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SMX II design issues
September 28, 2011
8:47 am
Steve Pooler
Forum Posts: 1127
Member Since:
November 12, 2008
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What you are describing is not really a design issue of the SMXII control...It is only doing what you ask it to do...Then responding to those set parameters.

Direct Expansion Air Conditioning relies heavily on air flow...Lowering air flow across the coil reduces the amount of heat being picked up...This lowers coil temp, which in turn will lower discharge air temp...

Slowing the fan enough (or restricting the design air flow) with any direct expansion A/C system will cause icing of that coil, and a major TD (temp differential) of the air in to air out...We shoot for a 15 to 18 degree TD with A/C (refrigeration is sometimes more but they use defrost timers & heaters)

So if your TD is more than 18 degrees you are not picking up enough heat & system performance will suffer (as you mention the compressor won't shut off when set low) and another side effect...As you are experiencing...Sweating metal grills due to that TD difference of Cabin vs Output temp..


Couple of thoughts here… 

Change your supply grills to Wood so they don''t sweat as easily…Cold metal sweats much easier than wood because it is able to pick up (absorb) the cold air across it easier as it is a better transfer medium....This big difference in cabin vs Metal grill temp causes the condensation...Just like what happens on the A/C coil...

Change to wood & maybe you can lower your low fan speed a bit more…But the compressor still may not cycle off as often... 

Second: Locating the temp sensor out in the room can be a challenge, because finding a spot that does not pick up radiant heat or cold from some other source such as a window, door, or heated engine room bulkhead, can affect the temp that the control sees…

Temp sensor cables Are available in lengths up to around 60 feet though…So getting one long enough is not a problem.

My guess is that your 1977 58' Hatt MY was retrofitted to SMXII, so another guess would be that they installed at least 10 foot sensor wires…What I would do first is remove that sensor from it's present spot & with the cable un-coiled… Temporarily test differing locations in the cabin/s to see what location/s provide the best overall performance…Then look at what is needed to permanently place it there…

Keep in mind that it sees as little as 1/8 of a degree difference…So touching any surface can affect it's reading…I will add though...That unless you want to tell the SMX to cycle the fan with the compressor...Locating the temp sensor in the return air path generally gives the best overall (average) cabin temp since it is getting an overall sample of all the air moved...Locate it out in the cabin & it can only sense where it's located  Cool



September 28, 2011
7:38 am
Knoxville, TN
Forum Posts: 17
Member Since:
September 6, 2011
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The SMX II has many settings but I haven't found the one I want.  In the cabins, I would prefer to reduce the fan speed very low when the compressor is not running, but I have to keep the low fan speed rather high in order to avoid condensation of the supply vent and to circulate enough air so the compressor will cycle off.


I tried using the fan off setting but as the manual suggests, the unit won't cycle enough because the remote temp sender is in the return vent.  How do you relocate the remote temp sender to an inside wall?  Does someone make a teak box with ventilation holes for that purpose?

1977 Hatteras 58' Motoryacht, galley up model with 4 staterooms

5 reverse cycle Cruisair compressors in the starboard engine room with SMXII controllers.  One self contained unit for the enclosed aft deck.  Total 87K btu