Max duct length for ZC fireplace central heating option

[babzog]

We're leaning toward the RSF Opel 3 but still like the look of the Napoleon NZ-26. One more quote to receive and the unit decision making will be complete.

One question that's come up is the max length of duct. We're planning to use the fireplace to heat the living rm where it will be and to duct some of the heat to the bedroom area using the central heating kit and that 630+ cfm blower. The duct will be run down into and through the basement, in the space between the floor joists and drop ceiling (a cavity about 4" high). Obviously, due to the space, the duct will be a custom 4x10 or larger square.

The length from fireplace to farthest vent could be 40', best case would be 30' (the difference between putting it near the door in the master BR, or beside a window on the north wall of the same room). There will be one vent in the basement, one in the master BR and possibly one elsewhere in the bedroom area - bathroom perhaps. The main concern is to get heat to the master BR.

Question is: are we going to be happy with the heat output at the farthest vent or are we not going to be able to realize our goal?

 

[begreen]

Do the floor joists run opposite of the duct run direction? It would be better if the trunk duct was larger than 4" tall. Otherwise it will have to be quite wide to handle say 3/4s of the airflow, assuming approx. 1/4 is diverted to the basement. If you are sending about 450cfm to the bedrooms the trunk duct should be roughly 6" x 20". To reduce heat loss the duct could be insulated, but that would increase its size.

An alternative is to box in the duct. If it was an 8" x 14" duct, that would only mean a 3" ceiling drop under the boxed run.

 

[babzog]

The main duct will run perpendicular to the joists so no chance of hiding it that way. I'd also rather not box in a big duct right down the middle of the basement ceiling.

How is duct size figured going from round to square.. is it purely based on matching square inches? If so, then the 8" duct coming off the fireplace has a cross section of about 50 sq in. A 4x13 square duct would have a cross sectional area of 52 sq in.

I would also prefer to insulate it to maximize the heat sent to the far end.

 

[ScotO]

you should be able to get enough heat with that 4x13 duct, but you could also wrap the duct with foil-backed fiberglass insulation, it would save some heat and should still give you clearance for your ceiling.....but you'd be pushing it to get that hidden by your ceiling...one other option is to lower your ceiling a couple of inches, to get you more clearance to insulate your ducts a little better....It would make a significant difference at the end of the heat run if you could insulate that duct a little better.....

 

[begreen]

Are you sure they are blowing 630+ cfm through an 8" pipe? 8" round pipe has a design airflow of about 180cfm. Your main issue is the length of the run. That is going to add resistance, which normally requires going up a size. Assuming that you are going to want at least 100 cfm for the smaller rooms and say 150cfm for the Mbr, that is 350cfm, or roughly a 10" duct. But the distance of 40ft dictates increasing the size. More air can be pushed with greater pressure, albeit with more noise.

This statement confuses me:
"We’re planning to use the fireplace to heat the living rm where it will be and to duct some of the heat to the bedroom area using the central heating kit and that 630+ cfm blower."

Edit: Never mind, it is an option for the RSF Opel 3. They are using a high velocity blower.

 

[ScotO]

Are you sure they are blowing 630+ cfm through an 8" pipe? 8" round pipe has a design airflow of about 180cfm. Your main issue is the length of the run. That is going to add resistance, which normally requires going up a size. Assuming that you are going to want at least 100 cfm for the smaller rooms and say 150cfm for the Mbr, that is 350cfm, or roughly a 10" duct. But the distance of 40ft dictates increasing the size.

This statement confuses me:
"We’re planning to use the fireplace to heat the living rm where it will be and to duct some of the heat to the bedroom area using the central heating kit and that 630+ cfm blower."

I think he is referring to the NZH62 kit from Napoleon, BeGreen....it was designed to use a single 8" pipe off of their ZC fireplaces, and you are right it seems like way too much cfm but they claim it will pull quite a bit of heat from that little pipe.....we are going to install two 6" pipes off of our NZ3000 and run them into one duct in the crawl space, and run that same 630cfm fan......

 

[aansorge]

Between the two stoves, the Opel has a much larger firebox....