectively, the duct is closed.

The particular AC guy who looked at this has been working on marine AC units longer than I have been boating; he knows his stuff quite well. Probably a revision of the way the duct and grille are installed will solve the problem, but it's kind of a fascinoma, as we say in my line of work.

Years ago, when I first got Blue Note, I found some things like that; they really made me scratch my head and wonder "what were they thinking?" Well, no one's perfect.

 

[Angela]

Rob, I think I DO need more condensing compressor capacity, as opposed to smaller fan coils, but I'm pretty much at my limits on amperage unless we hook up a second cord which is why we started eyeing a chilled water system. As soon as the fourth stateroom comes online, the temperature of the air coming out into all staterooms is just so-so - certainly not cool. All four staterooms run independently off of one compressor, by the way. Turn off a room, and the air gets cold again. I am running on R-22 (that I'm sure of as that's what the green tank says!). This is only a problem on hot days, and since we are in Miami, there are many hot days. The system can recover at night with manual switching of rooms on/off until I get there and then they cycle as needed and hopefully not all of them will come on at the same time during the night because when they do, none of the thermostats can get satisfied.

What I might be able to do is take the crew stateroom off that system and put it on a self contained unit - gotta go count my amps, though.

 

[REBrueckner]

Angela:

"....but I'm pretty much at my limits on amperage unless we hook up a second cord..."

I'm quite sure you have a very good chance to add a new/modern/high efficiency 29, 000 BTU compressor and use no more power than your old/original 24,000 BTU compressor.

In going from an old EER or about 12 to a new EER of about 16, you'd save around 30% in relative power.....in other words, you can power a bigger air compressor unit without using more electricity.

There is a least one long thread that discusses this. I considered it for my 1972 Hatt YF but decided against any change as I hardly ever used the air conditioning anyway...

 

[spcoolin]

Angela~

I know your dis-satisfaction with the Cruisair modulating system (still sorry about that) Even though the sticker said 24,000 BTU...Your original modulating condensing unit was actually able to handle that load of all 4 staterooms...It was actually a 28-29K compressor...They only made one size modulating unit back then but wanted it to be flexible...and It was.

Problem is that the new 24K unit that was installed is just that...24K and not able to handle much more...If your installer had known that, he might have installed a 30K modulating unit (they are available now)

The modulating units have the ability to "Throttle Down" but not Up...The up side is limited by the compressors pump rate & it's condenser's ability to remove that amount of heat....Extra capacity of a larger unit can be "Modulated" (bypassed) but not created by a smaller one.

The full load rating of your 24K is 8.0 amps...A 30K would be 11.6 amps.
Both will be less most of the time (after the heat load is reduced a bit)

Again I'm sorry for your experiences...I understand why you feel the way you do...I know with all the leaks of the older valve system & the failures you have experienced, plus maybe some poor service has caused that...As I have said before...Not many service folks understood your system (or the newer models) well enough to provide it...

I'm only suggesting this since you seem to have worked out most of your problems with that system...You are so close...
It may be better to step up to the 30K unit...Cost wise it would certainly be cheaper than a chiller system, it may be cheaper than converting the fwd strm to self contained, and the amp draw difference should slightly favor the 30K...adding 3.6 amps at max load.

Steve~

 

[sgharford]

Hi Angela - I use to sell commercial HVAC equipment for the Trane Company. We would put larger fan coil units on condensers from time to time and vice versa, usually not exceeding the nominal rated size of each component by 15%. If you put a larger fan coil unit on a smaller condenser you get a higher off coil air temperature which does not de-humidify as well. We usually did this out of necessity in retrofits because the larger condensing units were 2 circuits (basically two separate coils in the same box) for better humidity control. Additionally, sometimes we would put larger condensing units on smaller fan coil units to bring off coil air temperature down further for better humidity control. The lower the air temperature, the less water it can hold (dew point). You go below the dew point and the water condensates on the evaporator coil and falls off into drain pan, and therefore less humidity. Anyway - if your compressors aren't tripping on high or low pressure cut-outs, then one can argue system is at least working within design parameters. Chilled water systems are nice because you can infinitely vary the fan speed on the evaporator (fan-coil) unit without tripping the condensing unit (a chiller in this case). If possible, turn the fan down as low as you can for less air flow that will result in a colder off coil temperature which will wring out more humidity if humidity a problem. I was not an HVAC mechanic mind you, I just helped design and sell the systems.

One of the worst things you can do is put a HVAC unit in that is too big. Saw this happen more than a few times, especially on new Fire Stations. They would design these great party rooms on 2nd floor with a cooling design capacity for 200 people, yet 99% of time room is unoccupied. So you have this big honking HVAC unit that runs 5 minutes/hour and therefore cannot dehumidify because it's not running long enough. Next thing you know spackle tape is peeling off and drop ceilings are sagging. The longer your compressors stay on and with an off coil temperature of around 55 degrees or lower, the better your dehumidification will be. To solve above, we usually added smaller split system that would cool and dehumidify the room when unoccupied while the larger units were shut off completely until party time.

Another item of interest, the deeper the coil (more rows) and/or more fins per inch, the lower the off coil air temperature, and the better the humidity control. However, you pay a price with a higher air pressure drop across coil which requires a larger fan and therefore lower system efficiency. If you really want to increase EER, put a larger condensing coil on compressor. For example, with Trane, their 10 ton high efficiency 16 EER condensing unit was a 15 ton chassis and condensing coil with 10 ton compressor installed. Realize none of this solves your problem, but maybe interesting reading if nothing else.