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Old 11-13-2019, 03:24 PM   #1
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Furnace Vent with no air flow

My Columbus 340 RK has one vent that has a relatively long run from furnace. It has little to no air flow. Had it checked for obstructions and holes and dont see any. Tech suggested that because of length and curves the static pressure is preventing air flow. Does anyone manufacturer a 4" 12 volt inline duct fan? Only thing I see is a 4" Atwood Marine Bilge Blower. Would that work? It has a 160 degree operating limit. I have no idea how hot furnace air would be at approximately 2-3 feet from the furnace. Any other suggestions?
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Old 11-13-2019, 03:30 PM   #2
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I removed my vent tube and replaced with an insulated style commonly used for bath vent runs. It is encased in fiberglass batting. It helped get more heat out of the vent tube.

I did this because mice had chewed holes in the tube which was preventing/reducing flow.
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Old 11-13-2019, 05:23 PM   #3
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Is that duct rigid or flexible duct?
If it's flexible it may be too long and that will restrict a lot of flow. It should be as short as possible and all bends should be gradual and supported.
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Old 11-13-2019, 06:46 PM   #4
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typical problem, duct restrictions. Does the furnace cycle the heat on/off but leave the fan running? Place a thermometer in one of the supply grilles and watch the temperature rise, it should get up to about 120 or so and flatten out, if it keeps climbing and hits 140 or so and then the heat shuts off but fan runs it's cycling on the high limit.
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Old 11-13-2019, 08:21 PM   #5
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I had the same problem with the two registers in the living room at the far end of the TT V-lite. No air flow to 2 registers and furnace was overheating and tripping the high limit. Found that flex duct takeoff was on door side of the furnace plenum and made a 180 deg turn with flex duct around the plenum then an other 90 deg turn toward the rear of the TT. I cut open the Coraplast bottom cover to expose the furnace ductwork and plenum in a U shape. I could not cut in a takeoff on the opposite side of the plenum as there was no room. So I cut a 4" hole in the bottom of the plenum with a hole saw. Bought 3 four inch 90 deg aluminum elbows and a 2' length of 4'' duct. installed the new duct work which eliminated 6' of flex duct and patched the old 4" hole in the plenum. Taped and screwed the new duct work. Cleaned the coraplast and taped back together with gorilla tape which is holding up very good (about 18,000 miles last 1.75 years). the new duct work pushed the coralast down a little more than 1"and has not caused any problems. Now the furnace dose not trip the high limit and gives two times more air out the 2 register in the back of the trailer heating it much better.

Hope This Helps Tim
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Old 11-13-2019, 09:08 PM   #6
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On my trailer, furnace is at the back bumper. I had lousy air flow from the two registers in the kitchen/living area up front. The kitchen register didn't put out enough air to flutter a Kleenix.

On a trip the front half of my underbelly tore off. The ducts for those two registers was so long it dragged on the ground. I estimate one had at least 4 feet of excess length.
Because I took the entire underbelly off to continue on my trip, when I got home I replaced all but 2 feet of ducting with rigid. I now have acceptable air flow from the front registers.
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Old 11-14-2019, 08:31 AM   #7
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Originally Posted by Bama Rambler View Post
Is that duct rigid or flexible duct?
If it's flexible it may be too long and that will restrict a lot of flow. It should be as short as possible and all bends should be gradual and supported.
It is flexible. I didnt see it myself but tech that came to campground said it was quite long but was not restricted anywhere.
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Old 11-14-2019, 08:36 AM   #8
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typical problem, duct restrictions. Does the furnace cycle the heat on/off but leave the fan running? Place a thermometer in one of the supply grilles and watch the temperature rise, it should get up to about 120 or so and flatten out, if it keeps climbing and hits 140 or so and then the heat shuts off but fan runs it's cycling on the high limit.
I will check the cycling. If it is, would a inline fan help that situation? Our camper stays at campground year round and we will have to leave by end of November. So Im not real excited about having to run new duct work this time of year.
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Old 11-14-2019, 08:43 AM   #9
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Originally Posted by TimVWulp View Post
I had the same problem with the two registers in the living room at the far end of the TT V-lite. No air flow to 2 registers and furnace was overheating and tripping the high limit. Found that flex duct takeoff was on door side of the furnace plenum and made a 180 deg turn with flex duct around the plenum then an other 90 deg turn toward the rear of the TT. I cut open the Coraplast bottom cover to expose the furnace ductwork and plenum in a U shape. I could not cut in a takeoff on the opposite side of the plenum as there was no room. So I cut a 4" hole in the bottom of the plenum with a hole saw. Bought 3 four inch 90 deg aluminum elbows and a 2' length of 4'' duct. installed the new duct work which eliminated 6' of flex duct and patched the old 4" hole in the plenum. Taped and screwed the new duct work. Cleaned the coraplast and taped back together with gorilla tape which is holding up very good (about 18,000 miles last 1.75 years). the new duct work pushed the coralast down a little more than 1"and has not caused any problems. Now the furnace dose not trip the high limit and gives two times more air out the 2 register in the back of the trailer heating it much better.

Hope This Helps Tim
This is our first winter in it and we will be out of it in a couple of weeks. Based upon yours and others experience I may have to re-do that section next spring when its a little more agreeable to work outside on the ground. The area involved has 3 vents. Two are putting out a lot of heat and just the one is causing an issue.
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Old 11-14-2019, 08:52 AM   #10
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On my trailer, furnace is at the back bumper. I had lousy air flow from the two registers in the kitchen/living area up front. The kitchen register didn't put out enough air to flutter a Kleenix.

On a trip the front half of my underbelly tore off. The ducts for those two registers was so long it dragged on the ground. I estimate one had at least 4 feet of excess length.
Because I took the entire underbelly off to continue on my trip, when I got home I replaced all but 2 feet of ducting with rigid. I now have acceptable air flow from the front registers.
It looks like this is very common issue and most everyone is addressing with re-doing the duct work. If there is no other way to address it then I'll get by a couple more weeks and re-work it next spring. Thanks!
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Old 11-14-2019, 10:19 AM   #11
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very common problem as you can see. HVAC is my expertise and career, BS degree, etc. Fans put out air based on volume and pressure, just like a garden hose, more pressure in a given size hose means more fluid. Air is a fluid. The airflow (CFM) is inversely proportional to the static pressure (SP) so as static goes up CFM goes down but it is not directly proportional but is a square root function (fan laws). Reducing static pressure result when you decrease the pressure losses in the duct by either larger ducts or smoother transitions, elbows, etc. As you can see by others the best correction is to fix the crappy installation (imagine that!). Sometimes there's just no way to do that so might consider adding another supply duct, maybe in a cabinet right off the furnace compartment. They make the 3" round diffusers and one model has a butterfly damper so you can turn it on/off.
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Old 11-14-2019, 11:24 AM   #12
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One problem that's inherent to the ducting used in most trailers is that it's flexible duct, like a stretched out slinky covered with a vinyl material.

Moving air over all those ridges in the duct is extremely inefficient.

If possible any long, straight, run of the flex duct should be replaced with metal duct like that used for dryer vents (assuming standard 3" duct size). Just use the minimum amount of "slinky duct" to make necessary bends at beginning or end although metal 90's and 45's would be far more efficient. Seal joints with aluminum adhesive backed tape.

Also look for sharp bends, especially where the duct in question is attached to the furnace plenum. I just fixed the same problem in my smaller TT. Rear "register" was barely blowing air. Removed old duct and re-routed. Removed 3 90 degree bends and removed the kinked portions at both the furnace and the register. Whoever installed the duct had no clue about air flow.

Yes, one could install an axial fan that might move more heated air but that's just adding another source of noise and continuing maintenance.

Improving ducting will eliminate the need for a fan/blower.
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Old 11-14-2019, 11:29 AM   #13
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Originally Posted by tlhagood View Post
My Columbus 340 RK has one vent that has a relatively long run from furnace. It has little to no air flow. Had it checked for obstructions and holes and dont see any. Tech suggested that because of length and curves the static pressure is preventing air flow. Does anyone manufacturer a 4" 12 volt inline duct fan? Only thing I see is a 4" Atwood Marine Bilge Blower. Would that work? It has a 160 degree operating limit. I have no idea how hot furnace air would be at approximately 2-3 feet from the furnace. Any other suggestions?
Hi!

There are lots of computer case fans that are 12 volt that you can look at.
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Old 11-14-2019, 11:42 AM   #14
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Hi!

There are lots of computer case fans that are 12 volt that you can look at.
Just remember, rather than moving cool air to transfer heat out, the air in a heating duct can easily be well over 100 degrees. Plastic computer fans may not hold up to the heat.
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Old 11-15-2019, 12:46 PM   #15
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Others have given advice on how to fix an airflow problem by replacing the flexible duct with a shorter and smoother duct to reduce the airflow restriction in the duct to the affected vent. My trailer had a different problem. When my 5th-wheel trailer was brand new, almost no air came from the heat vent farthest from the furnace but there was way too much air flow from the bedroom heat vent. Since it was under warranty, I took it back to the dealer who put a restrictor in the bedroom’s duct. This dramatically increased the airflow from the rear vent and reduced the air volume to an appropriate level from the bedroom vent. I now have plenty of heat, even when it’s very cold outside.

If you have way too much air flow through some vents then your problem could be similar to mine and require you balance the airflow by inserting a restriction of some type into the high-flow duct, which will direct more air to the other vents. If the airflow to the other vents seems appropriate, make sure the duct in question is properly connected to the furnace’s air plenum and not torn or kinked.

I wouldn’t want a bilge blower running because they are too noisy and it would add an additional load on my battery while boondocking.
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Old 11-15-2019, 01:26 PM   #16
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So you're thinking of something like a duct booster.
Something like this.

Click image for larger version

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ID:	219214

These are normally 120V and this particular one is 6"
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Old 11-15-2019, 01:32 PM   #17
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Couple of reducers and it would probably work..
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Old 11-15-2019, 01:44 PM   #18
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So you're thinking of something like a duct booster.
Something like this.

Attachment 219214

These are normally 120V and this particular one is 6"

Doubt you'd find a spot for that, pretty big. And if too close to register would be noisy. I've not seen a 12v version
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Old 11-15-2019, 02:00 PM   #19
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. Since it was under warranty, I took it back to the dealer who put a restrictor in the bedroomís duct. This dramatically increased the airflow from the rear vent and reduced the air volume to an appropriate level from the bedroom vent. I now have plenty of heat, even when itís very cold outside.

If you have way too much air flow through some vents then your problem could be similar to mine and require you balance the airflow by inserting a restriction of some type into the high-flow duct, which will direct more air to the other vents. If the airflow to the other vents seems appropriate, make sure the duct in question is properly connected to the furnaceís air plenum and not torn or kinked.

I wouldnít want a bilge blower running because they are too noisy and it would add an additional load on my battery while boondocking.

This is a good way to balance air flow as long as it doesn't reduce air flow over the heat exchanger in the furnace. The duct requirements shown on the installation instructions ASSUME unrestricted air flow through the number of ducts they specify.

With larger trailers that may have more than just two ducts connected to the furnace, or a plenum on the back of the furnace allowing for distribution to under floor ducting and multiple other outputs, adding duct dampers may not be an issue at all.

It all boils down to how much air is flowing through the furnace and if insufficient other problems present themselves. (high limit cycling, Over-temp shut down, etc).
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Old 11-15-2019, 02:03 PM   #20
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So you're thinking of something like a duct booster.
Something like this.

Attachment 219214

These are normally 120V and this particular one is 6"
I used to have one of these in my basement in Colorado for radon gas control under the suspended wood floor. They are more annoying than a neighbors howling dog. Quiet should never be part of their product description.
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