Moving Air 60 Feet
I have a well insulated and well sealed house. We have some need for cooling, and added a 24,000 BTU mini split on the west side. I need to move roughly half of this capacity across the attic about 60 feet to our bedroom.
Any idea what size duct would work and what CFM fan size? It feels like 6 or 8 inch would possibly be enough at 450 CFM or more, but I am having a hard time getting the calculations. I may be able to get smooth duct in the attic, or I may have to use flex. I have to get up in there this winter and see what my options are, but it will not be an easy gig to get the duct across the scissor trusses. I sure don’t want to have to do it twice.
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The rule of thumb is 400 cfm per ton, so 24,000 BTU is two tons and half of that is one ton, so you need 400 cfm. That's assuming you can get air directly off of the air handler, 30F differential from room temperature.
Duct design is actually complicated, people who do it use specialized software. Basically you don't want to make the ducts any larger than they need to be, but if they're too small you run into two problems. The first is that the air starts flowing quickly through the duct, which makes them noisy. The second is the smaller the duct the more resistance it poses and at a certain point the fan won't be able to move the required amount of air.
The resistance of ducts is expressed as "static pressure," which is how high the air pressure is at the fan. The higher the flow, the higher the static pressure. The fan will have what's called a "fan curve," which shows airflow vs static pressure, the higher the static pressure the lower the flow.
The basic process is first you figure out how much flow you need. In this case, 400 CFM. Then you look on the fan curve and see what the highest static pressure you can have is and still maintain that flow. Then you find a duct size calculator which will allow you to calculate the static pressure at that flow for different duct configurations.
Fans are usually marketed at their zero static pressure cfm so my bet is you'll need something rated at about 600 CFM to move 400 CFM through 60 feet of ductwork.
I cannot help but think that one of those Pioneer small minisplits costs like 750 bucks delivered....
If the minisplit you have isn't capable of acting as a ducted unit, I wouldn't try this. If you're thinking you can put an intake grille somewhere like on the ceiling near the minisplit and then use a fan to "move half the capacity" 60 feet, you're going to be dissapointed with the very poor efficiency you get. That poor efficiency will be seen as one side of the room being much colder than the other, because you'll only be "moving" a small fraction of the expected capacity to the opposite side of the room.
Moving hot or cold air this way doesn't depend only on the CFM you move, it also depends on the relative temperature of the air in the duct compared to the air in the room. Basically if the duct is moving 65*F air to a 70*F space, you're not getting nearly as much cooling capacity going through that duct as you would if you were taking 50*F air directly from the output of your cooling unit. To make something like this work at all, you need to move massively more air than you'd expect for the amount of cooling you need. I've done things like this in datacenters before, where we usually have bigger thermal differentials than you would have in a house (hot side air is usually 100+*F, cold side is around 50ish). We usually use box fans (large air volume for little money), we move the air short distances, and it's a temporary setup, usually while the "real" cooling system is upgraded.
I would recommend just putting in a second minisplit on the opposite end of the room. I think you'll be much happier with the results this way. If you do try the ducted approach, expect to need to move much more air than you're thinking, and you absolutely want to use rigid ducting over that distance -- long runs of flex duct will significantly cut down on the air you can push through the duct. Commercial standard practice is to only use flex duct for the last 3-6 feet or so from a rigid plenum to a ceiling mounted diffuser. Office buildings with drop ceilings are almost always ducted this way.
Bill
Thanks for the input. Adding another mini split is possible, but will be a very big job in this floor plan. I had thought of that. There are multiple issues making that complicated. I need to drop about 5 degrees in the bedroom. I came up with 430 CFM with some online calculators as well. I think I have the information I need for now. We use the AC for about 5-6 weeks in the summer, so fortunately our cooling load is low.
The 430 CFM assumes you're able to move ~45F air coming right off the cooling head, and not ~70F air from the room. If the existing unit isn't ducted you're not going to be able to do it.
One thing to consider is replacing just the cooling head with a ducted head.
Another thing to keep in mind is that in houses built with trusses the insulation is usually right above the ceiling. So running a duct through the attic is going to be running it through very hot space, more of your cooling is going to go into the attic than arrive at the destination.
I'd be thinking of installing a ducted head in the hall between the two rooms and then a duct in a soffit along the hall ceiling into each room. You never regret doing a job properly.