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1970Columbia34 01-23-2007 10:53 AM

AC and Heat Duct Work?
As most of you know we are installing a heating and cooling system in our Columbia 34. I have having a hard time on how to run the duct work I am considering using the wall cavity (like used in houses) as my duct run since it will be difficult to run any ducts or flex pipe to this location.

I my picture below you can see a hatch pattern the represents a board that I would like to install to make the air run in the wall cavity, and how the air will be let out of the space through some custom window boxes in the areas the previous owner made to install some port windows. The unit is gettting installed in a locker fwd (to the right in these pictures) of the couch, there is no problem running the duct work fwd into the v-berth.

Is this a bad plan, anyone see a better way to to this? I want to do it the best way possible.

1970Columbia34 01-23-2007 11:08 PM

no help? or opnions?

Wayne25 01-24-2007 12:06 AM

I design HVAC systems for commercial and residential buildings. It is a very bad idea and against building codes to use the building structure for supply air ducts. Building structure can be used for return ducts for some designs. Why, because the large difference in temperature between the conditioned air and the structure surface causes condensation and a lot of it. You know what happens when things are always wet. Mold growth and worse. Properly designed supply ductwork can be designed so it is insulated from these temperatures and just as important covered with a vapor barrier to stop high humidity levels from infiltrating the insulation and causing condensation within the insulation. Use these priciples in designing your system. Furthermore, you must design the ducts so you don't go below the rated air volume of the A/C unit. Otherwise it will fail. The fan in the unit can only produce so much pressure. Make the ducts too small, too many bends, too long and you will reduce the rated airflow of the A/C unit.

Faster 01-24-2007 12:11 AM

Hey Scott

Wayne's got some good advice and valid concerns re the potential condensation when using the AC function.

My concern would be the elevated location of the duct outlet for heating. A heating outlet should be down low to let convection do your circulation for you. A fan or fans would be needed to get a uniform temperature when heating, adding to your electrical budget.

Wayne25 01-24-2007 12:21 AM

Faster, your concerns for heat stratification is correct in a larger space, and it would be best if he could supply heat low and A/C high. However, the amount of air needed to cool and heat this uninsulated harsh enviroment is substantial. The large air volume and probably high velocity will move the air around quickly and probably not produce the heat statification seen in large rooms we normally live in.

sailingdog 01-24-2007 12:47 AM

Aside from Wayne's and Faster's concerns, it would seem to me that the heating/cooling of the boat would be adversely affected by doing it this way. The space between the hull and hull liner is likely to be hot in the summer and cold in the winter, and if you're not using a duct, much of your cooling/heating capacity will go towards cooling or warming the hull and space between the hull and hull-liner. Even an uninsulated duct will tend to help keep the cooling/heating air flow directed towards where it will do the most good.

1970Columbia34 01-24-2007 10:17 AM

Thanks for the responses, ok I have come up with a new idea here i would build 2 of these that would fit in my windown slots and connect the with 4" pipe.

you mention insulating, are the products out there for insulating 4" pipe some sort of sleeve the pipe can slide into? or can i glue some sort of thin insulation to the pipe? what do you guys think of this idea. I know what your saying about the vents being high up but, the unit has a 10x14 return grill right at the floor which i hope helps pull the heat down.


sailingdog 01-24-2007 10:51 AM

The air return won't really help pull the heat down... heat rises. :D There should be some sort of insulation you can get for the ductwork, even if it is just a heat-reflective mylar barrier.

What you might want to do is have the ability to open vents at either the top or bottom, and then depending on whether you're heating or cooling, you can open the appropriate vent. It wouldn't be that hard to make a sliding collar type door for PVC pipe.

1970Columbia34 01-24-2007 10:56 AM

The problem is running duct runs on the floor is not possible, there is all kinds of things in the way like the water tank sit right next to the only locker the unit will really even fit in. are you saying use pvc pipe for my duct run vs, sheet metal?


sailingdog 01-24-2007 11:11 AM

I don't see why you couldn't use PVC pipe, rather than sheet metal, for the duct work... It really depends on how hot the air gets from the heating side of the unit... I am guessing that you're using a heat pump/AC heating unit rather than diesel or propane forced hot air, so it probably wouldn't get hot enough to be a problem. PVC pipe acts as a insulator to some degree, so the heat loss/gain would be minimized, and it is less maintenance than sheet metal, which will rust out...

Even if you use sheet metal for the ductwork, the vent openings could be made from PVC pipe relatively easily, and allow you to open and close the vents as needed.

One issue I see with having the vents directly beneath the ports is if the ports leak, it might cause some problems..

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