Ducted minisplit
Hi GBA community,
We were designing our new penthouse apt and wanted the flush linear slot diffuser look using the mitsubishi duct minisplits. We were going to have 1 head for each bedroom connected to each its own individual outdoor condensing unit.
The GC informed us that it may not work the best if we cant keep constant temperature in the apt the linear slot diffuser will start to sweat condensate. Is this true?
Will it be better if we put the unit right at the opening of the diffuser so there isnt much duct work?
However he said it might work better if were using the metal linear slot diffusers that go on the sheetrock that are more visible but still modern look instead of having the normal grate look most homes have.
Any thoughts.
Flush look we wanted: https://inviair.com/wp-content/uploads/2018/12/Linear-1s-Fin-Wall-1.jpg
Metal diffuser he suggested that might be better: https://i.pinimg.com/736x/64/ba/d8/64bad8ee62dd5c214f4939ae7739b685–linear-slot.jpg
Original Older look we want to avoid: https://www.austinacexpress.com/images/blog/12.jpg
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Replies
1. Diffusers are metal. If you have a humidity problem, they will sweat. It does not matter if they are linear slots, or typical residential type. If your home is properly built, and your HVAC system is properly designed, you should never have condensation on any type of diffuser.
2. The type of slot diffuser you want is very labor intensive because the duct has to be installed exactly correct, and then the gyp sub has to bed and finish around it with a special product before they mud & tape. This is an aggravation of coordinating trades and even very few commercial subs can execute it properly; it would be quite a challenge to get residential subs to execute that look. I suspect that is what your contractor wants to avoid. Watch this video:
https://www.youtube.com/watch?v=EFM1oOQla_Y
This is an expensive process and very difficult to get a good looking finished product no matter what you pay. The manufacturer of these things actually claims they reduce condensation issues - LOL.
The link you posted to what your contractor wants to install does not work, but I suspect a traditional slot diffuser with a trim ring that will allow them to install duct, then install, finish & paint gyp with a hole cut for the duct, then have the HVAC sub come back and slap a diffuser on it and the diffuser trim ring will cover the ugly sealant between the gyp and the duct.
3. I would be extremely surprised if you need a single ducted mini for EACH bedroom. The whole point of a ducted mini is that you can duct them, so you bring conditioned air to multiple locations via a duct. You wrote you are building out a penthouse apartment. Unless the roof is very poorly insulated so the loads are enormous (and if so you have a different problem) you should be able to serve at least 2 bedrooms with each mini split. Have you had a professional who is NOT your contractor and NOT your HVAC sub do a real manual J calc to determine your loads?
We havent had an independent HVAC sub look at this yet. We wanted independent control for each room.
This GC we work with should be able to do this as we have worked on many jobs with them in the past and their portfolio is the typical high end renovation in 1-10 million dollar range for units. I have seen their work and dont think they will have a problem.
In total we would probably have 5 indoor/outdoor units feeding the 4 bedrooms, 1 for living room, dining area, entry foyer, etc.
We have a large enough terrace where we can fit them anywhere and box it in nicely with a fence so not too worried.
Do you have other suggestions on what we could do for while achieving this look but not using any floor space in the condo.
Thanks.
You don't want an HVAC SUB to look at it, you want an engineer to calculate it - a firm like Energy Vanguard or one of their competitors (poke around on this forum) - a firm that does not install, and that specializes in optimizing energy efficient residential systems - not a typical consulting MEP that will oversize the system...
The GC is using all the same subs everyone else it. Based on experience I can assure you it would be a problem to get a sub to execute the diffuser you linked on a >$100 million project - the cost of the project is not the issue beyond the fact that it is a very expensive detail. But even if you are willing to pay for it, it is still problematic. You may not think the GC will have a problem, but I strongly suspect based on your GC's response that the GC does think it he/she will have a problem executing this....
Unless this is a 5,000-6,000 sf unit, you don't need 5 mini splits. Mini splits don't operate at peak efficiency when only a fraction of their capacity is called for. You can still have independent control for each room, especially if you can pay for it. That does not mean you should throw more mini split units at the issue, but maybe if you can get some very small ones...
Not sure what you mean by "not using any floor space"...are you referring to not giving up floor space for the HVAC equipment? Or that you want the supply & return diffusers on walls & ceilings? The condo construction will probably dictates that your diffusers need to be on the walls & ceilings. Mini splits are probably the correct way to heat & cool. You just need to get the details right for proper comfort. Get a professional. See 1st paragraph.
I can't see the first link being done in any reasonable way either. The one way I could see it is by making a cover out of moisture rated MDF flush with the drywall and skim coating the whole thing with mud for a clean look.
Once you figure out what your actual needs are, you can look at diffuser options. I would not be surprised that you need anything more than a one of these mud in vents:
https://ariavent.com/collections/drywall
Much easier to install and connects to standard plumbing in the back.
The one ducted mini in each bedroom is way beyond overkill in most cases. You can look at zoned setup using either Airzone system or Honeywell zoning kit using a thermostat interface module on the ducted mini split. With any ducted setup make sure to leave space for in-line duct silencer to reduce noise transfer between rooms.
EDIT:
I take it back, this doesn't look that hard to install but does need pretty deep walls:
https://inviair.com/wp-content/uploads/2018/12/SUBMITTAL-L100.pdf