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Old 09-06-2018, 09:48   #1
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What is the proper difference in temp. Return air vs supply

Have a mermaid 5200 BTU unit. Looking for the appropriate delta T. Friends boat is blowing air in the 35 range with a 12000 BTU. I'm at the same dock on the same day and blowing 60. Outside temp is about 90. temperature inside my boat is about 83. 76in his.

His pressure side is really Hot! Mine is about 107. either his is malfunctioning or mine is LOL .

Any help would be appreciated.
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Old 09-06-2018, 10:51   #2
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Re: What is the proper difference in temp. Return air vs supply

Sounds like you might be a tad low on refrigerant, or have a dirty coil(s). Generally you will wnat to see no more then 50-55 degree f discharge air. Any warmer and it's not dehumidifying at all.

It's also possible that your 5000 btu unit might be too small to cool your interior down. What size boat do you have?
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Old 09-06-2018, 13:31   #3
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Re: What is the proper difference in temp. Return air vs supply

The delta you're looking for should be 7-10deg F. Example: if your room is 95F, then the AC output will be 85-88F. As the room cools that output will decrease. You'll never get 35F air out of an AC UNLESS the room is 45F. Hope that helps
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Old 09-06-2018, 13:40   #4
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Re: What is the proper difference in temp. Return air vs supply

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Originally Posted by smattie View Post
The delta you're looking for should be 7-10deg F. Example: if your room is 95F, then the AC output will be 85-88F. As the room cools that output will decrease. You'll never get 35F air out of an AC UNLESS the room is 45F. Hope that helps


House yes, Boat is more like a car, my boat AC is stupid cold, like low 40 if memory serves with the Boat at 80 f or so.
Duct length is only about a foot and register is duct sized, so no restriction.
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Old 09-06-2018, 13:59   #5
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Re: What is the proper difference in temp. Return air vs supply

My expertise is with Dometic ACs. Interesting...
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Old 09-06-2018, 14:08   #6
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What is the proper difference in temp. Return air vs supply

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Originally Posted by smattie View Post
My expertise is with Dometic ACs. Interesting...


Should be the same, the more the difference, the less efficient. Therefore houses have the room to fit huge A frame evaporators and can move lots of air and drop the temp only ten degrees and Cool a house.
A boat doesnt have as much room, so evaporators have to be smaller and the temp drop higher to still cool the Boat, and just give up the efficiency.
However we all know that if a filter is clogged that your evaporator will freeze over, so if you dont mind turning down the air flow, freezing temps are possible.
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Old 09-06-2018, 23:49   #7
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Re: What is the proper difference in temp. Return air vs supply

A 20 degree split is optimal. I just measured my air temps at home, 20 degree difference.
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Old 10-06-2018, 00:00   #8
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Re: What is the proper difference in temp. Return air vs supply

Quote:
Originally Posted by mjgill1 View Post
Have a mermaid 5200 BTU unit. Looking for the appropriate delta T. Friends boat is blowing air in the 35 range with a 12000 BTU. I'm at the same dock on the same day and blowing 60. Outside temp is about 90. temperature inside my boat is about 83. 76in his.

His pressure side is really Hot! Mine is about 107. either his is malfunctioning or mine is LOL .

Any help would be appreciated.
If your boat interior temp is 83 and your A/C is blowing 60 then you need to check the airflow.
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Old 10-06-2018, 04:34   #9
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Re: What is the proper difference in temp. Return air vs supply

Hmmm, quite a diversity in answers... Home A/C and marine A/C are different animals. Marine A/C uses the water has a heat sink, so the water temp has some to do with it. I do know that the unit is way too small for the boat. I should have a 12-16 BTU and it's a 5200. God only knows what someone was thinking there.

In case someone in the know picks up on this. The high side is running at around 107 degrees and the suction line was around 55 when I got done messing with it. The unit was overcharged by an HVAC guy thinking he could use gages on it, so I bled off gas and was able to get the output temp to 55 before getting nervous and stopping. Too much refrigerant can cause damage to the compressor if liquid gets back there. No enough and well....Air flow is maximized with combed fins on the evaporator, clean filter and unobstructed ducts. I still think I should be blowing lower temps at the supply register.

My neighbor's 12k BTU unit is blowing around 35 degrees, but had a clogged filter, LOL.

The manufacturer recommends pumping it down and filling it just until the evaporator sweats to the top since there are too many variables to use gages once it leaves the factory. Factory charge is ~ 9 oz.
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Old 10-06-2018, 06:28   #10
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Re: What is the proper difference in temp. Return air vs supply

Vacuum it down if you can and put 9 oz in it and see.
Many people undersize an AC because they dont understand the heat load, many boats deck can get quite hot in the sun and you can feel the heat on the underside, plus the R value of fiberglass isnt all that good. Plus they go on the theory that they want the compressor running 100% of the time cause it dehumidifies better
I oversized mine, which to means I run them at their lowest fan speed and the noise isnt so bad, but they are still noisier than a home unit of course.
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Old 10-06-2018, 06:54   #11
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Re: What is the proper difference in temp. Return air vs supply

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Vacuum it down if you can and put 9 oz in it and see.
Many people undersize an AC because they dont understand the heat load, many boats deck can get quite hot in the sun and you can feel the heat on the underside, plus the R value of fiberglass isnt all that good. Plus they go on the theory that they want the compressor running 100% of the time cause it dehumidifies better
I oversized mine, which to means I run them at their lowest fan speed and the noise isnt so bad, but they are still noisier than a home unit of course.
I'm replacing the aged (25 years), obsolete, non-working, environmentally unfriendly (r22) 16K unit on my C30 with a new 14K unit from Marinaire. I used their numbers to select this size.

My old 16K unit (according to PO's logs) always had issues. Then again, it was installed by an idiot. The ductwork was a joke - with a 6" outlet immediately stepped down to 4", then through a 90 elbow, into a "T", more 90"s, and only two outlets - one at the end of a 12' long winding duct. Needless to say the airflow was anemic.

Even here on Lake Ponchartrain, most guys with similar size boats are happily running 12K units. I've had some other owners say "I wouldn't go less than 16K!"

We'll see how the 14K does. I think with the better location and proper ducting I will install, it will shine.

Factory recommends strongly against oversizing, at it will shut off before properly dehumidifying. They endorse undersizing A BIT to ensure efficiency, dehumidification, and longer life due to less frequent cycling.

BTW, I could have had the 16K for only $90 more and the same physical size. So the extra couple of BTU wasn't prohibitive. I'm pretty sure a proper installation will make all the difference.

Another BTW - even though I work for a master A/C technician who was more than willing to work on the soon-to-be phased out R22 system - I just didn't feel like putting a couplea hundred bucks into a system that has served it's time, led a hard life, has never worked properly, could break again in 2 months, and won't even be "legal" to service in less than two years.

On my boat as I write this, gonna unload the new A/C from my truck right now!
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Old 10-06-2018, 06:56   #12
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Re: What is the proper difference in temp. Return air vs supply

Oh, one more piece of evidence for the "idion installation" accusation: solid wire household Romex - and no dedicated circuit breaker...

(Did I mention the installer also seemed to own stock in duct tape?)
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Old 10-06-2018, 10:27   #13
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Re: What is the proper difference in temp. Return air vs supply

Quote:
Originally Posted by mjgill1 View Post
Hmmm, quite a diversity in answers... Home A/C and marine A/C are different animals. Marine A/C uses the water has a heat sink, so the water temp has some to do with it. I do know that the unit is way too small for the boat. I should have a 12-16 BTU and it's a 5200. God only knows what someone was thinking there.

In case someone in the know picks up on this. The high side is running at around 107 degrees and the suction line was around 55 when I got done messing with it. The unit was overcharged by an HVAC guy thinking he could use gages on it, so I bled off gas and was able to get the output temp to 55 before getting nervous and stopping. Too much refrigerant can cause damage to the compressor if liquid gets back there. No enough and well....Air flow is maximized with combed fins on the evaporator, clean filter and unobstructed ducts. I still think I should be blowing lower temps at the supply register.

My neighbor's 12k BTU unit is blowing around 35 degrees, but had a clogged filter, LOL.

The manufacturer recommends pumping it down and filling it just until the evaporator sweats to the top since there are too many variables to use gages once it leaves the factory. Factory charge is ~ 9 oz.
Too many variables to use gauges once it leaves the factory?
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Old 10-06-2018, 11:14   #14
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What is the proper difference in temp. Return air vs supply

I measured mine, its 77f on air return, and Im getting 51f output.
I thought it was lower than that, but I do have a real clean filter just now.Click image for larger version

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Old 10-06-2018, 13:04   #15
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Re: What is the proper difference in temp. Return air vs supply

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I measured mine, its 77f on air return, and Im getting 51f output.
I thought it was lower than that, but I do have a real clean filter just now.Attachment 171411
How many BTU is your unit?
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