Return air filters

CUFFS054 · 10-29-2012, 12:41 PM

In my reserch I'm seeing that most (all?) gas RV furnaces don't use return air filters. Is there a reason?

lorax614 · 10-29-2012, 02:02 PM

Where does the return air enter the furnace? Ours doesn't have return ducts. At least I don't think it does. I assume that it draws air directly into the furnace, in my case from behind the couch.

SteveC7010 · 10-29-2012, 02:49 PM

Quote:

Originally Posted by lorax614

Where does the return air enter the furnace? Ours doesn't have return ducts. At least I don't think it does. I assume that it draws air directly into the furnace, in my case from behind the couch.

You are correct. Typically, there is a large vent in a wall between the furnace and the living area. The furnace pulls cold air in through that vent and then distributes the heated air out through the ducting.

Bushman512000 · 10-30-2012, 07:03 AM

I do not think any amount of air returns to the furnance .They have a fresh air to the furnace outside where the exhust is look and see the 2 vents one is hot one is cold.But if You like You can install a filter in the furnace install service cover inside the trailer...not that it will do any good ..I asked about that years ago and was told a closed system why?I do think some thing to do with size of area and safety not a lot of air to reuse that makes Me think there right.Fresh air from out side yep!...Bushman

Paddler · 10-30-2012, 09:26 AM

I'm pretty sure they are closed systems. The furnace intake is cold air from inside the trailer, which is ducted around the outside of the furnace combustion chamber, where it absorbs the heat then returned to the trailer.

On the exterior wall of the trailer there maybe a cold input duct and hot exhaust duct but the cold input duct is for providing air to the furnace combustion chamber. Think about it, would you want the fresh air intake for your heating system to be within a foot of the furnace exhaust? Carbon monoxide poisoning would be a major risk.

geo · 10-30-2012, 11:08 AM

Paddler is correct. There is an outside fitting with two ports - one for intake air to the combustion chamber and one for exhaust from the chamber. Some on the Forum have had problems with the "sail switch" that is inside of the air intake - it wants to make sure that the intake fan is on and running before allowing gas to the combustion chamber and ignition.

These two ports have nothing to do with the circulating air inside the RV. If you look at your furnace unit, one end of it is basically a distribution plenum with all sorts of aluminum ducts connected. On the Alpine, four of these ducts run over to a "bulkhead connector" that passes through the floor to connect to insulated flexible ducts. This is probably a safety feature in that the plastic with insulation ducts are kept away from the intense heat coming off the furnace. Only the aluminum ducting gets the "real heat". I would bet that there is some variation of this type of connection on all the RVs.

At the other end of the furnace - then end that is closest to the outside wall - the outside walls of the furnace are basically all vented. (By the way, these little slat openings are pretty darned sharp! I've got a nasty cut on my finger to prove it! Got it this past weekend while trying to change fuses in the convertor!) If you peer into these slat openings, you will see a squirrel cage fan - right there on the other side of the slats! That is the "return air" point for the furnace. No filter, no way to attach a filter that wouldn't get fried sunny-side up, notta. And the slats discussed earlier under the stairs or elsewhere to allow air into the basement area . . . well, they are pretty worthless without any modifications. By looking around, you will see that there are plenty of cut openings in the floor providing air flow access to the underbelly. And speaking only of the Alpine, the whole area behind the WaterWorks is just a huge air communication pass-through between the underbelly and the utility basement. If you block the "return air slats" as Michele has suggested, you will not really cause any harm nor keep the furnace from getting any circulation air. You will stop some of the cold drafts caused by lack of insulation and air restriction from the underbelly. But the furnace will just draw air from the basement and underbelly and force it into the living quarters of the RV. Now, depending upon how well sealed the living quarters are, this will just force air out the holes around the slides, plumbing, etc. to the outside. There will not be any air returned to the basement, or a percentage of the air returned to the basement, that is warmed to be warmed again and recirculated.

So, the "return air" back to the furnace is basically the whole side of the furnace - the left wall side as you look at it from the outside.

Ron

SteveC7010 · 10-30-2012, 11:09 AM

Quote:

Originally Posted by Bushman512000

I do not think any amount of air returns to the furnance .They have a fresh air to the furnace outside where the exhust is look and see the 2 vents one is hot one is cold.But if You like You can install a filter in the furnace install service cover inside the trailer...not that it will do any good ..I asked about that years ago and was told a closed system why?I do think some thing to do with size of area and safety not a lot of air to reuse that makes Me think there right.Fresh air from out side yep!...Bushman

Those are for combustion air only. The furnace pulls air for combustion in one of the ports and exhausts through the other. The heated air for warming the interior of the trailer is circulated through a different part of the mechanism as already described..

Bushman512000 · 10-30-2012, 02:50 PM

Quote:

Originally Posted by SteveC7010

Those are for combustion air only. The furnace pulls air for combustion in one of the ports and exhausts through the other. The heated air for warming the interior of the trailer is circulated through a different part of the mechanism as already described..

Air in to burn propane not to change air in trailer Gas needs air for fire I have had furnaces out for fan repairs and seen no air return at all.But I just may have missed the vent Think Not lol Bushman not into a chewing match...THE END

Brantlaker · 10-30-2012, 02:51 PM

The combustion part of the furnace is a closed system the hot combustion goes to a heat exchanger were the fan circulates air around it and then pushes it through the ducts to heat the RV. You can as have secured a filter (cut to fit) type securing it to the back of the cabin (RV) inlet grill.I use a good commercial type that I get at a plumbing supply store.

chuck&gail · 10-31-2012, 04:54 PM

RV furnaces should not use filters, ask the manufacturer and I'll bet they say SHOULD NEVER. Why? Because the furnace is designed for a certain air flow over the heat exchanger. If the airflow is too low, the heat exchanger can crack, leaking poisonous gas into the RV, or, if you are lucky, the furnace will just refuse to run because there is too little air flow to enable the sail switch.

Why are they built that way? Simple. If designed to have a filter, the size of the FAN would need to be increased. That would, of course, require the RV to have larger batteries. It is simply a cost and weight issue.

Since most boondockers are already unhappy about the power it takes to run the furnace, requiring a lot more power just so a filter could be added would be somewhat self defeating I think.

**JRTJH** · 11-01-2012, 06:40 AM

Quote:

Originally Posted by Brantlaker

The combustion part of the furnace is a closed system the hot combustion goes to a heat exchanger were the fan circulates air around it and then pushes it through the ducts to heat the RV. You can as have secured a filter (cut to fit) type securing it to the back of the cabin (RV) inlet grill.I use a good commercial type that I get at a plumbing supply store.

Brantlaker,

You may find in your specific RV that the furnace is mounted in one cabinet only, but the way most RV furnaces are installed, there is an opening along the bottom of the cabinets where the furnace is secured. There is, as you say, a grillwork in front of the furnace where the majority of the air comes from to flow over the furnace heat exchanger, however, any time you open one of the cabinet doors along that row of cabinets you get airflow that would be unfiltered. Granted, you may be able to filter most of the air using your method of installing a filter, but it's not going to be a foolproof way of assuring all the air is filtered.

Additionally, as indicated by Chuck, installing a filter makes the system work harder to draw air, may increase the temp of the heat exchanger and cause furnace damage. In 40+ years of RVing, I've never had a heat exchanger plug from too much dirt/dust. Filters in homes work to help clean the air and keep dust to a minumum, but in an RV, with the "open wall build system" (all the holes and leaks from construction) I'm not sure that you can keep the dust and dirt out no matter how many filters you install.

I'd be careful with a filter in the return air system because of decreased air flow and check your cabinet construction carefully, you most likely will find that the grill is only one of many places where furnace air comes from.

SteveC7010 · 11-01-2012, 07:19 AM

Quote:

Originally Posted by JRTJH

I'd be careful with a filter in the return air system because of decreased air flow and check your cabinet construction carefully, you most likely will find that the grill is only one of many places where furnace air comes from.

Fully agree!

In my Cougar, only a flimsy piece of 1/4" luan separates the furnace from the basement area of the fiver, and that is not air tight due to gaps in the visible framing and a long open area that terminates by the water pump. There are several obvious openings into the underbelly area, too.

I would suggest that anyone desiring air filtration in their TT consider that your air conditioner does have a rudimentary filter on it and a ducted circulation system totally separate from the furnace ducting. It should be possible in place a better quality filter there without the problems associated with doing so on the furnace.

There are also free-standing filtration units available.

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