mcewena wrote:
#1 Yes. To be honest it's been so long since I've run it (COVID etc) that I'd have difficulty describing the sound now but it's the sudden jet engine sound as it kicks on as opposed to any squeak so I assume it's combustion & air movement/pressure in the plenum.
#2I didn't suggest slowing down the fan, although if possible a larger fan could spin slower and produce the same volume. I was wondering if there was a replacement fan that drew less current and as a side benefit was quieter (better winding/bearings?). Thanks for the part #s Old-biscuit.
#3I considered baffling it with an office ceiling tile, there really isn't much room for anything thicker that would be both heat/sound tolerant (it's under the dinette bench) but it's hard to buy 1 ceiling tile and without blocking the air intake I wasn't sure that was going to be effective anyway.
#4The Winnebago plenum design doesn't look that efficient but by the time I fabricate an alternative I might be into the diesel heater price range.
#1, "jet engine" sound IS normal and to be expected.
RV furnaces basically are "turbo charging" the burner by forcing a high speed "draft" past the burner and through the heat exchanger via the dual sided fan. The downside of draft inducer system like this is it will increase the sound level of the burner and create a pretty good burner "rumble". That sound travels right through the heat exchanger walls and right into your RV interior. Some of that rumble is passed through as a vibration which can be transmitted to the wall and floor of the RV adds to the noise level.
It IS normal and to be expected on these furnaces.
The exhaust draft inducer IS a vital part of the burner and heat exchanger system, the heat exhangers the way they are fabricated and bent do not have enough natural flow for the exhaust gasses to move on their own which is why the inducer fan is included on the burner side.
#2 Can't just change the fan wheel size, while in theory the outside of the larger wheel would turn a bit slower, in reality what will happen is the larger wheel most likely will have MORE "FINS" resulting in MORE AIR FLOW resulting in MORE AIR NOISE. Technically, going with larger wheel in order to keep the CFM same, you would have to slow the RPMS of the motor. Now you could go with a larger and thinner wheel but then you have to alter the housing size to make that wheel work efficiently..
Brings up the next issue, increasing Wheel size and you would have to change the HOUSING of the wheel for the new wheel to fit and for best efficiency of the wheel.
The current draw of the fan is dependent on the speed (RPM) and the load on the motor. To reduce the current draw you would have to reduce the speed and the load with your current motor.
If you were to use a motor from a smaller furnace you would find that it will not produce enough airflow for the burner and interior fan.
Once again changing ANY of the CFMs of airflow WILL require changes to the burner and even the sail switch. You HAVE to reduce the burner flame and unless you have knowledge and equipment it is not a DIY change. Good chance you would end up burning more fuel (lower efficiency) and STILL the furnace will sound like a "jet engine".
#3, Never, ever block the cold air intake unless you have a death wish with taking chances of your furnace burning down your rig.
If you dig up the installer manual on your furnace it will give you very specific information on how many square inches of open space is required for the intake.
You may redirect the intake opening to another nearby cabinet or side of the existing cabinet as long as you have sufficient square inches of open space all the way to the new inlet opening.
#4 can't help you on the plenium issue vs cost, redirecting the cold air intake to a bit more remote area might take some of the edge off the sound but since it is under a dinette bench you have two other directions you could create a new opening under the dinnette or the backside of the dinnette if the current opening is on the end..
Redirecting the opening may help reduce the sound level a bit..
Adding some carpet on the outside dinette bench can absorb considerable amount of noise (soft fuzzy texture absorbs noise, solid hard surface reflects noise).
Redirecting opening to be a bit less direct and adding something like carpet to the dinette booth may prove very helpful..
I would not recommend using carpet inside the bench (although that would be effective for noise) due to possible encroachment of the furnace minimum required air space and flammability issues..
There are high temp noise control products but they tend to be on the expensive side..
