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Old 08-26-2014, 09:13 AM   #1
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Air Conditioning Opinions Needed

Figured I'd start a new thread. I'm installing 2 self contained units after my one self contained died from low voltage compressor failure. While I was researching my ancient split unit has finally gone to the grave. It lasted for 35 plus so no complaining here, oh wait, why did it have to go NOW
I'm installing 2 16000 webasto cause I'm cheap and they looked pretty decent after all my research and they were cheap.
Now the Questions! I read on another thread about using wrapped insulated 4" pvc instead of the usual flexible hose because of greatly improved air flow and cooling. Anyone else try this? How big of a pain is wrapping and then trying to maneuver rigid pipe through tight spaces and cabinets? If it's worth it I'm all for the extra effort, just would like to know before i buy/start.
I have a March 1020 gph pump servicing both AC's now, but they are 5' apart. The salon AC is just a remove and replace and add an extra vent to the aft cabin. The split system cooled the aft and v berth. So now the other unit will go beneath the v berth and run new duct to the galley, front bathroom, and v berth. I'm hoping this is enough going to the aft as the split unit was crappy and it cooled ok until it gets really hot out. Anyhoo, is that pump still going to be able to service both units as the v berth is 20-25 feet away from the pump now? I have a bad feeling that more money is coming out of the down island fund.
I've got more questions but that last thought has me woozy!
Thanks
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Old 08-26-2014, 09:47 AM   #2
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Why don't you just call March Pumps at 847-729-5300 and ask them? Be ready with the specs on your new systems, and perhaps the age of your existing pump. If you don't have a lot of elbows and unnecessary fittings to impede the water flow, you might be OK. I think 16K units need around 500GPH so 1020GPH should be OK, if you don't have a lot of piping losses or additional head. Remember to make that new run as smooth and straight as you can with a continuous, gradual slope down to the pump, and no sags. If the pump is old, maybe a wet-end rebuild kit and you're good for another decade or so.

Good luck

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Old 08-26-2014, 10:03 AM   #3
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Thank you. Was just trying to pick brains while waiting for new AC's to get here!
Calling is probably the best answer :face palm:
Any thoughts on the pvc ducting?
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Old 08-26-2014, 10:14 AM   #4
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Thank you. Was just trying to pick brains while waiting for new AC's to get here!
Calling is probably the best answer :face palm:
Any thoughts on the pvc ducting?
I've never tried it, but it sounds like a great idea. Some have also mentioned using A/C duct board to fabricate their ducting. I guess it all depends on how much room you have to work with.
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Old 08-26-2014, 11:03 AM   #5
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I don't think you will see significant air flow differences between corrugated 4" duct and 4" pvc pipe. And why does it have to be insulated. It all runs in a conditioned space doesn't it?

A single pump if sized right should be able to handle two units even if one is 24' away. You may have to put a balancing valve in the closer unit to get the flow even between the two units. I don't think the 500 and 1,000 gph spec is right. Should be a fraction of that.

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Old 08-26-2014, 11:23 AM   #6
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The specs on the march pump say 27' max, which is in the limits but I'm running 2 AC's so I don't know how much that effects things. I guess I'll find out. If not maybe running 2 pumps parallel will over come the length without me having to haul out and put a new hole in the boat!
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Old 08-26-2014, 11:46 AM   #7
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I don't think you will see significant air flow differences between corrugated 4" duct and 4" pvc pipe. And why does it have to be insulated. It all runs in a conditioned space doesn't it?

A single pump if sized right should be able to handle two units even if one is 24' away. You may have to put a balancing valve in the closer unit to get the flow even between the two units. I don't think the 500 and 1,000 gph spec is right. Should be a fraction of that.

David
David,

A/C supply ducting is insulated to prevent temperature loss and to prevent sweating due to condenation. In some installations, the ductwork can run through equipment spaces or other non-airconditioned areas. In these cases, the heating/cooling losses can be significant.

As for the pump capacities, I was referring to this chart, and others like it published by Dometic, etc. When you buy a 16K Btu A/C kit, most manufacturers supply or specify a March LC-3CP pump or equivalent, which is rated at 510 gph. Per the chart, at 250 gph/ton, 400 gph is the minimum requirement.

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Old 08-26-2014, 02:13 PM   #8
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Larry:

You are right about raw water flow needs. 400 gph for 16,000 btu/hr is about a 6 deg rise which is about what I see on my unit.

Insulation is good.

Do you agree about there being little air flow difference between PVC and flexible insulated duct?

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Old 08-26-2014, 02:32 PM   #9
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In my opinion the advantage 4 inch PVC would have over the flexible duct would be in time the PVC would not collapse. I have flexible duct and in a couple of places it has the potential to collapse because it shares the compartment with other storage that could mash the flexible ducting.

As for water pump size, there are two considerations: one is about 250 gph per ton and the second is head of pump pressure. 27 feet is about 12 psi. If you have more losses that 27 feet or 12 psi then you need some more pump in terms of pressure. I have an air conditioner located on the fly bridge and 12 psi was a stretch from the bottom of the boat. I solved the problem by connecting two pumps in series to give me the pressure I needed. Anyway good luck with your project.
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Old 08-26-2014, 04:21 PM   #10
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The pump ypu have should be fine. If it's the March AC5CMD model I've run 3 A/Cs off those all the time.

PVC is great to use as duct work. But it can be a pain to run and insulate.

As others have mentioned. if you use flexible duct support it properly and watch out for and sharp bends.
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Old 08-27-2014, 06:04 PM   #11
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I read on another thread about using wrapped insulated 4" pvc instead of the usual flexible hose because of greatly improved air flow and cooling. Anyone else try this? How big of a pain is wrapping and then trying to maneuver rigid pipe through tight spaces and cabinets?
The insulation I used is foil-sided bubble wrap - Google: Lowes/reflective insulation. It is easy to wrap long lengths of straight PVC pipe (4" rainwater pipe not Sch #40 pressure pipe) Acrylic basting tape or double sided carpet tape works well to secure it. I used a stretchy clear Scotch tape to make the joints look neat. Wrapping elbows takes more time, depending on how neat you want the finished item to be. Threading PVC through holes in bulkheads is considerably easier than doing the same exercise with floppy accordion ducting. for funny shapes and around cabinets, cut and assemble as you go.

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I don't think you will see significant air flow differences between corrugated 4" duct and 4" pvc pipe. And why does it have to be insulated. It all runs in a conditioned space doesn't it?
I don't have any numbers to back it up, but it seemed logical that air would flow more easily through a smooth-walled pipe than through the inside of an accordion. Similarly, PCV bends would be less restrictive than sharp changes in direction. Insulation is a must - the pipes get colder than the ambient air and run with condensation if not insulated.
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Old 08-27-2014, 11:01 PM   #12
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Thank you for the info! Your thread is the one that I had read. What vent endings did you use that fit into the pvc? Been going back and forth over pvc and flex. The Webasto has a 5" blower exit and that size is not pvc friendly! Go down to 4 or run flex?
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Old 08-27-2014, 11:33 PM   #13
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Thank you for the info! Your thread is the one that I had read. What vent endings did you use that fit into the pvc? Been going back and forth over pvc and flex. The Webasto has a 5" blower exit and that size is not pvc friendly! Go down to 4 or run flex?
Definitely do not use 4" duct work with a 5" blower. You could go up to 6" as long as your registers are the proper size which will keep the static pressure up and "throw" sufficient to mix with the ambient air. You could use a common 6" x 5" reducer like this to make the transition.

Thermaflex makes flexible ducting that has fairly a smooth bore. Check out the link for information on the proper bend radius and transitions.
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Old 08-28-2014, 07:33 AM   #14
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IF the AC is to work well there are frequently return duct requirements.

These are as important as the feed ducts , since its suction , smooth with gentle radius is even more required.
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Old 08-28-2014, 09:52 AM   #15
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IF the AC is to work well there are frequently return duct requirements.

These are as important as the feed ducts , since its suction , smooth with gentle radius is even more required.
+1 Very important! And make sure the return air grille and ductwork is large enough, no cheating here. Per Webasto it has to be 121 sq. in. minimum, 144 sq. in. might even be better if the grille has a filter. And a total of 24 sq. in. for your supply registers. Just follow their installation instructions to the letter and you will be happy, happy!

New air conditioning is so cool . . . . .
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Old 08-28-2014, 06:31 PM   #16
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. . . . You could go up to 6" as long as your registers are the proper size which will keep the static pressure up and "throw" sufficient to mix with the ambient air.
Larry, could you please explain the meaning of the static pressure and "throw" sufficient to mix with the ambient air. Thanks.
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Old 08-28-2014, 10:16 PM   #17
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Larry, could you please explain the meaning of the static pressure and "throw" sufficient to mix with the ambient air. Thanks.
I'll try . . . When you move air through a duct, it passes along the sides of the duct and through friction builds up resistance to the flow. The rougher the duct, the more friction and resistance. This resistance to the flow of the air is called static pressure. Additionally, every shape transaction, elbow, branch and filter also adds static pressure. It can be compared to your own blood pressure. Here is one of the best explanations I have ever found. The ducts must be sized and designed for an optimal static pressure when used with the fan's rated CFM to produce adequate air volume and circulation.

The throw is a term used to describe how far the conditioned air can be felt from the register. In a residential setting, I believe this should be about 75% of the distance across the room. The air flow must have sufficient velocity and volume to break up the stratified air in the space and circulate or mix it with the conditioned air. Ideally, it should do this without creating an uncomfortable blast of felt air. In a boat, the same principles apply, but I don't have any rules of thumb for those. Just make sure the area of the supply registers and return air are adequate for the heat load and area you are trying to cool.

If you haven't already done so, you can check the calculations for your boat using a planning guide like this one. It will give you a rough idea of what you need, or if the original system was adequate for the way you now use the boat.

Sorry for the lengthy explanation . . .

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Old 08-29-2014, 06:21 AM   #18
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Good explanation, thanks.
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Old 08-29-2014, 09:24 AM   #19
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+1 Very important! And make sure the return air grille and ductwork is large enough, no cheating here. Per Webasto it has to be 121 sq. in. minimum, 144 sq. in. might even be better if the grille has a filter. And a total of 24 sq. in. for your supply registers. Just follow their installation instructions to the letter and you will be happy, happy!

New air conditioning is so cool . . . . .
Just as important is making sure it's pulling return air form the area it is cooling and not from some where else. I've seen A/C installed where they were actually getting return air from outside the boat!
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Old 08-29-2014, 09:57 AM   #20
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Just as important is making sure it's pulling return air form the area it is cooling and not from some where else. I've seen A/C installed where they were actually getting return air from outside the boat!
Absolutely right, another sneaky one is air leaks from adjacent spaces, particularly from penetrations around hoses, cables etc. I never encountered return air from outside the boat though, that one takes the cake.

I once saw a water heater installed right next to the A/C unit in a small locker. There were fairly large holes from the locker into the engine compartment for the coolant hoses, and into the bilge for the raw water and drain hoses. The water heater was between the A/C unit and the return air grille. They could never figure out why their cooling was so bad underway and the humidity so high. Oh, another good one is large solar-powered vents, and on and on.
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