I am planning out how I am going to heat and cool my attic that I am finishing. I have a 19 by 12 room and a 8 by 8 bathroom. I want to put the unit on an interior wall in the 19 by 12 room. The other side of that wall will be unfinished attic space. It will also put the unit only about 5 feet from the bathroom. I have been looking at different mini splits and have yet to find one that will allow me to run a duct to the bathroom. I understand that these units are generally not made for the friction that occurs in ducts but it will be such a short run I would think I could find a good solution for this. Any help would be appreciated. Thanks.
Ducted mini split
- Thread starter Steven JP
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Ducts can be designed to accommodate even the wimpiest of mini-split ducted cassettes- it's not rocket science. It's only a matter of keeping the duct sizes sufficiently large for the cfm. The static pressures increase as a function of the V^3: Increasing the the duct's cross sectional area by 2 cuts the velocity in half, which lowers the static pressure by a factor of 8, all else being equal.
A typical 3/4 ton cassette puts out no more than 350-400 cfm at high speed, so if the ducts are sized using typical crummy rules of thumb for 800-1000 cfm air handlers, even long ducts don't present much static pressure- most of the "friction" is in the cassette's own coil and the air filter. Using an oversized pleated filter like an Aprilaire 2400 or similar also keeps static pressures down.
All that said, Fujitsu's xxRLFCD series have a bit more beefier blower that most of the competition, able to accommodate 0.5" w.c. static pressures about half what a typical split-system AC air handler can deliver. Their even beefier xxRGLX series can handle 0.8" w.c., which is pretty much the same as any split-system air handler.
Just about any 3/4 ton mini-split would handle the cooling & heating loads of ~300 square feet of conditioned space you're describing. Avoid the temptation to oversize, which leads to lower comfort, lower efficiency, higher up-front cost. I like the Fujitsu xxRLCD series in part due to their fairly low minimum-modulated output, so that it runs very long, quiet duty cycles rather than doing a lot of cycling on/off, but there are cheaper models out there. Midea's Premier-Hyper MEHSU-09CHD2 has almost the same modulation range and about the same capacity as the Fujitsu, but like most ducted mini-splits needs to keep the design static pressures on ducts & filters under 0.2" w.c. @ the rated 335 cfm high speed flow.
Is the unfinished attic space insulated?
A typical 3/4 ton cassette puts out no more than 350-400 cfm at high speed, so if the ducts are sized using typical crummy rules of thumb for 800-1000 cfm air handlers, even long ducts don't present much static pressure- most of the "friction" is in the cassette's own coil and the air filter. Using an oversized pleated filter like an Aprilaire 2400 or similar also keeps static pressures down.
All that said, Fujitsu's xxRLFCD series have a bit more beefier blower that most of the competition, able to accommodate 0.5" w.c. static pressures about half what a typical split-system AC air handler can deliver. Their even beefier xxRGLX series can handle 0.8" w.c., which is pretty much the same as any split-system air handler.
Just about any 3/4 ton mini-split would handle the cooling & heating loads of ~300 square feet of conditioned space you're describing. Avoid the temptation to oversize, which leads to lower comfort, lower efficiency, higher up-front cost. I like the Fujitsu xxRLCD series in part due to their fairly low minimum-modulated output, so that it runs very long, quiet duty cycles rather than doing a lot of cycling on/off, but there are cheaper models out there. Midea's Premier-Hyper MEHSU-09CHD2 has almost the same modulation range and about the same capacity as the Fujitsu, but like most ducted mini-splits needs to keep the design static pressures on ducts & filters under 0.2" w.c. @ the rated 335 cfm high speed flow.
Is the unfinished attic space insulated?
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Thanks for all the great info! Right now it's a completely unfinished attic space. The whole attic won't be finished when I'm done. I am leaving a space for storage. This is the way I look at it. With a ducted unit will have to put it in the unconditional space and run ducts off it which will not only be alot more work but will not be as efficient as a unit that resides in the conditioned space. With a ductless unit I will have to have an air handler in the main room and an air handler in the bathroom. I have not been able to find any handler that is smaller than the normal ones so the one in the bathroom would be over sized. What are your thoughts?
Why do the ducts and cassette need to be in unconditioned space? If you calculate the omparative loads and cfm and the size of the duct needed for the bathroom you'll find it's quite small. A 3/4 ton Fujitsu can be mounted vertically, and with a commoned return grille can be installed in a mini-closet/cabient taking up about 5-7 square feet of floor area. If that micro-closet is on the partition wall between the bigger space and bathroom the duct runs could be essentially zero, but you may need to select a register grille with higher "throw" for the bigger space to ensure mixing.
An oversized ductless head cycling on/off at some ridiculously low duty cycle rather than modulating is extremely inefficient, no matter what the nameplate SEER & HSPF might be.
This is a 1.5 ton Fujitsu heating and cooling an entire house in the SF Bay area:
The grille is the common return, and the longer duct runs are run in soffits below ceiling level. This is what it looked like before it got closed in:
If the cassette and ducts are installed in the unconditioned space it shouldn't be too tough to build an insulated kneewall around it to turn it into conditioned space.
For very short runs the duct size doesn't need to be very big at all if using hard-piped ducts, and tapered duct boots deliver higher exit velocity for better mixing.
I am kind of confused as to how I can route my air correctly to each space with a ducted mini split. I want most of the air going to my main room which 260 sq ft. and then the rest going to my bathroom which is only 70 sq ft. Is there a correct way to come off the unit to make sure I am heating and cooling each room accordingly. How can I calculate what size ducts I'll need to use to split off from the unit to each room?
On another note how well do mini split cassettes with branch ducts work? This is another option I have been exploring. I could put the cassette in the ceiling and run a duct off that for about 15 to 20 ft to the bathroom.
The last option is to put a regular wall mounted unit on the farthest wall away from the bathroom and try and fit a transfer grill somewhere around the bathroom door but I don't think Ill have much more than a square foot of space because I have to raise the floor on the bathroom to accommodate for plumbing. The thinking behind this would be that the air would be blowing straight towards the bathroom and the transfer grill would move the air to the bathroom.
I added a diagram of what I am planning on doing with the main bedroom at the top of the diagram and the bathroom to the bottom right and an unfinished space next to the bathroom. I was planning on putting the ducted mini split in that unfinished space if I go that route because it is between the two rooms.
Any thoughts would be very helpful.
I have had several contractors out to look at it and every one has had a different idea.
On another note how well do mini split cassettes with branch ducts work? This is another option I have been exploring. I could put the cassette in the ceiling and run a duct off that for about 15 to 20 ft to the bathroom.
The last option is to put a regular wall mounted unit on the farthest wall away from the bathroom and try and fit a transfer grill somewhere around the bathroom door but I don't think Ill have much more than a square foot of space because I have to raise the floor on the bathroom to accommodate for plumbing. The thinking behind this would be that the air would be blowing straight towards the bathroom and the transfer grill would move the air to the bathroom.
I added a diagram of what I am planning on doing with the main bedroom at the top of the diagram and the bathroom to the bottom right and an unfinished space next to the bathroom. I was planning on putting the ducted mini split in that unfinished space if I go that route because it is between the two rooms.
Any thoughts would be very helpful.
I have had several contractors out to look at it and every one has had a different idea.
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Start by running a room by room heating/cooling load calculation. The loads are not likely to be directly proportional to load area.
Then look at the maximum (highest blower speed) numbers for the cassette in question, and divide the cfm proportional to the loads. (If the difference in room load is bigger for cooling than heating or conversely, use the lesser proportion.) eg: Say the heat load of the 70' room is 650 BTU/hr and it's cooling load is 1100 BTU/hr, and the heating load of the 260' room is 2400 BTU/hr, with a cooling load of 4100 BTUhr.
The ratio of cooling loads between the rooms is 4100/650= 6.3:1, the ratio of heating loads is 2400/650= 3.7:1. The ratio of heating less, so we'll divide the max cfm into a ~3.7/1 ratio.
Say you're looking at a 3/4 ton Mitsubishi -KA/KD09 as the solution. The max cfm is 317 cfm @ 0.2" w.c. static pressure across the system. Dividing that proportionately, the 70' bathroom gets 317/(3.7 +1)= 67 cfm, the big room gets 317 - 67= 250 cfm. The ideal duct system would be designed to deliver those proportions almost exactly when the registers are wide open, tweaking the flows with the registers when changing from heating to cooling mode.
Duct induced static pressures are largely about two things- velocity, and turbulence around turns. You may find these discussions useful. But at the bottom line, if the ducts are large enough, with a low enough velocity and radiused rather than sharp-throated turns, most of the static pressure will be induced at the filter and the heat exchanger in the cassette.
How low a velocity is low enough? Assuming it's all within conditioned space, shoot for something less than 400 feet per minute and the absolute lengths will matter a lot less. Using this handy nomograph (developed for this purpose by a home energy retrofit company in CA) you'll see that the KD/KA09 example above, the room getting the 250cfm would be best served with a 12" round duct, the room getting the 67 cfm would take a 6".
As in the above, try to keep the velocities in each trunk and branch in the 250-400 cfm range. This will mean narrowing down the trunk after branches to keep the same velocity range.
As a general rule, hard-piped ducts are more consistent, but flex can be used if it's fully stretched taut (except where it must turn). Floppy flex induces far too much turbulence, increasing the friction/static pressure.
Also, installing conical reducers ahead of the register boots helps in two ways- it increases the velocity for better "throw" into the room, and allows use of smaller registers/diffusers. The only place you want the velocity to be higher than 400 cfm is where it enters the room, to improve the mixing within the room. While that short narrowing induces some amount of static pressure, it's far less than if it were a high velocity along the full length & every twist & turn of the duct path.
So much great information. Thanks so much. I really enjoy learning about all this. What are your thoughts on the last idea I had. I would have a wall mounted unit on the farthest wall away from the bathroom so the air would be blowing straight towards the bathroom. Then install some kind of transfer grille above the door and maybe a grill on the bottom of the door too. I know it won't be perfect but was wondering what your thought were.
Installing a ductless wall coil head 20-25 feet away isn't going to have sufficient throw to move much through a transfer grille to the bathroom. Unless the bathroom door stays wide open most the time that isn't going to cut it.
Is there enough flat ceiling to go with a 3/4 ton Mitsubishi MLZ? These ceiling cassettes are mono-directional blowing toward just one side, but (unlike others) will fit between 16" o.c. joists. Mounted roughly across from knee wall at the stair well blowing toward a transfer grille it might get you there. Even with a transfer grille near the top of the partition to the bathroom, there still needs to be a return path to get any flow, such as a louvered panel on bottom half of the door or something.
As with the KD09 mini-duct the MLZ-KP09's cassette's minimum modulation is still pretty high though. Have you bothered to calculate the room loads yet?
This really does matter, and getting those numbers are key to getting a solution that's really comfortable and/or efficient.
Yeah I've got room. Its completely unfinished right now. That's kind of what I was talking about in that earlier post. I'm really thinking I'm going to go that route but use the branch duct option. They are just twice as much as the wall mounts.
It seems like every website I go to gives me a different load. I've got a 22 by 12 (260 sqft) room and a 70 sq ft bathroom. A window in each. Seems like 9k should be plenty.
It seems like every website I go to gives me a different load. I've got a 22 by 12 (260 sqft) room and a 70 sq ft bathroom. A window in each. Seems like 9k should be plenty.
Most online load tools are crap. LoadCalc will usually overshoot reality by a double-digit percentage, but is better than most.
Coolcalc is a full-on ACCA certified Manual-J tool, not quite as flexible as a true professionals tool, but most newbies use conservative input assumptions rather than using aggressive assumptions (as prescribed by the instructions in the Manual), and end up with some crazy- big load numbers.
Yes, any 3/4 tonner would MORE than cover the loads of a room that size, assuming it's insulated and has substantially less glass area than a greenhouse. Having enough capacity is not the issue. The goal (from both a comfort and efficiency point of view) is to select a unit with a minimum modulated output that is well BELOW the design cooling/heating load so that it modulates with load rather than simply cycling on/off. If the minimum-output is 3/4 the design load or greater the comfort under average conditions is noticeably lower, and if it's doing way more cycling than modulating the as-used efficiency is lower than it's SEER & HSPF numbers too.
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