AC and Heat Duct Work? - Page 2 - SailNet Community

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Old 01-24-2007
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good idea with the pvc pipe, I am not sure how I could put a control valve in other then on the grate its self. I could make my stacks that come up out of wood and hook pvc pipe into those. its not easy trying to add this to a 1970 sailboat. I plan to run 3 discharges of the 4" to the rear and run one 4" into the v-berth.
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Old 01-26-2007
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What type of heating unit are you installing and what are it's ratings? I am currently installing a heated water furnace rated at 40,000 btu with a 3 speed fan blower. 5 seperate euro vents with 2 vents sharing the same 3" duct as this was the only possible way to get a duct in the head and at the inside helm windshield. I have cut 12 holes either 3" or 3.5" through grp depending on whether it was a euro vent or the actual hose that needed to go through it. I am using 3 way ball valves to route heated engine water from the already installed 5/8 inch hose that is connected to the heat exchanger on the heated fresh water supply. I like your diagrams and it appears as though you have some serious challenges with your plan. Knowing what the weather is currently like in Bay City, the air temperature isn't helping much either. Sure hope that you have heated storage, or maybe this is a spring project?


Thanks for the post. I appreciated your reply.

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Old 01-27-2007
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In reference to PVC, check with your local plumbing supply for sewer and drain pipe. This is a much lighter pipe than the regular plumbing PVC, and the cross section can be flexed to fit in spaces other pipes cannot. I have used this stuff in the past for ductwork, and it works very well. Also, I second the need to insulate, or you will make a water still on the outside of your duct.
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Old 01-28-2007
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1970 Columbia34
You can purchase sleeve insulation made to slide over different diameter round ducts. The sleeve has fiberglass insulation on the inside and a vapor barrier made from vinyl or aluminum foil on the outside and is easy to install. The advantage is they have no lengthwise seams that could void your vapor barrier. They come in 5' lenghts and you tape the joints with foil tape, NOT DUCT TAPE. Duct tape is good on everything but ducts. It is available at any HVAC supply house. United Refrigeration has stores all over the country and world. Make sure the insulation doesn't get wet. If it does, it a perfect place to grow mold. DWG PVC is a good applicationas mentioned earlier, as is electrical PVC conduit. You can buy havy cast aluminum "blast/slide gates" to use as dampers if you can get access to them. Place them at the takeoff of your main duct for less noise at the outlet.
I'm comcerned about the 4" size of your ducts. Look at your specs for the evap fan. If the ESP (external static pressure) capability of the fan is .5" water column then your 4" ducts are probably too small and you will not get your rated cfm for the unit and it will freeze up. If you have a high pressure evap fan that can produce 2" ESP, then you probably have a chance.
There is a product call felxible duct that may be recommended to you. Its easy to install and can snake anywhere and comes already insulated with a vapor barrier. Problem is, it has a very high static pressure loss which reduces airflow and it looks like you already have small ducts. It is used in the HVAC industry to connect ceiling diffusers to the round branch ducts. Good design doesn't allow more than a few feet of it. Cheap designs and installation that use a lot of it usually have airflow problems due to high static pressure losses from the flex.
If you can fit a 10" wide x 4" high register for example, in your design, you can purchase it made from aluminum so it doesn't rust and specify the hoizontal blades to be in the rear of the diffuser. If your mounting high, turn the diffuser so the blades can point down to push heat to the floor. Reverse the blade angle for summer.


Last edited by Wayne25; 01-28-2007 at 12:14 AM.
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Old 01-28-2007
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I called the 10" x 4" a diffuser. It is actually a register.
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