Filter Location Options for Ducted Minisplits
I’m looking for thoughts on how to provide filters (ideally media filters) for our HVAC system.
This will be new construction, single story ranch home, slab on grade, no attic. So all mini-split equipment and ducting will be in dropped soffits (9 ft ceilings dropped to 8 ft).
The HVAC designer is specifying three ducted units for the home, with several different return air approaches.
For a zone with three small bedrooms, he has individual return ducts from the three rooms, with a filter in each of the three return air grilles.
In a common area zone, he has jumper ducts from the smaller areas into the central hallway, with the PEAD unit mounted in the soffit above the hallway. There will be an access panel below the PEAD unit. He is suggesting creating a large drywalled plenum within the soffit where the PEAD is placed, with a large return air grille and media filter built into the access panel. So the PEAD will be sucking air from the drywalled box “plenum”, through the media filter in the access panel.
I hope this makes sense.
I’m not really crazy about either of these approaches. Having separate filters at each return air grille seems like a maintenance headache (so many filters to change). And the central plenum also feels out of place. Frankly, the whole thing feels like a compromise that one might encounter in a remodel, but should not have to do in new construction.
But I’m not sure what other (better) options there are for including filters in a ducted mini split where all air has to go through soffits. Are there any better ideas?
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We just went through deciding on filtration for 3 fujitsu slim ducted units we installed on a not quite gut remodel.
For the finished basement we went with a 16 x 25 filter grille that accepts a 2" filter. This is similar to your plenum filtration except ours in the wall with the unit mounted vertically. There are two bedrooms and a living area for a total of around 700 sq feet. We're using the undercut of the doors for return air. It works great but you definitely hear the air rushing through the return.
For the main floor living area which has 3 bedrooms, living room, a couple of bathrooms, foyer, etc. We went with ducted returns in each bedroom and the living area. We had a sheet metal shop build a filter box with a 14x25x1 filter on a 45-degree slant. Something similar to this: https://www.mitsubishitechinfo.ca/sites/default/files/PG_FB-Sereis_Filter_Boxes_ME_AC_0860_1-20.pdf
It works great and the return air is nearly silent. The pressure drop across the filter was very low using a Merv 8 so overall we're very pleased. The one drawback is the return ducts will over time get dirty. Someone recommended a pre-filter on each return grille but I've not checked into that.
For the last unit which is the main bedroom, bath, and office above we went with two 16x25x2 filter grilles. One is in the office and the other is in the bedroom. They are quieter than the basement but since the air return run is shorter they are noisier than the main floor unit.
I've done something pretty close to what your designer is suggesting but instead of a drywall plenum, converted the ducted unit to bottom intake (some Mitsubishi units need an adapter plate for this) spaced the access panel 4" off the bottom of the unit. Installed into the access panel a 4" filter grill directly under the unit. The grill seals directly against the bottom of the unit with some weather stripping, using the unit housing as part of the return.
About the only issue with this setup is that the blower is right there and you will hear it if you are near the unit. Fine at low/med, but I find it a bit too loud on high. The sound is perfectly fine about 10' from the unit, so it really depends on where it is and where you might be sitting.
If you have the space, a quieter option would be to get a sheet metal adapter fabbed that goes from the back of the unit to say a 20x24 sheet metal air return boot and install the a filter grill into that. Line the inside of the return boot with fiberglass duct liner. This would put a 90deg bend between the blower and the room and would be much quieter.
As for the rooms, you don't need individual returns for each room. Door undercuts are the simplest, but if you don't want door undercuts, you can use something like a Tamarack R.A.P. 12.6 for jumper duct.
If you do keep the individual returns, one option is to include only a washable filter for each room return and install a proper Merv8 right at the unit. The washable filters would keep the ducting mostly clean and would also reduce how often you have to change the main filter.
The reason why some of this feels a bit too simple is that because of the multiple air handlers, the ducting tends to be very simple. Nothing wrong with that.
The one suggestion I would make is to make all the filters the same size even if they are oversized for some of the units.
My ducted mini is in an encapsulated attic.
The return is DUCTED down a stud cavity into the vestibule between the bedroom and bath door. The return air filter is accessible standing on the ground with no ladder.
The bedrooms return via under door cut. Not sure how well that works, but I can always add jumper ducts, which don't need filters.
Thank you for the responses, they are very helpful. Akos, you are right, I am more used to large FAU installations with a supply plenum and a return air box housing a 4" media filter. But these are usually installed in separate mechanical rooms (or attic/basement), with easy access - we don't have that luxury in our design, where the unit is up in the soffit. So I need to adjust my thought process.
Based on your replies and the designer's comments, I see these general options (cartoonish images attached):
1. Use a central filter mounted in the access panel. I think that is not going to be an option for us for a couple of reasons - 1. noise due to the filter being right below the unit, and 2. I don't want a large access panel in exposed hallways or baths, and would rather tuck it away above closets - and we obviously can't have return air being pulled from closets.
2. Use a filter in a 45 deg box as suggested. That gets the filter opening a little further away from the PEAD unit, so the noise may be slightly attenuated. But it's not clear to me how I will access the filter for replacement, it looks like they have to slide out the side, which would make for a very large area needed in the soffit. Am I missing something? And this also probably won't work if the PEAD unit is over a closet, since I need to place the filter a little further away.
3. Use a large central filter grille, connected to the PEAD unit with some ductwork. This helps with noise attenuation, and allows me to place the filter grille in an open area like a hallway.
4. Use separate return ducts from different rooms, each with a filter at the grille, and then connect the ducts to the return side of the PEAD. Seems like the pro of this approach is not needing a large central filter grille (aesthetics), but the con is probably higher cost of ductwork.
Does this sound right, or am I misunderstanding some of the considerations? It seems that a combination of Options 3 and 4 might be best suited for me.
3 and 4 sound good. If you go with 2 a sheet metal shop can make the filter box so the access panel is on the bottom. This is what we had to do as we didn't have enough clearance to pull it out the side.
I would go for option 3, this way you can have a standard design for all of your units. You can do door undercuts / transfer grill / jumper duct for the bedrooms.
If you have a bit of space and can build a thicker partition wall or vertical bulkhead for your bedrooms, a quiet jumper duct option is to install two 90 deg duct boots, one the top on to the inside and one to the bottom outside connected by 7" flex duct. This is good for around 70cfm at 5PA.
The 45deg filter box similar to the FMB ones works well when the unit is installed in a basement or crawlspace where you have space to remove the filters. Also I would rather have a filter grill with a standard sized filter (16x25x4 or 20x25x4) instead of the 14x20x2 .
I wonder how large the house is, and what the Manual J load calc came out to?
Will it be (3) 1:1 systems, or a single outdoor unit Multisplit?
If 3 slim ducted systems are needed for capacity, I might start looking back at a conventional central ducted Heat Pump, with ductwork ran in the soffit.
The house will be about 3500 sq ft, I'm attaching an image of house layout below, along with Man J calcs for each area. Also shown there are the proposed zones.
The plan is to have one ducted unit(single zone) for the ADU area in Yellow; one ducted unit (single zone) for the guest bedrooms in Pink; one ducted unit (two zone) for the common and primary bedroom areas in Green and Light Blue. The Gym area in Gray will actually be part of the Common zone in Green. Each indoor unit will have its own outdoor unit.
Do you think this is more suited for a conventional system? We don't have gas at the property, so it'd have to be all electric.
Well, that’s a pretty spread out floor plan, so it’s going to be interesting.
Zone 1 is a completely separate ADU, so probably just a 6k ductless head.
Unit 3 likely can’t be zoned because the loads are drastically different between zones 1&2 (6k vs 21k), which makes airflow drastically different. Plus you can’t zone a slim Ducted mini split.
Zoning a single story home isn’t all that practical anyway, except that you do have a long floor plan.
What is the sq footage, roughly, of each unit, or zone you have laid out? For each of those Man J calcs.
Which way is North on that drawing?
Do you have a target Blower Door test leakage number for the contractor to hit, and did they input that CFM50 number into the load calc, or are they using the generic input like “loose, average, semi tight, tight”?
I'm attaching the actual Man J short form with the details you are asking for.
The difficulty with using a single unit for all areas other than the ADU is that the common area in the center has a 14' high clerestory ceiling - so it'd difficult to run ducts from one side to the other. Not impossible, but probably difficult enough to make two minisplits a better option.
I hope you are wrong about not being able to zone a ductless minisplit, because that is what my HVAC designer is proposing. Can you point me to a source that suggests that air zoning a PEAD mini-split cannot be done? I obviously would like to know if that's the case.
On the drawings, North is up.
I’m not sure if you had seen my other comment (#12)?
I think you’ll definitely need 2 systems in the main house.
My suggestion would work with the 14’ ceilings as well.
With the long walls, and lots of glass, facing North/South, plus very limited exterior walls and windows facing West, and West shielded by garage, with only an 87 degree design temp, I wonder if the A/C load might be slightly less than shown, even with new construction code minimum windows and insulation, being pretty adequate now.
Ductless minisplits would be zoned with heads of course, but Ducted minisplits cannot be zoned with traditional zone boards, and separate Tstats. They can’t handle the higher TESP from closed zone dampers. No system should ever be zoned with a 2 ton/800 cfm zone, and a 1 ton/400 cfm zone, or less. You’re asking for a terrible TESP (static pressure) problem when the bigger zone closes. And with a PEAD and it’s very low TESP capabilities, it’s barely possible to manually damper a single room off, let alone half the rooms.
I like the Fujitsu RLF series of AOU/ARU combo for a slim Ducted mini, with its .36”wc TESP capability, for the Guest/office area, but a Mitsubishi 2 or 2.5 ton SUZ/SVZ combo with upflow air handler for the main/master suite/gym area, rest of the house.
I’m starting to think a 9k Fujitsu slim ducted unit for that guest wing in pink.
Then, a Mitsubishi 30k btu SUZ/SVZ traditional style air handler/mini split outdoor unit for the entire rest of the house, green, blue, gray.
With an upflow air handler in the closet for unit 3, a 20”x30”x2” filter/grille, and if you have soffits above that closet, using 1 way sidewall registers along the top/side of that wall/soffit, blowing across the ceiling, towards the guest wing. Then a 4” to the laundry, nothing in the mud, and correctly sized ductwork to the master suite and gym. Pwdr 1 won’t need anything either.
David, I appreciate your suggestions. Perhaps I should start a new thread on the equipment selection, as we are getting away from the original filter question.
I'm definitely concerned by what you are saying about zoning the ducted minisplit for the common area - we are paying thousands of dollars for the HVAC designer to design the system, so if he's proposing something that won't work well, I sure would like to know that.
I do like the idea of the SVZ, as it looks better suited for zoning, and it's probably easier to attach a nice media filter to it (as in conventional systems). Just wondering if we can use it for everything except for the ADU, or just for the common area.
I am attaching a couple of images of that central clerestory area, which makes running ducts across it from one wing to the other a little challenging. Probably nothing can be routed along the south wall (with the kitchen nook), but we could potentially soffit the ceiling over the dining and computer desk areas at the north wall, and run ducts across there.
Do you have thoughts on what would be better - a single SVZ to cover everything except for ADU, or like you suggested, a ducted unit for guest rooms and office, and an SVZ for the common area and primary bedroom? And when I say "better", I'm curious if the advantages are cost, efficiency, complexity of ducts, etc?
Edited - I asked my HVAC designer about this, he says we cna do either, but he has full confidence in the PEAD approach. He says they have 0.6" static capability, which he believes is plenty.
I also prefer options 3 and 4, just need to figure out which is better (maybe different solutions for different parts of the house).
The designer also proposed a fifth option, for the two units that are mounted in soffits that border full-height ceiling (image attached). Basically the return air would come in from the soffit's vertical "wall", with a filter presumably right against the PEAD's return inlet. But that would result in a pretty small filter, or at least one that is less than ~12" high. Is that going to cause difficulty finding replacement filters?
12" filters are not stock size around me. Findable but not readily available, especially if you want 4" for low restriction.
There another option if the unit is mounted say in the ceiling of a closet or laundry room. You run an 8x24 duct out from the back of the unit to the wall to the living space. Inside the wall transition to a standard filter size and install the filter grill on the outside wall surface. You can see the return transition in the wall in the picture before it was drywalled over.
Since you are looking at a new build, you can hide most of the ducting with a bit of upfront design. You can build double stud partition walls with 2x4s on flat to run ducting along the house. You can also run ducting in the ceiling of closets. Above or behind built in cabinetry/shelving is also a good spot. Oval ducting is also your friend.
Running bulkheads along the inside of rooms directly above the doors also tends to make them disappear since they are out of sightlines.
Keep in mind that ducting inside conditioned envelope doesn't need to be insulated, so it takes up less space.
Be ready that this won't be a cheap install. Two slightly larger units with a bit more ducting would probably be 30% cheaper.
That's an interesting idea about turning the return 90 deg to have the filter grille in the wall rather than ceiling. Maybe I'm not seeing it right, but won't that have a very thin profile if in a 2x4 wall? How's the air going to flow through a filter (2"?), then make a sharp turn to flow up through the remaining area? Or maybe this is suited for much thicker walls?
You make a valid point that we should be able to get creative with routing ducts through chases, false walls, etc, for new construction. I couldn't agree more. Unfortunately our architects appear to be "done" with the design (without having really given HVAC much thought other than "we'll have a mini split ceiling cassette in every room"), so I enlisted an HVAC designer to design the system.
But he's not the architect, so I am not counting on him to find creative ways to route ducting through the house. Ideally I would get him and the architects on a call and have us all brainstorm how to do this collaboratively - sounds simple enough, but I am finding that there are huge gaps between how things "should be" and how things play out in the real world, at least with the team that I have assembled.
Nonetheless, I will try to get us all on the same call and talk about how we can route ducts where needed, without it looking like a remodel job.
It is a 2" filter in a 2x4 wall, so only 2" gap behind the filter. As long as velocity is low, air will easily make sharp turns. When you are looking at such a large area the velocity will be under 200FPM, you can do pretty much anything and it will flow well enough. It definitely works as I recently had to change the filter after a of a lot of sawing and it had a pretty even buildup.
Designing in space for mechanicals is part of an architects job. I would have very low tolerance for any visible ducting in the living space on a custom build. Do the design work up front, you'll save a lot of time trying to figure it out during the build and end up with a much nicer install.
There is a poster in here that did install the ceiling cassette in every room. After two years, the whole system was ripped out and replaced with a ducted unit because of all the issues with noise and performance.
I see, so you mean something like what I sketched in the attachment (red is supposed to be the drywall of ceiling and soffit)?
I couldn't agree more about mechanicals being part of the architects' job. We're paying them a ton of money, and I am finding that I am doing more planning and thinking and designing than I ever wanted to. That was the whole point of getting these architects, but at this point we are where we are. I just hope to be able to get them and my HVAC designer on the same phone call so we can all brainstorm how to incorporate the HVAC into the house design, without changing the structurals.
Given some of the responses I'm seeing here, I am not sure that our HVAC designer is giving us the best possible advise either.
Edited, any chance you can find that thread with the guy who had to rip out the single minisplit units?
Pretty much it. I would try to get a flat 90 deg bend between the PEAD and the transition and line the long radius with sound absorbent material. Besides reducing the noise, this has the added benefit of making the rest of the supply ducting easier as it avoid a lot of bends.
Look at comment #9.
https://www.greenbuildingadvisor.com/article/planning-a-furnace-to-ducted-heat-pump-retrofit