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Old 10-16-2017, 10:25 AM   #1
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Low forced air furnace flow

Have an 08 Voyage with a Suburban SH 35 propane furnace. This is new motorhome for us. It seems that when we run the gas furnace we do not get much air volume (low air flow) from the floor heat ducts. Ducts just don’t seem to blow much warm air. Is this normal? Or should we be feeling good flow of hot air from the ducts? Is there a chance something is blocking airflow? We do get heat but seems very inefficient. Any suggestions before we go to dreaded RV service shop?
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Old 10-16-2017, 10:32 AM   #2
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Not normal duct closes to furnace should have highest air flow. Did you check the filter?
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Old 10-16-2017, 10:45 AM   #3
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Low air flow from RV furnaces can also be caused by distribution hoses (usually 3-4 inch diameter) coming loose and falling off the connections at the furnace or floor register. Sometimes there is a smaller diameter hose (2") attached to the furnace as well to provide warm air into the water bay/underbelly.

If you can reach/see the backside of the furnace you can determine if any of the hoses have fallen off or if you have the 2" water bay hose.

Secondly, the fan/blower motor may be getting old and turning slower. It is a 12V and turns slower with lower voltage - does it increase in speed while running on only battery and then with plugged in to shore power? Voltage usually goes from ~12.5V to 13.6/14.2 when the converter kicks in and there may be a discernible increase in fan speed. If not the blower motor may be getting weak/restricted.

Not sure if you have an inlet filter.

As mentioned - air balancing can be an issue with ducts closer to the furnace providing more air flow and ducts farther away having lower air flow. I balanced previous SOB RVs with adjustable floor registers, forcing air to farther away ducts.

My thoughts.
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Old 10-16-2017, 11:51 AM   #4
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Airflow can be subjective since you are talking about a 12 volt blower designed to run for extended periods of time with the lowest possible current draw and still get the job done when on just the coach batteries. You won't get a hurricane blast of heat and get two batteries to run it all night when dry camping. Plus high volume blasts coming out of the ducts can drastically reduce the efficiency of the furnace by cooling the air as it exits the vents.

That said does the 08 Voyage have Basement Air? If it does you should hear a definitive clunk as the metal flapper switches between the Cold Air and Hot Air plenum however if its bound up with dust it won't switch over completely and will reduce air flow. A dirty air filter that will only moderately reduce airflow on the Basement AC can greatly reduce airflow from the weaker 12 volt blower.
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Old 10-16-2017, 12:36 PM   #5
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Or, torn/kinked hoses.....The plenum that the hoses in mine connect to, has 4 outlets.....two on the back and one on each side.......the squirrel cage type fans have way better flow thru the two off the end, as they're inline with the flow.....the side ones, not much.....
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Old 10-16-2017, 01:03 PM   #6
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Thanks for the info. Do you know if the furnace is easily accessible under the bed?
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Old 10-16-2017, 01:55 PM   #7
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Does bed have storage underneath?
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Old 10-16-2017, 02:54 PM   #8
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The air being pushed out the furnace by the blower is going multiple places and at any given place you are not going to feel a lot of air movement.

For your type of Class A Winnebago, for the hot air side, chances are you have a rectangular metal duct embedded in the floor running from a rectangular floor vent in the bedroom to a rectangular floor vent in the front end living room. In my 30' Winnebago, there are ducts on both ends of the living room, for a total of 3 rectangular heat ducts. The furnace output is connected to the rectangular duct somewhere in the middle right above the furnace. Chances are there is no easy acess to view or inspect where the furnace output connects to the floor duct, although there may be a hard to get to removable panel held by screws in the basement behind the furnace. There will be a small round duct conneted to the rectangular duct in the floor feeding a small round vent in your bathroom. There will be a small round hole connected to the rectangular duct in the floor that blows into the basement to keep the basement above freezing . You can unscrew the rectangular floor ducts and check for and remove any debris in the floor duct. When I did this on mine debris had fallen into the duct during assembly and I got out some stuff, mostly wood debris and some screws when I vac'd it out.

For the return air side, one side or the other of the furnace box is open to draw return air, this connects to the basement and to dead space in the wall where the big return air vent is. In my RV a lot of this dead space is under the shower pan and under cabinet floors in the bathroom. In my RV there is also a round return air vent in the bedroom wall that connects to the dead space below the bathroom sink cabinet floor.

I don't believe that Winnebago puts any filters on the furnace at all, at least not on the low and midrange coaches. However, since yours is used, sometimes people take off the big rectangular return air vent and add a filter behind it so you might check that out.
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Old 10-16-2017, 10:45 PM   #9
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Your 2008 32H is a Basement Air Coach which owners of only the newer coaches won't have much experience with. The heat is in the back with its service panel there on the outside of the coach and possibly with some items serviceable from underneath. There is a central duct running the length of the coach under the floor from back to front and the regulated floor outlets should be in the back of the coach which you close to force more heat up front when needed. If you leave the back outlets wide open you won't get much airflow up front however if you close them too much then most of your heat could end up flowing to the basement to keep the tanks from freezing. There should also be a regulator on the heat outlet in the bathroom that may need adjusting. The 14" X 20" residential style air filter should be accessible via a panel in the storage area under the bed and just drops into place. I use a good quality pleated paper filter to keep the AC coils cleaner and since its mounted on the floor expect to replace it every 2 weeks especially if your in a dusty location or have pets.

You may need to turn the basement airs blower to the on position at the thermostat instead of auto and then switch back and forth from cool to heat a few times waiting for the system to change modes in order to loosen up the damper separating the two systems. You should hear it clanking when you switch over from cooling to heating.

As I mentioned previously this damper is known to hang up. If the damper does not open fully when in heat mode it will restrict air flow into the furnace as its purpose is to allow both the basement air and the heat to use the same air filter. This hanging up issue is especially true if the furnace is seldom used as is the case with my coach since I am in the Deep South and rarely need heat and even when I do a wall mounted electric heater is usually sufficient or simply baking some muffins or corn bread in the convection oven. The furnace can eat up a lot of propane so one does find ways to limit its use whenever possible however economizing should be easier on your coach since the basement air on yours is the heat pump model which should still be sufficient this time of year with 70/80 degree daytime temps and lows in the mid 40's to mid 50's at night in Box Elder, SD.
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Old 10-17-2017, 07:12 AM   #10
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Thank you all for the help and especially NeilV since you have described my coach perfectly. I will try the heat/cool off and on and see if that helps. But from what you say it cannot be expected to get much hot air flow from the vents especially up front so I guess that is normal. I do note that the floor in the kitchen area (because it is vinyl?) seems to be warm on thru the hallway. Also did not know that my filter needed that much changing so will take care of that as well. Thanks for the help and will work on it today.
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Old 10-17-2017, 08:19 AM   #11
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The basement HVAC and the propane furnace only share the thermostat. Each has it's our ducting. The coach heater and propane furnace do share the sane duct, propane furnace blows into the rear, the coach heater into the front. Only the basement HVAC has a filter on the return duct. The return path for the propane furnace does not (and should not per the manufacturer).

Hard to find a good image of the system. The parts diagrams have some, but lack a good overall view. I'd find the return path and make sure it's clear. Ours is under the desk in our bedroom. Real easy to accidentally block that grill with a box or something. It then flows behind and around the drawers in the wardrobe and down into the floor. If something falls out of a drawer, it could block flow in ours.

Just need to dig around a bit, you'll find how it works.

Looking at it more, maybe you only have one small return register? The rearmost one shown in the parts diagrams?
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Old 10-17-2017, 12:37 PM   #12
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My compressor heat pump and propane floor furnace are separate systems. With a common control in the hallway off the kitchen. The heat pump blows air from the ceiling vents, and the propane furnace uses the floor vents.

I get plenty of hot air from all my floor vents. My slate floor slowly gets warm all over, not in just one area. Can pretty much tell it's warmed by the air from above, not a leak from below. If you know what I mean.

I would suspect, based on your description of the problem, that one of your ducting pipes has a tear or an open seam allowing too much air out before it gets to your front vents.
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Old 10-20-2017, 08:14 AM   #13
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Thanks for all the good information. I tried switching between the AC and Heat and that seemed to improve the air flow in all the ducts. I think it worked well.
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Old 10-21-2017, 05:15 PM   #14
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32h Propane heat flow fix.

I have an 2009 adventurer 32h (gas not diesel). I also notice that the heat flow wasn't very good and on my system and the furnace even would hit the overheat cutout and stop the burner at times.
I realized that there was only one in-floor register for return cold aire in the rear center of the coach and it was the same size as all of the other output registers. It only goes to reason that it was being starved for return air and wasn't pushing through the air needed.
I pulled off that one return register and realized that I could easily make it bigger. Went to the home store and picked up a bronze grate that was 2x bigger. It made a big difference in the heat output.
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