Miller cmf2 80 po conv ( venting)

DownDraft

cmf2 80 uses a 4" flue roof jack that max out at 60inches with the srj5, problem is i have 70inches in attic to get through trying to stay within miller install with the jack but its looking almost impossible, cant powervent due to my guidlines so thinking about moving to an intertherm m5 roof jack that gives me 51-96 but runs a 5" flue thinking of bumping the 4 of unit into 5 before im out the ceiling. Only other option is to double wall direct vent it and then im way out of their install range.

EBEBRATT-Ed

I would use the intertherm and leave room for a barometric if you need one. You may have more draft with a taller flue

Those thing are always a problem

DownDraft

Was thinking along same lines, my options are just limited and i think thats the best one, get a better draft maybe a little bit more condensate but its reallly not going to be used alot so not a huge issue their, not to mention finding rated double or even triple wall rated pipe is almost impossible besides in pellet stove pipe. Not a normal install I am on a cottage base running duct off it which is within spec but its a really small area an I am fighting for every inch of clearance I need, and running off the cottage base like i am they spec running the combustion vent also. thanks for the feedback.

EdTheHeaterMan

EBEBRATT-Ed said:

I would use the intertherm and leave room for a barometric if you need one. You may have more draft with a taller flue

Those thing are always a problem

Do not use a barometric on a mobile home furnace. The blower intake is in the same location as the vent. A barometric opening will allow combustion exhaust to enter the blower intake. That furnace is designed to operate without a barometric.

As long as the roof jack is protecting the combustible surfaces, you can add a few inches of 4" galvanized single wall 26 Gauge pipe to the bottom of the adjustable 4" sleeve on the roof jack.




If you are having this problem as indicated in the section I added to the illustration, where the red line indicates the ceiling of the furnace closet, then the roof jack is not protecting the combustible surfaces of the ceiling. You can either raise the ceiling inside the closet as indicated by the green lines in the illustration... OR... if you use the Intertherm roof jack, be sure to cap off the 9" diameter outer pipe at the bottom and add a 4" pipe to the side of the 9" outer pipe. Run the 4" to the lower compartment (the burner compartment) to have combustion air flowing in thru the outer part of the roof jack. That is how that roof jack was approved for use, by having cooler outside air enter around the center, hotter, exhaust pipe.

Respectfully submitted,
Mr.Ed

EdTheHeaterMan

Is this on new construction or a new modular home? It appears that you have a roof or attic that is outside the design criteria of the CMF design.

Do you already have the furnace? Miller makes a different furnace that can fit in the same location and operate similarly. Miller M5 series furnace is the same footprint for the bottom duct connection but uses the 5" exhaust inside the 8" intake roof jack. I have offset those with galvanized 26 gauge 5" adjustable elbows and used an 8" flexible aluminum duct to make the 8" offset. The roof jack does not need to be directly over the opening that way and you get the lower clearance roof jack because of the pipe in a pipe vent design.