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Old 01-08-2014, 05:37 AM   #1
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Furnace Ducting Design Review - 66 Trade Wind

I read the "not enough heat in back" thread with considerable interest. I have set my new Suburban NT 30 SP furnace under the galley sink cabinet. My idea is to use under floor ducting to floor registers on the curb side of the living room and bathroom. I have the belly pan off to fix floor rot in the bathroom area.

I need some of you experienced trailer HVAC experts to take a gander at my ideas and see if this heating system will work, I can change directions at this point if needed.

I planned to dump the furnace output into a 19 by 19 by 5 high sheet metal plenum box under the floor. Then I planned on running a 6 by 2 1/2 oval sheet metal duct over to the curb side, and then tee to the living room, and run back to the bathroom. I have to transition the oval duct to 4" round to sneak through the holding tank pan. I will dump some of the heat into the holding tank pan, and the rest to the bathroom floor register.

I can add additional ducts to the plenum if necessary. Two 6 by 2 1/2 oval ducts are about 30 sq inches of heat exhaust area. The duct runs are long. The main duct will be about 5 feet to the curb side tee, and then 4 feet to the living room floor register, and a 10 foot run to the holding tank pan. Then round ducts through the pan and another 5 feet of 4" round to the bath floor register.

I will insulate the ducts the best I can. I must stay between the frame rails of the trailer.

I could run 4" flex above floor on the street side. The flex duct takes storage room away.

Any chance this heating plan will work?

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Old 01-08-2014, 06:35 AM   #2
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In my whole career, I've only designed HVAC dutwork for two small office buildings, and never for a travel trailer, so don't take this as gospel without independent confirmationů

Your concept should work, with a few modifications. You don't really need a plenum if you've got a main duct. You just need a transition section, that matches the furnace shape on one end and the ductwork shape on the other. In general, the main ductwork cross-sectional area should be about the same size as the furnace output, to avoid either accelerating the flow (causing increased noise) or creating back-pressure that would reduce furnace efficiency.

If your furnace is in the middle of the trailer, then the furnace transition section goes straight to a manual diverter valve that splits the flow between two main ducts, one running in each direction. This lets you use smaller main ducts, by the way, since each only handles part of the total furnace output flow.

In a main duct, you will want manual diverter valves for each branch duct, so that you can adjust the volume of flow going to each branch. This procedure is called "balancing" the HVAC system, and only needs to be done once, when you test the system after installation.

If you want some of the heat to go to the belly pan, you will have one branch duct that terminates in the belly pan. But remember, anyplace that you're blowing hot air, there has to be someplace for cold air to leave, that will handle an equal flow volume. Make sure there is someplace for air blown into the belly pan to get back out, on the other side of the tanks from where the duct is.

Now for the numbers:
In the main duct, you want the flow velocity to be between 780 and 1380 feet per minute. Based on the cubic feet per minute that your furnace produces, that lets you determine the size of the main duct you will need. Remember, if you have two main ducts running in opposite directions, you will use the total square inches of BOTH main ducts to derive your ft/min flow velocity from cu.ft./min flow volume.

In each branch duct, you want the flow velocity to be between 200 ft./min. and 590 ft./min. The slower the flow, the quieter it will be, and the less of a breeze you will feel by the duct. But remember you're still pumping the same cubic feet of heated air even at a lower velocity.

The floor registers should have valves to close them off. This not only allows you to regulate the flow in real time (unlike the diverter valves or check valves, which are set once when you balance the system and left alone thereafter) but also allows you to close the ducts in summer when the furnace is not in use.

Since your trailer moves, and vibration can cause the manually-set diverter valves and check valves to vibrate to a different position, safety-wire the adjustment levers in place once they're set the way you want them.

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Old 01-08-2014, 07:33 AM   #3
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Hi David

Airflow in the oval ducts is roughly the same as airflow in 4" round flex, maybe a little better.

You will need at least 3 oval ducts coming off the plenum in order to provide sufficient airflow for the furnace to work properly. 4 would be better.
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Old 01-08-2014, 08:53 AM   #4
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I had originally planned three, with a separate duct going directly to the holding tank pan. That's why I planned a plenum. I have to get through the holding tank pan with 4" flex, squished down to 2 1/2 going through 3 stringers. Then I thought why not just tee off a 4" flex with a 3" or even 2" flex and dump into the holding tank area, and route on to the rear bath. The holding tanks just have to be above freezing, not 68 degrees. I figured cold air would escape the holding tank pan through the 3" drain manifold and dump valves. This area will not be sealed up very good. The excess heat would spill outside contributing to global warming!

The furnace bottom duct is a "4 by 10" square. Maybe I can drop down, then just run a 4 by 10 across the trailer curb side. I probably don't need a plenum to branch my three ducts. I'll just tee into the 4 by 10. But I have to use oval ducts to fit in the oval holes in the stringers.

I thought I could balance the system with two simple floor register dampers. But with them both shut, I could overheat the furnace.

I figured exhausting warm air on the curb side with the furnace on the street side would help air circulation some. The old mid sixties furnaces had sheet metal oval ducts inside the stringers under the floor. This was for return air to the furnace. Heat just came out one opening in the sink cabinet. The original oval ducts are what gave me the idea of under floor heat distribution.

Jammer, if I could get anywhere near the delta temps you achieved in your 2010 Airstream I would be so happy. My 86 has two furnaces, three ducts each, and I have a hot living room and a cold side bath. And 70 degrees at head height, 60 degrees at the floor. It isn't the greatest.

The input received so far sounds like under floor heat distribution in sheet metal oval ducts might work okay.

Thanks for your review and suggestions.

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