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14 hours ago, LakeBob said:

For my clarification, the delt T was between the return and vents?

 

Generically and specifically.  The DelaT measurement is supposed to be between the first or closest air outlet and the “return”.  In a car dash system, that is measured based on NOT being in the recirculation mode and allowing the unit to run and be stable. The outside temperature is typically used.  When folks drive in on a hot day and the MH has been sitting out, then a mid day recharge and test, you will get a much lower DeltaT than the “charts”.  Personal experience….I added 24 OZ to my Camelot….which it typically needs every few years.  It was hot as blazes and the results were less, per the DeltaT, than our published charts and guidance and instructions.  BUT, after driving on a trip and letting the MH stabilize….it was the same as the OTR tech’s reduction….happy was I.

Per the LD folks, the MH should be inside overnight or have had the AC on and running and got the rig “stabilized”.  Gut feel or not an exact science….but on an ambient day in the mid 80’s….maybe an hour or so….whenever one felt “cool” or comfortable.  Then put the thermometer in or on the rerun grill.  Stabilize.  Put it on or in the closest supply or outlet…stabilize.  Then do the math.

NOW…..a comment from a long time Dometic Factory tech when I was evaluating or “is this unit working OK?” comment.  He said that for some unknown, but widely known condition, the FRONT unit, in the HP MODE, would always run 10 degF “cooler” if you were on the roof and had the cover off and did laser or contact measurements on the various line and valves and such.  Go do the same on the rear unit…10 degF higher.  Therefore, I CONCLUDED….and maybe falsely, that the rear unit was providing MORE a heat….and would cycle less.  He also, and I chased this down, said that on my 2 HVAC unit, that there was a difference in the ducting.  

EDIT….If you look, on the 3 AC or 42/45 models, there are probably two versions….and the shower layout or location makes them different.  Read on in a subsea post for the details.

IF there is a rear shower and no open, skylight shower in the middle. Then, all three AC’s supply air into the  TWO ducts running from the front to the rear….inline….so each AC/HP I’d dumping or supplying air….so they are, in effect, a “central” system with three “supplies” and the overall impact is more consistent or homogeneous mixture or conditioning.  In the shorter 2 AC/HP systems, the center or the, typically, passenger duct is blocked due to the shower.

 If a 3 AC with a middle or hall shower, then the middle AC kicks in and is also connected to the front on that side.

IF you follow that….then one would have to measure the Delta T up front or each side….then average as that is what is going on “Up Front”. On a 3 AC system….the rears are in the same duct on both sides….so it wouldn’t matter.  From a “Engineering” lab experiment….if you measured the rear and front on either side…or from both ducts….average the reading and do the DeltaT.  Same for the front,

GOD…that was heady….but you asked….

PS….one final “thought”.  As long as the DelaT’s were within say 10%…then, in theory, for a 3 AC system….use the average. But, if higher, per my esteemed Thermodynamics Prof, for my only entry level cases some 50+ years ago, one would also then measure the airflow or CFM.  THEN do a weighted average…that, in theory….would be the correct or exact DeltaT.  A tech MIGHT put his hand near or over the outlet on either side…MAYBE.  As long as they “feel” the same…flow and or cooling…then test only one side…

There are, folks I know of and believe are here that would be, out of curiosity, at this level.  Random thoughts.  Reality. my OEM Dometics are marginal for heat of the summer travel in the SW. When one dies…out both goes and then whatever 15K unit that works off a multiplexed phone line control circuit and is compatible with the original Monaco install circuits will be there with the correct thermostat….

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3 hours ago, Tom Cherry said:

 

   In the shorter 2 AC/HP systems, the center or the, typically, passenger duct is blocked on either side of the center exhaust fan in the hallway.  That means that the rear AC, which is covering a smaller volume has less impact on the front.

 

Hmm. Never knew that!

So, your two ceiling HVAC ducts aren't wide enough apart to clear the centerline mounted vent fans?

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44 minutes ago, 96 EVO said:

Hmm. Never knew that!

So, your two ceiling HVAC ducts aren't wide enough apart to clear the centerline mounted vent fans?

Good question.  Made me think..The Dometic tech told me that mine was blocked and if I had a center AC, I would have two ducts that were in line.  It has been a while since I looked, but I did do an experiment and turned off the front AC and put the rear fan on high.  The “drivers” side…and we know that my 2009 40 is a mirror image of what was done in a 2008 40….don’t ask me…??  So, yes, you get air out of the front and private bathroom vents.  But none on the passenger side in the living galley area.  OK….looked at the 2008 & 2009 Camelot floor plans.  Vaguely, maybe, whatever….the answer is IT DEPENDS.

All the 2 AC 38-40 will have a blocked vent….on whichever side the shower is on.  That is obvious from knowing what I know and now looking at them.

There we’re TWO plans  for the 3 AC or 42 ft units.  One has a rear shower.  IF that is the case, then the rear AC will pump or force air all the way to the front….and I suppose, from an HVAC standpoint is the most efficient or the best design for even cooling.

Next up is the center shower 42.  The middle AC is not connected or ducted to the bedroom….because of the shower.  BUT, it IS connected to the front AC’s duct.  So, with all AC’s ON….the center and living area has two running… more air…a smidge less than the other with 3.

I hope that made sense.  It also reinforces my thinking that my front AC probably DOES have different, or maybe the POTENTIAL, for different DeltaT and if I ever test again….I’ll find out.  That also flagged a haunting thought or “THATS WHY”.  I always notice or knew, but figured it out…and then forgot…that the full length duct put out a smidge MORE air in the front…especially the two forward most vents, than did the other side.  It has two fans…and the longer one has only one.  Unconsciously, I have throttled down the rear vents a smidge in the rear galley area to kick more air up to the hot spot…the cockpit…

Again…great question….and I will check the previous post or edit it to go here for a better explanation….

From a comfort standpoint, on some really sweltering days. I did consider retracting the slides and going to DUMP for more cooling 😎 

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11 minutes ago, Tom Cherry said:

 

From a comfort standpoint, on some really sweltering days. I did consider retracting the slides and going to DUMP for more cooling 😎 

Ha. Circled right back to the topic of the thread.

I use my air dumps often enough I won't consider installing those aftermarket air silencers. They eliminate the option of dumping the cool air straight down.  

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Guest Ray Davis
12 minutes ago, Tom Cherry said:

From a comfort standpoint, on some really sweltering days. I did consider retracting the slides and going to DUMP for more cooling 😎

It's cooler where you dump?  😁  So does your A/Cs have the ability to dump air?  Mine doesn't

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3 minutes ago, 96 EVO said:

Ha. Circled right back to the topic of the thread.

I use my air dumps often enough I won't consider installing those aftermarket air silencers. They eliminate the option of dumping the cool air straight down.  

BINGO…a winner…no more radical ideas and experimentation with DeltaT and airflow….mine are used all the time….thanks.

YES…the Dometic Penguins used, at least on the Camelots from, I assume 2007/8 (memory) but YES on the 2009…have dump register or louvers….  I thought ALL RV AC’s did.  This is how we learn and are a little more reluctant to make generalizations….

THANKS!

 

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Guest Ray Davis
8 minutes ago, Tom Cherry said:

BINGO…a winner…no more radical ideas and experimentation with DeltaT and airflow….mine are used all the time….thanks.

YES…the Dometic Penguins used, at least on the Camelots from, I assume 2007/8 (memory) but YES on the 2009…have dump register or louvers….  I thought ALL RV AC’s did.  This is how we learn and are a little more reluctant to make generalizations….

THANKS!

 

That really upsets me,  you know how hot & humid it is here in E Tx?  It's hard to cool a coach right now and I think if I could dump air straight down it would help a lot.

I was thinking I would add a 3rd A/C where my Coolamatic is but a different intake with dump would be cheaper.   Have either of you got a picture?

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7 minutes ago, Ray Davis said:

That really upsets me,  you know how hot & humid it is here in E Tx?  It's hard to cool a coach right now and I think if I could dump air straight down it would help a lot.

 

On hot days, our little dog loves when I open the front air dump!

She finds the perfect spot to lay on the floor and get max cold airflow 🙂!

https://parts.unitedrv.com/products/dometic-duo-therm-rv-air-conditioner-filter-replacement-14-x-7-5-ac105g

Ray, just scroll over one image to see them.

Edited by 96 EVO
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Guest Ray Davis

Thanks Ben,  I suppose the dumped air comes out where the square pattern is?   You guys have probably seen pictures on here of the square 14 x 14 in intake grills that Monaco made.  That's what I have, it has a wooden frame around it and is hinged to swing down for filter replacement.  

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1 hour ago, 96 EVO said:

On hot days, our little dog loves when I open the front air dump!

She finds the perfect spot to lay on the floor and get max cold airflow 🙂!

https://parts.unitedrv.com/products/dometic-duo-therm-rv-air-conditioner-filter-replacement-14-x-7-5-ac105g

Ray, just scroll over one image to see them.

Good find.  Amazon sells the OEM filter and new grill assembly for $11 or so.  For me, the air flow might be an issue, but for others with 40% more capacity…not such, as long as the DeltaT of 20 or so is maintained.

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Guest Ray Davis

I don't know what my delta t is but i do know when I'm hot,  that's the real measure for me.  LOL

I would probably to be like Ben's little dog with cold air blowing down on me,   just kidding it would probably make me feel bad or sick.

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Guest Ray Davis

When I lay on the floor I need help getting up.  and I used to spring up like a cat,  well,,,,, I could get up without help.   😄

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7 hours ago, Tom Cherry said:

 

All the 2 AC 38-40 will have a blocked vent….on whichever side the shower is on.  That is obvious from knowing what I know and now looking at them.

 

Tom, you piqued my interest when you mentioned the shower skylight.

I always assumed mine had two parallel ducts running the length. When you mentioned the shower I decided to check mine. Going by the line of diffusers, there was no way the duct didn't either end, or jog around the shower skylight.

Pulled the closest diffuser to the shower, pulled out the mirror and flashlight, and sure enough, there's a break in that HVAC duct!

There must be a crossover duct at the rear of the coach, because when I run my mid A/C, I get a little airflow out of the bedroom diffusers, on the shower skylight side of the coach.

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  • 1 month later...

Just an update on my ac silencer……we have been on a few trips and currently on one now using the rear ac all the time and it has preformed very well. Much quieter and doesn’t bother us when we sleep at night. The biggest problem before I installed it was when sleeping you could hear when the compressor kicked on and off very distinctly. Now it sounds pretty much the same all the time so just a constant white noise that doesn’t wake you up.

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