Helping the Air Conditioner(s)

[Robev]

I have a 2020 3120rl and have just pulled the LP furnace out and am replacing it with a diesel heater. The air to be heated is drawn thru under the stairs on ours also and so of that air is used to supply the flame in the primary side of the heat exchanger as well. To compliment that primary air source, some air is also drawn from around the outside of the exhaust port that sticks out the drivers side of the rv, next to the water heater. Leaving the the stairs blocked off during winter use, while using the furnace creates many safety issues. Remembering to remove it at the end of summer is very important.

[Carl n Susan]

Originally Posted by McRod

The suburban furnace air intake is clearly labeled on the exterior of the vehicle - along with the exhaust. There is no magical way to suck warm air from inside your RV back into the furnace.

True the exterior ports bring in outside air and exhaust it out. But that is only air for the sealed combustion unit.

Suburban furnaces installed in recreational vehicles are classified as Direct Vent Sealed Combustion Furnaces. A forced draft furnace utilizes a sealed combustion chamber which is vented to the outside atmosphere. The intake air for combustion is also taken from outdoors and is completely isolated from the room air. A motor is used to drive an impeller wheel to draw intake air into the chamber to support combustion and force the exhaust gases through the furnace chamber to the outside atmosphere.

Ref: Page 3 - Suburban RV Furnaces Service & Training Manual (see link to PDF below)

The furnace blower box has several ducts of various sizes that are ducted to push heat to directed areas within the rig. In my rig, one of the 2" ducts goes directly to the water closet, which is the backside of where you connect your water lines and houses the water pump and water lines

Also true, there are output vent lines to the tank area along with the floor ducts and bathroom floor vents.

The fallacy in your thought process is that the stairs act as some sort of "return air" for the furnace. They do not. The furnace gets it's air from the air intake on the exterior of your vehicle. So no. You don't need to unblock them for the furnace's sake.

Not exactly, actually not at all.

The cabinet that the furnace may be installed in will have louvers or openings for the return air back to the furnace. When the furnace is installed, it is imperative that the return air louvers on the furnace cabinet opening are not obstructed. Usually, these furnaces are installed under a counter, sofa or bed in order to be out of the way. A grille or opening must be built into the cabinetry or into the base area of the sofa or bed. Return air from the living area of the trailer is drawn in through the grille and into the return air openings in the furnace cabinet. Figure 20 illustrates the return air circulation of the furnace.

Ref: Page 19 -Suburban RV Furnaces Service & Training Manual (see link to PDF below)

You may want to consider reviewing the way your RV heats the basement areas if you plan on camping in freezing weather. The stair louvers assist with keeping that area warm.

I am fully aware on how the RV furnace heats the basement and the coach. I dry camp a lot in the winter and I rarely use the RV furnace due to its inefficient use of propane and the consumption of battery by the fan.

https://manuals.heartlandowners.org/...11-05-2015.pdf

[pitman44]

Originally Posted by fulltilt

Wow - I spent 45 years of my working life in Commercial Heating, & Airconditioning...I am speechless !

LOL I'm thinking the same thing remembering all the failed furnaces I've seen with plugged filters or returns covered.

[pitman44]

Originally Posted by McRod

The fallacy in your thought process is that the stairs act as some sort of "return air" for the furnace. They do not. The furnace gets it's air from the air intake on the exterior of your vehicle. So no. You don't need to unblock them for the furnace's sake.

You may want to consider reviewing the way your RV heats the basement areas if you plan on camping in freezing weather. The stair louvers assist with keeping that area warm.

That small vent on the outside of the camper is for combustion air, not return air. The stair louvers are indeed return air for the furnace.

[SteveandTerri]

Carl's reply is well done.

Terminology gets confusing when describing furnaces.....

Furnaces have 2 air entry pathways: Combustion Air and Return Air.

(a) air for combustion is drawn from the outside into the combustion chamber/burner. Propane is mixed with the combustion air and it burns before passing through the heat exchanger. After passing through the heat exchanger, the products of combustion are then expelled directly back to the outdoors. The products of combustion never enter the occupied space. If it did, carbon monoxide poisoning would be a problem.

(b) Return air comes from the occupied space, passes through the heat exchanger, and is then blown back into the occupied space by the circulation fan.

Combustion air and return air never mix in the heat exchanger. The combustion air pathway is completely sealed. The louvers in the face of the stairs are in the pathway through which 'return air' flows from the occupied space back to the furnace.

...I was an HVAC Engineer for 40 years... now retired....

[McRod]

Not exactly, actually not at all.

What your looking at labeled return air has nothing to do with the furnace combustion. 100% of the combustible air comes from the external air intake. All heating devices require spacing to prevent fire. These furnace's that we, as Montana owners, have installed are in huge basements with sufficient air to circulate between the heated box of the furnace and the surrounding areas.

The stair louvers in question are not the louvers cited in the manual. The louvers cited are built into the furnace unit and should not be blocked off. Your basement has more than sufficient air from holes into the underbelly and door gaps to keep the furnace operating safe.

If your furnace is under a small cabinet or seat, as in many small campers, then the louvers on the cabinet enclosure would be necessary. In our case, it's not needed!

[MARK A]

The stair 'vents' are a passive type vent. If you had NO windows open, and didn't open the door or anything the interior would pressurize and inhibit the efficiency of the furnace. But they aren't necessarily for the furnace.

Mark

[McRod]

Originally Posted by MARK A

The stair 'vents' are a passive type vent. If you had NO windows open, and didn't open the door or anything the interior would pressurize and inhibit the efficiency of the furnace. But they aren't necessarily for the furnace.

Mark

Yes, theoretically in a vacuum. - but if true you would eventually run out of air even from the cabin space. We all know there are sufficient air gaps built into the Montana RVs, that the OPs question can be answered such that there is no need to worry about the stair vents. The conjecture by many to prove points on this web site is frustrating to readers and can result in lack of participation and new readers.

The OP wants a simple answer, yes or no, and doesn't need to be schooled in the aerospace engineering of hermetically sealed vacuum spaces of heating elements. It's pointless.

[DebNJim B]

All I know is our first winter there was a nasty draft coming from those stair vents so I blocked them off. Then we got a night that dropped into the 20's and the pipes behind the convenience center froze. When I got in there to thaw them out I discovered a large gap in the J wrap that the factory failed to seal with insulation. I plugged it and the basement warmed up and the pipes thawed. Then I read that the stair vents were for return air so I unblocked them and there now was no draft and furnace performance returned and no freeze ups since.

Just sayin' that's my experience.