Duct sizing for Fujitsu 9RLSFCD concealed duct unit
Hi All,
I am planning to install Fujitsu 9RLFCD concealed duct unit and can’t figure out way what size duct should I use and how to determine how many CFM does each room need. I have a heat loss done for each bedroom.
The plan is to put unit in the lower level aka basement (it’s split entry home) and run the duct work to the 3 bedrooms thru the floor. All three bedrooms are next to each other and would not require more than 4-6ft of ducts to each bedroom.
I would also like to use and add register in the basement ceiling to help heat the basement. Getting return air from each bedroom would be complicated and might require long duct runs, so was thinking to use basement as point for return air.
If anyone can help guide me at last with some “rule of thumb” idea that would be great. I know these are not good ideas, but I need to start somewhere.
Thanks
Alex
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Replies
Alex,
I would start by consulting the manufacturer's installation instructions. Do you have a copy?
As a rule of thumb, it's not a good idea to use rules of thumb for HVAC design. It's especially not a good idea to use rules of thumb when designing with an air handler (the Fujitsu ARU series) that requires a relatively low pressure duct system (compared to standard residential design).
I think the best place to start to learn about residential duct design is here:
http://www.amazon.com/Manual-D-Residential-Duct-Systems/dp/1892765500
Hi Martin,
Attached is the submittal sheet from Fujitsu. Is that what you meant by installation instructions?
Thanks
Alex
Alex,
As far as I can tell from visiting the Fujitsu web site, Fujitsu does not make its installation instructions available publicly. You have to be a contractor to read the installation instructions.
If you are planning to purchase, or have purchased, a Fujitsu minisplit, you need to convince the distributor that you need to see a copy of the installation instructions. (The submittal sheet that you shared is not the same as installation instructions.)
John Semmelhack is correct: to design a duct system, you either need to understand the necessary calculations or have access to Manual D software.
Ducted minsiplits have low air flow rates, and therefore require duct systems with low static pressure. Whereas a typical furnace might have a blower that operates at 1,200 cfm, it isn't unusual for a ducted minisplit to deliver only 350 cfm. So you can't use rules of thumb developed for a U.S. furnace and apply the rules to minisplit ductwork.
As Marc Rosenbaum noted in a GBA blog, "The [ducted minisplit] blowers have significantly lower static pressure capability than what most HVAC folks are used to. My air handler allows the external static pressure to be selected at the controller (not by the homeowner) and the maximum is 0.36 inches. So ducts may need to be larger than you think if the distances are significant or there are a lot of fittings. ... Ducted systems need well insulated ducts and they need to be airtight. My system has a design air flow of 353 cfm. Lose 100 cfm of that into the basement, and have ducts wrapped with bubble wrap, and half the output might not reach the living space."
I did get a copy of the installation instruction from distributor and there is no much of information about ducts part except static pressure data.
Attached is the file for reference.
Alex,
Ah, yes. This is a site-assembled heating system. Fujitsu sells you the heating appliance, but they don't sell you the distribution system. That's handy, because it means that Fujitsu is not responsible for the design of the distribution system. They only sell you one part of your site-assembled HVAC system.
Your engineer has to design the system, and your HVAC contractor has to install the system according to the mechanical engineer's specifications.
You can see why ductless minisplits are so popular, can't you?