Montana Owners Club - Keystone Montana 5th Wheel Forum - View Single Post - Something else to do before the furnace comes on

Irlpguy · 01-16-2013, 06:20 AM

I don’t know where your furnace is located in your model Greg, however I suspect it is very little different than my 3402RL which is located next to the hot water heater.

Sadly these furnaces are not built like the ones in our wooden houses, if you take off the cover of your furnace on the outside you will see a squirrel cage fan with a 6” diameter fan, the fan itself is made of plastic and only has ¼” high vanes, therefore it must turn very fast to provide the air required to move it to the back of the RV. You will also notice that on each side of the burner compartment there is an opening for air supply to the fan, there is also an opening behind/above the circuit board as well. These do not go anywhere except into the large compartment behind the furnace. That means they can draw air from the storage area, the whole area under the shower bathroom and the underbelly itself.

If you consider the amount of space there is in the basement and compare it to the rather small openings to the living area at the two steps you will find that it is minimal compared to the overall area. It is also a considerable distance from the furnace air intake.

You can put filters on the back of those step openings which will eliminate any dust and so on coming from there, however the vast majority of air is coming from the large open spaces around the furnace and is not coming from your living area.

There are actually two fans combined in one on these furnaces, the larger one pushes air past the burner tubes for heat and the other one in the lower sealed compartment blows air out the exhaust vents in the bottom of the vent panel. This is meant to move the exhaust away from possible intake into the furnace.

You would have to be very small and dexterous to get in the area of the furnace to modify the intake to resemble what we have come to know in our S&B’s. It could be done but would be a very difficult modification.

Because of the size of the furnace fan we have to put up with the noise of it turning so fast, a larger fan turning slower would produce the same amount of CFM and would be much quieter. It would not be all that difficult to build a dual voltage feed to the motor to control the speed of the motor and place the switch in a convenient location, however while it would slow down the motor thus reducing the noise, unfortunately it would also reduce the air flow proportionately.

I hope the above gives you a bit more of a perspective as to where and how the furnace gets it’s return air, and a better understanding on the limits of return air filtration.

01-16-2013, 06:20 AM	#24
Irlpguy Montana Master Join Date: Oct 2012 Location: Chilliwack Posts: 1,520 M.O.C. #12935	I don’t know where your furnace is located in your model Greg, however I suspect it is very little different than my 3402RL which is located next to the hot water heater. Sadly these furnaces are not built like the ones in our wooden houses, if you take off the cover of your furnace on the outside you will see a squirrel cage fan with a 6” diameter fan, the fan itself is made of plastic and only has ¼” high vanes, therefore it must turn very fast to provide the air required to move it to the back of the RV. You will also notice that on each side of the burner compartment there is an opening for air supply to the fan, there is also an opening behind/above the circuit board as well. These do not go anywhere except into the large compartment behind the furnace. That means they can draw air from the storage area, the whole area under the shower bathroom and the underbelly itself. If you consider the amount of space there is in the basement and compare it to the rather small openings to the living area at the two steps you will find that it is minimal compared to the overall area. It is also a considerable distance from the furnace air intake. You can put filters on the back of those step openings which will eliminate any dust and so on coming from there, however the vast majority of air is coming from the large open spaces around the furnace and is not coming from your living area. There are actually two fans combined in one on these furnaces, the larger one pushes air past the burner tubes for heat and the other one in the lower sealed compartment blows air out the exhaust vents in the bottom of the vent panel. This is meant to move the exhaust away from possible intake into the furnace. You would have to be very small and dexterous to get in the area of the furnace to modify the intake to resemble what we have come to know in our S&B’s. It could be done but would be a very difficult modification. Because of the size of the furnace fan we have to put up with the noise of it turning so fast, a larger fan turning slower would produce the same amount of CFM and would be much quieter. It would not be all that difficult to build a dual voltage feed to the motor to control the speed of the motor and place the switch in a convenient location, however while it would slow down the motor thus reducing the noise, unfortunately it would also reduce the air flow proportionately. I hope the above gives you a bit more of a perspective as to where and how the furnace gets it’s return air, and a better understanding on the limits of return air filtration.