Furnace Ducting for Balanced Air Flow

[dznf0g]

Here is a service manual excerpt for Atwood, covering several different models:

#3 - Minimum Ducting Requirements
The various BTU sizes of 85, 89 and 2-stage series furnaces require a minimum number of square inches of heat ducting.
The most common size of soft ducting is 4˝ diameter. Therefore, since a 4˝ duct is equal to 12 square inches, we can call
out the minimum number of ducts needed for the models of furnaces noted below.
Furnace Model Minimum Minimum
Ducting Ducts
8516, 8520 24 sq. in. 2
8525. 8531 36 sq. in. 3
8535, 8935, 8940, 1522, 2334, 2540 48 sq. in. 4
CAUTION - Ducting Installation
4˝ Flexible Hose
• each 90° bend adds the equivalent of 10 feet of ducting
• ducting should be securely attached to furnace
• each run should be as straight and short as possible
2˝ Flexible Hose
• 2 - 2˝ duct runs do not equal one 4˝ duct. A 2˝ duct is only 3 sq/inches
• 2˝ duct adapters are available
• 2˝ ducts are ideally suited for bathroom and holding tank compartments
Closeable Outlets
• a closeable outlet does not contribute to the minimum of total outlets recommended for a furnace.
Bottom Discharge
• furnace must be completely sealed to floor and plenum with a bottom discharge gasket, with no air gaps.
• if furnace is installed in middle of run, the main duct run must be a minimum of 24 sq/inches.
• if furnace is installed at the end of the run, the main duct must be 48 sq/inches.
#4 - Return Air
This return air requirement can be met in a couple of ways.
• The return air grille mounted on an inside wall of the trailer, exposed to
the cabinet area of the furnace is the most common mounting used on
the 85, 89 and 2-Stage Series furnaces.
• Another option is to provide openings at various locations in the furnace
cabinet area capable of drawing air from inside the trailer (ie. rowtered
holes at bases of sofas or walls, etc.).
The total square inches of openings must meet minimum requirements.
Do not place any types of air filters in front of or behind the return air door.
Blocking this area will substantially decrease the return air causing - less
air delivery to the heat registers - short cycle of the furnace - limiting of
the furnace.
We recommend electrical air filters that can be placed anywhere in the
open living area of the recreation vehicle. They can be purchased in most
hardware stores.
Outside of RV
Duct #1
Duct #3 Duct #2
Duct #5 Duct #6
Duct #7
Bottom discharge opening 5" x 10" (127mm x 254mm) Duct #4
Ducting Locations
85, 89 and 2-Stage
Series Furnaces
65 in2
80 in2
85, 1522 and 2334 Series
89 and 2540 Series
33 in2
79 Series
28
NOTES

[DRPREECE]

I have worked in the energy efficiency and HVAC business for years, and when it comes to getting the most out of a furnace, airflow and intake air system integrity are the two keys. Simply put, it is better to have more available duct capacity on the output than on the input and however the air gets into the input (the cold air return) needs to be sealed air-tight except where you want the air to enter it. The latter is typically not a problem in a trailer since the cold air return has no ducting (just a hole or holes in the side of the furnace box).

The critical issue for occupant comfort is the integrity of the envelope the furnace is heating - the box that it is blowing hot air into. The fan on the furnace creates a slight vacuum inside the RV. This slight vacuum will suck cold air inside the RV through ANY opening to the outside, reducing the indoor temperature and humidity - both bad for the comfort of the occupants. However, if you seal up the envelop air tight, there will be excess humidity issues - so you must ventilate it in a controlled manner.

Unfortunately, most RVs are the equivalent of a screen door when it comes to envelop integrity (expanding foam is the greatest invention - well - maybe the second after duct tape).

The energy efficiency (and comfort) mantra is:

Insulation, Infiltration, Ventilation.

So, having said that...

Before you start restricting air flow or adding another duct, make sure the ones already there are working the best they can.

While working on winterizing our new 2017 26U, I discovered the existing ducts were kinked, bent, and compressed - not good for air flow. They all were too long and the extra length was the cause of the problem.

I opened access to the ducts and pulled them straight, rearranged how they went around plumbing and wiring, how they went through bulkheads, and the position of the 2" adapters. I then shortened each duct to the length needed at the vent boot - allowing just enough slack to enable removal of the vent but not enough to become kinked when installed. All of this significantly improved air flow and balanced the system. No extra ductwork required.

As to using insulated ducts - this should not be required since all of the ducting used is inside the conditioned space. And as others have stated, the heat loss of the duct is beneficially used to heat areas behind cabinets and under the floor (tanks).

To the point of the flex duct installed by Airstream. This is not dryer vent hose. This is UL listed with a temperature and flame resistance rating. It is amazingly difficult to cut or puncture and the wire used is a strong steel. The material is almost like a heavy canvas that has been coated on both sides.

I hope this helps.

Good luck with your challenge - and there are a lot of good suggestions in this thread.

P.S. Now for the rant...

The real point to be made is - I, or any of us, should NOT need to do ANY of this. Airstream is a premium brand and we should be getting a premium product - from design to delivery. Unfortunately they have re-defined "premium" to be just "sometimes better than the lower cost brands". I was shocked at some of the things I found during winterizing (the duct work being only one). And every one of them is an issue with quality control and pride in workmanship at Jackson Center. I should not find handfuls of aluminum drill shavings anywhere inside my Airstream. Nor should wiring harnesses and ductwork that is obviously too long just be shoved into a compartment to get it to fit. There should not be screws and pop rivets rolling around behind cabinet drawers or inside of storage compartments. The list goes on. And unfortunately warranty does not cover bad quality control issues until something breaks.

[KJRitchie]

Originally Posted by KJRitchie

I have a similar issue so I purchased 3 vents that also can be opened/closed to baffle the air. Still haven't installed them yet as Airstream is in storage. I purchased from Dyers RV. My rear dinette vent blows strong. I believe that is a separate circuit from the furnace output. The other furnace output is shared by the hall way duct, the bathroom duct, the holdings tanks, and the bedroom. My hallway duct blows strong so I'm going to try closing off the hallway in increments to see if I can get more to blow out the bedroom and bath. I've purchased three of these vents to replace the three main vents in my Airstream

http://www.dyersonline.com/d-w-beige...th-damper.html

Kelvin

75F this weekend so I took my Airstream out of storage to caulk the rear windows and install my new baffled furnace ducts. There are 3 circuits: one for the dinette area, one for holding tanks and bathroom and one for the hallway and front bedroom. I closed off the hallway and there was a little more airflow into the front bedroom. Opening the hallway a little, the front bedroom duct was able the same as with the hallway wide open.

My Airstream was renovated from queen to twins and the bedroom vent was extended from the shower wall another 6 to 7 feet to the head of the street twin. The extra length seems to restrict the airflow. I checked the duct an tried taking out the right angle bends but it didn't seem to make that much difference.

My conclusion is don't waste your time replacing the vents with baffled versions.

Kelvin.

[wcrockett]

We have a 27'fb with a queen and also find that the bedroom will stays warmer if the hall way door is closed.

[KJRitchie]

When I had my queen to twin bed remodel done on my 2008 Classic 25fb I had the shop doing the work run the bedroom ducting to the front of the bedroom next to the night stand.

On our trip to Yellowstone a couple of weeks ago the flow from the front duct was weak. I had installed some baffled vents before the trip. On my furnace there is a dedicated duct for the rear dinette which has a strong flow. There are two ducts running forward to the hall duct and the bedroom duct. The hall duct has strong flow. While at Yellowstone it got cold a I ran the furnace. With the hall duct baffled down there was a slight boost to the bedroom duct. However I noticed something the furnace burner would cycle off before the desired temperature was reached. I decided to open up the hall baffle and noticed the furnace didn't cycle off. Not sure if it was the altitude (7900' at Grant Village) or the baffled hall register.

Now that I'm back home I decided to look at the ducting under the fridge. What I discovered was the long run from the furnace to the bedroom also had a 4" T fitting to funnel off the air to the bathroom on the other side of the trailer. The hall register had a straight through duct to it's register.

I decided to switch the ducts so the bedroom register had the straight run with the hall duct split with the T. I rigged up a temporary connection and turned on the furnace. Much better flow now from the bedroom register and adequate flow from the hall and bathroom registers.

I will need to get a short 4" connector and a couple of 4" clamps. I'll need to cut the duct shorter and there should be enough of that duct to connect to the hall register from the galvanize T connector.

Kelvin

[KJRitchie]

Today I purchased a 4" duct connector and a couple of clamps. I shortened the duct and joined the duct from the front bedroom to the duct that originally ran to the hallway register. I used the excess ducting to join to the T fitting and routed that to the hall register opening.

Turned the furnace on and now find the air flow from the front bedroom is much better. I can feel air coming from the street side bedroom register under the street twin bed across the aisle on the curb twin.

Photo attached shows the bottom ducting was originally running to the hallway register which was a direct connection is now switched to the front bedroom using the 4" connector and two clamps. The upper duct connects to the T fitting to feed the hall way, bath registers and holding tank. Flow is still good out the hall way register since its a shorter run to the furnace. The bath flow seems as good as before.

The furnace ducting is now better balanced.

Kelvin

[Adriel]

We're in Florida so never needed anything more than the heat pumps. But a trip through Texas during a cold winter storm has had us running the heater all day and night with the dinette too warm and the bedroom way too cold.

The bedroom vent had very little air movement and the vent under the closet seemed to have almost none. I opened the service door and discovered the duct hose completely off the vent, pointing into the floor.

I connected the duct to the vent and secured with duct tape. So far, everything is working much better. The dinette is still warmer but not uncomfortably hot. The vent under the closet is putting out decent warm airflow and the bedroom vent seems to have increased warm airflow as well.

Hopefully, we'll move out of this cold weather and this simple fix with do the trick for us.
-a-