Furnace Air Flow

Question

During our Texas freeze this winter I noticed the furnace had very little air flow and the ducts appear to have collapsed. The floor vents at the front and back have no air flow. They barely get warm after running for a while.

The service tech said it's typical for the ducts to 'bow' as they did and the only thing he can do is replace them. And then the new ducts will eventually do the same. The small amount of air flow is considered normal.

Is it typical for rigid heater ducts to bow in the center and pinch off the air flow?

How much air flow is considered normal?
Is there a minimum CFM measurement at the vent that defines 'normal'?

What's the best fix for this? The bedroom and basement gets zero heat.

Unfortunately, the furnace is buried in a cabinet under a counter and the underside is covered with coroplast. Nothing is easy to get to.

Any help on this is appreciated

schlep1967 · Answer

I would start with a borescope inspection of the vents. You can probably "borrow" one from autozone or some other local auto parts store. You will likely have to give them a refundable deposit. Look into the vents to see if you can find a restriction point. Once you find the problem then you will know where to start removing the bolts that hold the coroplast up. They are generally self drilling/tapping and if one breaks off it's no big deal to run a new one in a few inches away from it.
I doubt rigid ducts would collapse. They may come apart. I would guess you have flexible ducts and it may be laying over top of something like a wire or water line. As the flex duct settles it could pinch off the air flow.

dougrainer · Answer

1. It is NOT typical for rectangular front to rear floor ducts to BOW.
2. You usually determine the condition of the floor duct from just removing each floor vent cover and use a mirror and flashlight to look forward and aft between the vent covers to see if and where the "bowing" has happened.
3. If you have a bow, you can make a shim to slide in the floor duct covers(if the covers are installed with the long side front to rear and not side to side. Usually you can use a flexible type wood shim to get into and push to the bowed area to unbow that area.
4. As another posted, the other way is to remove the bottom frame plastic cover and that will reveal the ductwork except if you have fresh/black/gray holding tanks below the ductwork. Doug

dougrainer · Answer

Last, when you state bedroom, are you talking above the 5th wheel hitch bedroom? If so, usually they run 4 inch round flexible duct to those bedroom vents and they may indeed be pinched. The also usually just add a 2 inch round vent to the under storage area visible metal duct to get warm air to the storage area. If so on your RV I would close it off because that will diminish the force/volume of air to the front bedroom. Doug

2112 · Answer

Thanks Doug,This is looking towards the furnace about 10' away. This register is the only one I feel air blowing out of. This is looking towards the rear register about 6' away. Sorry for the poor quality. I see the foil tape blocking the path. I'll clean that up.Unfortunately, the registers run left to right. It will be difficult get anything down in there. There are two round flex ducts running to the front bedroom and basement. I can barely see them behind a panel. It would take some effort to get my hands on them. I doubt I could fit my big self through the basement hatch.  Would I learn anything from cutting a window in the coroplast under where the duct connects to the furnace? Maybe the flex duct disconnected or pinched? I'll be working right above the axle.

mfl · Answer

I am thinking if the bedroom and basement get zero heat, likely a flexible duct is ripped open, or disconnected. If this happened close to the source, it would slow air flow to all registers.

My FW has 4 main registers, plus the small duct to basement. I only have one floor vent, and the rest go through various cabinets. I can see most of the flex duct, which is routed to keep lines, and water pump from freezing.

All my vents blow similar amount of air (testing with hand). IMO, the air flow is substantial, and can be felt several feet from vents.

Jerry

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