Furnace Ducting for Balanced Air Flow

[kscherzi]

I have a question, has anybody tried to modify their furnace ducting to better balance heat throughout the trailer? My 27FB furnace is at the rear of the camper while the bedroom is towards the front. The vent under the dinette next to the furnace blows hot air like a hurricane, while the vent in the bedroom has a gentler flow of lukewarm air. The bathroom vent appears to be for looks only, I can hardly tell if anything is coming out.

I've looked, and the ducting is this cheap un-insulated spiral plastic flexible pipe. There's a 4" pipe to the bedroom and a 2" pipe to the bathroom. I've thought about a putting a restriction under the dinette, but I worry about limiting overall air flow and effects of the system.

Mostly I'm curious as it does not look like an easy project to replace with solid or insulated ducting, but if someone else has tried with good results I'd like to hear about it.

[DaveP]

If you travel in cold climates, don't forget that the uninsulated ducting helps keep your tanks from freezing.

[dznf0g]

Yes, but not in your layout. Well covered recently, for rear bedrooms.

[HowieE]

Since all of the duct work is inside the trailer any heat loss during transmission is still additive to the trailer.

I would not suggest attempting to throttle one section to increase the delivery to another. These heaters work on the edge of efficiency to begin with and you may end up running the heater exchange at too high a temperature and causing problems.

If you can put a Y in the duct, close to the heater, and running an additional duct, increasing total flow, to the area you want more heat in that may help. The biggest loss is the length and friction within the duct.

[dznf0g]

What HowieE said...do not restrict flow to less than total area dictated in the installation manual. I ran an ADDITIONAL 4" duct to the bedroom and installed a grille with an adjustable shutter on ONLY ONE of the two living room ducts.

[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

[dkottum]

What Howie said. Don't restrict air flow over the furnace fire chamber.

We have the same issue and live in this Airstream all winter in the Southwestern states. Generally we want more heat in the living area than the bedroom, so it works fine as is.

Our primary heat is from a Dyson space heater, and the furnace is a supplemental heater. We move the space heater around so during the evening it blows air towards the living area, and towards the bedroom area at night. We also use an electric blanket. When boon docking we turn the furnace way down at night, and have an extra blanket for the bed.

[Shiloh4570]

On my 27 FB, I accessed under the closet through the panel on the closet floor and discovered someone had stepped on the 4" duct about 10 inches before it reached the louvered outlet. I thought that odd, so straightened it out, opening it up and re-attached to the outlet. I get more air flow in the bedroom now but can't help wondering if I did the right thing. Maybe the stomp on the hose is the factory fine-tune method to balance out air-flow?? If you start adding hoses and outlets, you likely won't have uniformity in your temperature from front to back.

[HowieE]

Airstream has some crude construction techniques but I don't think 'Stomp' is one of them.

Restore the duck to the original shape.

[wcrockett]

Originally Posted by dkottum

What Howie said. Don't restrict air flow over the furnace fire chamber.

We have the same issue and live in this Airstream all winter in the Southwestern states. Generally we want more heat in the living area than the bedroom, so it works fine as is.

Our primary heat is from a Dyson space heater, and the furnace is a supplemental heater. We move the space heater around so during the evening it blows air towards the living area, and towards the bedroom area at night. We also use an electric blanket. When boon docking we turn the furnace way down at night, and have an extra blanket for the bed.

We have a 2012 27' FB and I have been blocking the vent under the dinette on cold nights (temps below 32F) for the last 2 winters. Doing that increases the flow to the bedroom and to the vent going into the tanks. Seems to work just fine for us. We also use an electric heater to partially heat when we have shore power but we are careful about making sure the furnace also cycles on as needed to heat the tanks. We set our thermostat to 50 every night then I warm up the trailer when I get up each morning.

[Bigventure]

The correct solution is to replace the grills with one that have dampers on them. You are not restricting the air flow only redistributing it.

[dznf0g]

Originally Posted by Bigventure

The correct solution is to replace the grills with one that have dampers on them. You are not restricting the air flow only redistributing it.

No, if you have reduced the total opening square inches, you are restricting airflow. Read the installation manual.

[kscherzi]

Reading about flexible ducting installation they say to pull the ducting taught. So I went to the trailer today and yanked about 4 feet out by stretching, cut it and reattached it. Maybe that'll help. I also noticed that the duct to the bedroom is split to also serve the tank area. That too cuts flow to the bedroom. I might be tempted to block that diversion somehow. I've never camped, nor intend, where it's below freezing for days at a time.

[Bigventure]

Originally Posted by dznf0g

No, if you have reduced the total opening square inches, you are restricting airflow. Read the installation manual.

There are a lot of factors involved. Restricting air flow to certain outlets only raises the pressure to the remaining outlets.

Ideally the furnace would be in the middle of the coach and the duct runs would be all equal lengths. But that's not always possible. So you end up with runs that are 6" and other that are 15'. The flex that Airstream uses is also restrictive and noisy. The long runs should be in smooth pipe to increase air flow and lower the noise.

In are trailers there are no dampers to allow for balancing. So the next best thing is to dampen the outlets.

For a detailed explanation see the link provided.

https://www.titus-hvac.com/file/1107...ng_AG_2016.pdf

[Jim Foster]

Several years ago this issue was discussed regarding warm air to rear bath Airstreams. Same issue, different layout.
Air does not push well at all but is pulled very easily. I installed a 12v boat bilge blower fan in the duct hose at the outlet in the bath. I put the switch in a convenient place in the bath room. I turn the furnace on, then the blower. The bath warms up nicely. Pulling the warm air into the bathroom robs some warm air that would have gone to the living room, but that is exactly what I wanted.
When you push air it will escape at it's first opportunity.
Down side, the bilge blower is far from silent, which for warming up the bathroom is not an issue. Might be too noisy for a bedroom.

[Jammer]

Originally Posted by kscherzi

I have a question, has anybody tried to modify their furnace ducting to better balance heat throughout the trailer?

I believe I was one of the pioneers in this area. I added ductwork to my then-new trailer in 2010. There are photos and a writeup on the forum here somewhere. At least one other owner has copied the mod.

Since your trailer has a different layout, the details will be different, but the principles remain the same. In my trailer, Airstream installed the minimum amount of ductwork the furnace manufacturer recommended. I would imagine that is the case for yours. Therefore, you have to add ducts before you restrict existing ducts, or you'll reduce the airflow, and internal temperatures will go up, causing safety problems and possibly causing the limit switch to trip under some operating conditions.

I've looked, and the ducting is this cheap un-insulated spiral plastic flexible pipe. There's a 4" pipe to the bedroom and a 2" pipe to the bathroom. I've thought about a putting a restriction under the dinette, but I worry about limiting overall air flow and effects of the system.

The 4" uninsulated ductwork is widely used. In most cases it is the best product that will fit the space and withstand the temperatures. There is very little airflow in 2" duct, less than 1/4 what you get with 4", so it's not good for much except keeping pipes from freezing.

Mostly I'm curious as it does not look like an easy project to replace with solid or insulated ducting, but if someone else has tried with good results I'd like to hear about it.

It can be done.

Broadly speaking, here's what you do:

1) Find room to run another duct to the far end of the trailer, and choose a location for another air outlet. Use the same spiral flex duct. It is OK to mash the duct out of round a little bit if necessary to fit the space.

2) Get another duct collar for the furnace, a length of duct, another air outlet, and a clamp. I got my parts at http://www.dyersonline.com/ but there are plenty of places that have them.

3) Cut holes, run ducts. A hole saw works best. I used a hole saw and a right-angle drill to cut holes in bulkheads that otherwise would have been inaccessible.

4) Restrict existing outlets that are right next to the furnace. If you have room to replace the short duct connecting these to the furnace with something longer with twists and turns, you'll reduce airflow and quiet things down, or you can get an adjustable vent.

5) If there are any Ts in your ductwork see if you can remove them and replace them with a "home run" to the furnace.

6) If you are stuck with the 2" run then try to change it to stay in 4" duct from the furnace as far as possible then put on a reducer when you're at the point where you can't get past a restriction with out it. You can get 4"-2" reducers at home centers or on line.

There's a lot of loss of both airflow and temperature over the length of the duct and there's not much you can do about it other than add more ducts to compensate. One fact to consider though is that the temperature loss is greatest when the furnace first starts. Over time as the furnace runs the floor, walls, cabinets that the ducts adjoin will warm up considerably and this will improve the temperature at the distant outlet.

I experimented with non-flexible metal ducts and found that there was minimal benefit and considerable difficulty of installation.

[KJRitchie]

So you are saying punch out another duct hole in the furnace casing and run a dedicated flexible duct to the bedroom? I believe there is only two air ducts in my furnance, one to the dinette and one that goes forward which supplies the hallway, bath, holding tank and front bedroom registers.

Kelvin

[SSquared]

Originally Posted by KJRitchie

So you are saying punch out another duct hole in the furnace casing and run a dedicated flexible duct to the bedroom? I believe there is only two air ducts in my furnance, one to the dinette and one that goes forward which supplies the hallway, bath, holding tank and front bedroom registers.

Kelvin

I have never seen a 25FB Classic, but I suspect you have more than two ducts.

I have a 2013 25FB Flying Cloud. The furnace is street side under the bench. I have found four ducts.

There is one duct that runs a short distance to the rear. This dumps out under the dinette.

There are two 4 inch ducts that run to the area under the closet. One dumps into the hallway, the other runs under/behind the shower and dumps into the bedroom, under the street side twin bed. Under the closet, one of those two ducts forks off a smaller duct that runs under the floor. I believe this feeds the (weak) bathroom air register. I don't know if this dumps any air into the belly.

There is a hidden fourth duct. It comes off the end of the furnace that is facing the center of the trailer. It dives under the floor. I got a picture of it with my cell phone by opening the cabinet that is forward of the furnace, and reaching my hand in and up to where the camera lens and flash could see into the hidden space.

[kscherzi]

I think the hidden duct goes to the bathroom. On my 27FB I traced the bathroom duct rearward to under the sink. Then it dives under the floor. The duct leading to the bedroom is split, with a 2" piece shoved into the tank area. It terminates just under of the floor.

[dznf0g]

Originally Posted by Bigventure

There are a lot of factors involved. Restricting air flow to certain outlets only raises the pressure to the remaining outlets.

Ideally the furnace would be in the middle of the coach and the duct runs would be all equal lengths. But that's not always possible. So you end up with runs that are 6" and other that are 15'. The flex that Airstream uses is also restrictive and noisy. The long runs should be in smooth pipe to increase air flow and lower the noise.

In are trailers there are no dampers to allow for balancing. So the next best thing is to dampen the outlets.

For a detailed explanation see the link provided.

https://www.titus-hvac.com/file/1107...ng_AG_2016.pdf

And that higher air pressure is caused by greater restriction. That higher pressure will not deliver the same cubic feet/ minute of air as a greater opening surface area at lower pressure. Again, please read the installation manual for your particular furnace. If AS built to the minimum duct opening surface area (4-4" ducts in my case) there should be NO shuttering or restricting in any of those four ducts. The only safe way to close off (even partially) on of those 4 ducts is to add another in the location you want more heat and airflow.

Yes, in your link....if a house is built with twice the minimum duct surface area required by the furnace manufacturer, one can indeed balance by baffling and shuttering registers. Your AS isn't built that way.