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Old 11-20-2017, 11:38 AM   #1
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Heated underbelly?

Just wondering if the sealed underbelly has a open duct to warm it with the heater. If air from the heater is going into the underbelly how is it being replaced in the trailer? Total air output = total air input. Is it sucking cold air from the outside.
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Old 11-20-2017, 12:08 PM   #2
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Originally Posted by Highway 4x4 View Post
Just wondering if the sealed underbelly has a open duct to warm it with the heater. If air from the heater is going into the underbelly how is it being replaced in the trailer? Total air output = total air input. Is it sucking cold air from the outside.
My understanding is that the heat is just ducted through the underbelly. In other words, it blows past the three liquid tanks to reach the various floor vents. All air sucked in via the cold air return (below the fridge for our model) gets vented into that cavity to eventually come out the floor vents.

[Edit] You can see the black/gray knife valve ducting here:
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Old 11-20-2017, 12:21 PM   #3
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Yikes, if that's the way they're made I can see how that would be really inefficient. By the time the warm air passes over/around the tanks and exits the floor ducts it would be cooled off significantly. To have that much temperature difference throughout the trailer is not a good design.
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Old 11-20-2017, 01:24 PM   #4
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Originally Posted by Highway 4x4 View Post
Just wondering if the sealed underbelly has a open duct to warm it with the heater. If air from the heater is going into the underbelly how is it being replaced in the trailer? Total air output = total air input. Is it sucking cold air from the outside.
In ours there is no return air duct. With the number of openings in the basement a return air duct would an opening for cold air to enter the trailer.

Our tank heating duct enters the basement between the black and grey tanks. The FW tank is 4'-5' ahead of the grey tank. With the number of openings in the basement and with no insulation on the FW pick up line we never use the FW below 32F.

Nobody said having FW was part of 4 season use
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Old 11-20-2017, 01:57 PM   #5
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My son just bought a used Crusader 5th wheel with the heated basement & underbelly. We blocked both of those off to have more heat in the coach. Nothing in the basement needs to be kept warm. If the holding tanks need to be kept warm it's time to find another place to stay!
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Old 11-20-2017, 04:16 PM   #6
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It is my understanding that an RV Propane furnace does not have a return cold air vent as it draws air from outside.
If you take the grill panel off inside the coach and reach in around the furnace, you should be able to find a 4" duct that actually goes through the floor.
This is the heating vent into the Basement area and Wet Bay.
I also use a basic indoor/outside thermometer with a remote sensor ($20.00) positioned beside the fresh water tank,which let's me know at a glance what the temperature is in the basement.
Just checked it last week and the coach the temp was around 36 deg., the basement temp was 33 deg.
I started the furnace and the coach rose to 64 deg. and the basement rose to 54 deg.
Perfect!
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Old 11-20-2017, 08:24 PM   #7
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It is my understanding that an RV Propane furnace does not have a return cold air vent as it draws air from outside.
If you take the grill panel off inside the coach and reach in around the furnace, you should be able to find a 4" duct that actually goes through the floor.
This is the heating vent into the Basement area and Wet Bay.
I also use a basic indoor/outside thermometer with a remote sensor ($20.00) positioned beside the fresh water tank,which let's me know at a glance what the temperature is in the basement.
Just checked it last week and the coach the temp was around 36 deg., the basement temp was 33 deg.
I started the furnace and the coach rose to 64 deg. and the basement rose to 54 deg.
Perfect!
Hey B1
The only air that is drawn into the heater from outside is for the burner.

If you look at the outside cover for the furnace there are 2 openings. One is the combustion air inlet and the other is the exhaust.
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Old 11-20-2017, 09:38 PM   #8
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Just looked at the service manual for our propane furnace and it does show a Cold Air Intake on the top side of the furnace.
I could not see that from the position I was viewing it.
Thanks for correcting me.
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Old 11-20-2017, 09:46 PM   #9
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Hay I do,nt care how it works. I have down to 5 and its nice to have warm floor and the top is warm also. The furnace did.nt ful time.
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Old 11-20-2017, 10:37 PM   #10
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Originally Posted by Highway 4x4 View Post
Just wondering if the sealed underbelly has a open duct to warm it with the heater. If air from the heater is going into the underbelly how is it being replaced in the trailer? Total air output = total air input. Is it sucking cold air from the outside.

Excellent question 4X4. In 20 years of reading trailering stuff, I've never heard that asked, or answered. I believe the answer is straightforward, and you have answered it.

It would be great if Mr. ORVPDMAN could chime in. Have we heard from him lately?

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Old 11-21-2017, 12:27 AM   #11
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I have had all the floor grates off and inspected the ducts as best I can and cleaned them of dust. Can't really see where the heat enters the belly. My FW tank is in the rear and there is no opening to the belly in that area from the ducts. The ducts get very hard to see through toward the center even with a flashlight an mirror. Mine came with a cold weather package that had a electric tank heater that is switched on the main panel. If I could I would block off the heat going into the belly as I really don't need have heat go there, but can't locate that duct opening, if there is one. The heater return is below the fridge and I inspected that area also, nothing but sawdust there. I wish I would have been taking the tour the day that trailer was built.
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Old 11-21-2017, 12:51 AM   #12
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Many designs and many different solutions to this one. In my 5th, the furnace blows heated air into multiple ducts. At least one feeds a metal duct under the floor coming up through four floor vents. Another goes direct to the kitchen vent and two drop down into the lower basement area the black and gray water tanks are. The FW tank is above this level and is in an air space common with an enclosed portion of the basement. This basement space serves as the "cold" air return from the coach. There's a large grill cover in the lowest stair riser that ports air from the coach into the basement. The furnace takes this return air to reheat. I think some heated air is lost in the belly closure for the waste tanks and that air must be made up from leakage from the outside coming into the coach. RVs are not exactly air tight construction.

I also have remote temp probes in the FW tank area and outside in one of the propane tank compartments. So at a glance I can get a fair idea what's going on temperature wise. We are one of those RVers that will go out boondocking with temps well below 32 and so I really care about those temperature readings. Never had any problems, just keeping them propane tanks full! Hey, this winter we will be getting warmed up in Quartzsite, AZ for a while before going to Alaska.
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Old 11-21-2017, 07:12 AM   #13
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I asked one of the techs about how the tanks were heated via the furnace at the camp out this year and the short answer is they drill holes in the bottom of the vent plenum that carries heated air from the furnace to the floor vents. It is a simple and apparently ineffective method. Here i sit in northeastern Montana where it got down to 15 degrees last night and my water tank is frozen up. Pump works fine it just canít draw any water. It should warm up today and i have some spare water in a jug so we are fine but note that four season apparently stops at 15 degrees in my trailer. I did not get the heated tanks but i guess I will need to make that modification.
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Old 11-21-2017, 07:49 AM   #14
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I asked one of the techs about how the tanks were heated via the furnace at the camp out this year and the short answer is they drill holes in the bottom of the vent plenum that carries heated air from the furnace to the floor vents. It is a simple and apparently ineffective method. Here i sit in northeastern Montana where it got down to 15 degrees last night and my water tank is frozen up. Pump works fine it just can’t draw any water. It should warm up today and i have some spare water in a jug so we are fine but note that four season apparently stops at 15 degrees in my trailer. I did not get the heated tanks but i guess I will need to make that modification.
Ah, thanks for that. Like Highway 4x4 I've inspected my vents and could not find an exit to the basement. But a few holes drilled in the bottom of the floor plenum would be easy to miss.

I've also noticed that air rushes past the first open vents and mostly exits at the end vent. We often cover the end vent to force the air to exit at earlier vents. Perhaps that would also force more air into the basement (but make the cabin colder)?

Although really, at 15F (-9.4C) I would expect problems. Skirting, tank and hose heaters, supplemental insulation ... might be necessary.

Good luck!
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