You wanted to see what you could "get by with". Just why would you want to do that?
Doesn't make logical sense if you are in the design and engineering mode to design and engineer something to "just get by".
"Wow...if I use the smallest duct possible, I can be on the jagged edge of it not working, yet it may still work!" Yeah...that's the ticket.
A 5200btu unit is going to have a fan that will be pushing 120-150cfm. But this cfm is figured with air traveling through an orifice (fan housing) that is 8-10" in diameter. Basically, an 8-10 inch duct.
Boxing that up and only allowing the air to move through a 4" opening or duct, you no longer have the 120-150 cfm you started with. The fan is met with resistance because it's path has been choked down to a fraction of what it started out with.
An 8" hole provides 50.25 sq inches of air flow area.
A 10" hole provides 78.5 sq inches of air flow area.
A 4" hole provides 12.5 sq inches of air flow area.
Do you see what you are doing to the air flow by choking it down to 4"?
The unit may work, and it may appear to be fine, but it is working harder to accomplish the task, and as a result the unit will not last as long as one that has not had to labor it's whole life. Before long there will be a line met and the unit will start to cycle and the wet side will be less and less cool.
So, the unit does not work perfectly as described when no fan is helping.
And yes, you can get the air volume back by adding a fan to help, but there are a few things to consider when adding a fan. First off, CFM does not mean CFM at the exit of your duct run. It means CFM at the exit of the fan.
Take a 250CFM duct booster for example....if you place a 10" piece of flex duct to the exit of the booster, the resistance the air meets from the head of the duct causes the fan to labor, and it can no longer maintain it's 250cfm rating. With a duct booster, I could easily see it being cut in half. They are not very powerful at all...and why they cost $30.
Now, place a 250CFM S&P inline in front of a 10' flex duct run and it will barely know the duct is there, and there will be very close to the 250cfm coming from the exit of the duct.
A fan may be rated at a specific CFM, but at what power?
Nothing has changed, and the unit may work...but it won't work as long as it would, had you decided to not "just get by".
Doesn't make logical sense if you are in the design and engineering mode to design and engineer something to "just get by".
"Wow...if I use the smallest duct possible, I can be on the jagged edge of it not working, yet it may still work!" Yeah...that's the ticket.
A 5200btu unit is going to have a fan that will be pushing 120-150cfm. But this cfm is figured with air traveling through an orifice (fan housing) that is 8-10" in diameter. Basically, an 8-10 inch duct.
Boxing that up and only allowing the air to move through a 4" opening or duct, you no longer have the 120-150 cfm you started with. The fan is met with resistance because it's path has been choked down to a fraction of what it started out with.
An 8" hole provides 50.25 sq inches of air flow area.
A 10" hole provides 78.5 sq inches of air flow area.
A 4" hole provides 12.5 sq inches of air flow area.
Do you see what you are doing to the air flow by choking it down to 4"?
The unit may work, and it may appear to be fine, but it is working harder to accomplish the task, and as a result the unit will not last as long as one that has not had to labor it's whole life. Before long there will be a line met and the unit will start to cycle and the wet side will be less and less cool.
So, the unit does not work perfectly as described when no fan is helping.
And yes, you can get the air volume back by adding a fan to help, but there are a few things to consider when adding a fan. First off, CFM does not mean CFM at the exit of your duct run. It means CFM at the exit of the fan.
Take a 250CFM duct booster for example....if you place a 10" piece of flex duct to the exit of the booster, the resistance the air meets from the head of the duct causes the fan to labor, and it can no longer maintain it's 250cfm rating. With a duct booster, I could easily see it being cut in half. They are not very powerful at all...and why they cost $30.
Now, place a 250CFM S&P inline in front of a 10' flex duct run and it will barely know the duct is there, and there will be very close to the 250cfm coming from the exit of the duct.
A fan may be rated at a specific CFM, but at what power?
Nothing has changed, and the unit may work...but it won't work as long as it would, had you decided to not "just get by".