Choosing a Ducted Minisplit System
Head is spinning with Oil; Heat Pump conversion options
My head is spinning with options. I have done a lot of reading and education about the options out there and now I’m even more confused than I was when I started. I’m hoping some of the smart people here can give me some guidance.
I’ve had several quotes done now, and for the sake of argument, let’s say they all cost about the same. They are all clearly oversized, but I’ll get them down to the correct size. For mini split systems, they all want to put a head in every room.
We purchased the house in spring of ‘22 so only have 18 months of usage data to go off of. We live in a 1960s 2000 Sqft. single level ranch in climate zone 4a (rough floorplan attached). Our 99% design temperature is 8f / 88f We’ve had the attic air sealed and R49 blown in. Walls have original R-10, floors are R-19. Basement is drywalled in so moving ducting into the basement isn’t an option. Our windows are triple glazed U factor .21. I did a coolcalc manual J, and came up with a heating load of ~35,000 BTUs/hr (attached). This feels a little low based on some of my actual usage data, but I’d bet it’s somewhere between 3 – 3.5 tons.
We currently heat the house with oil / hydronic base-boards. We also have a wood burning stove we use for extra comfort / enjoyment, it’s not our primary heat but gets the house toasty. Cooling is done via central A/C with ductwork in an unconditioned attic. I have to assume the previous owner did the ducts the cheap way, as he did everything the cheap way. They have lots of bad things – runs with big bends and tied to the roof deck, etc. The living space has ~7 rooms: 3 Bedrooms, a den, living room, dining room and kitchen. Only the bedrooms and bathrooms have doors, everything else is open, although the openings between rooms vary in size from 5 feet to 2.5 feet. I’m excluding bathrooms intentionally as they get their heating/cooling from the rest of the house as it is. They are small.
I have solar on the house with excess capacity for heat pumps.
Option #1: Use the existing duct vents and replace the ductwork, heat and cool this way. We’d probably have two air-handlers and compressors. I like this for aesthetic and simplicity reasons, but I’m very reluctant to put heating through unconditioned attics. I spoke with a consultant who made the good point that it’s maybe worth spending extra in energy costs (via inefficient ducting) to get effective dehumidification and filtration.
Option #2: Hybrid ducted and minisplit systems. A ducted system for the bedrooms and ceiling cassette mini splits for the rest of the house. I have the same concerns about ducting in the attic as I did before, but this solves the aesthetic problem that is very important to my partner. It would also allow all of the bedrooms to get heating and cooling directly.
Option #3: All multi splits. The bedrooms would each get a head on one compressor, at my partner’s insistence, and the rest of the house would be on another compressor. Many installers are suggesting a head in all of the dining room, den, living room and kitchen but this seems like overkill to me. I think two would be fine, but I really have no way of knowing that. I’m concerned about humidity control and short cycling in this setup, as well as comfort since people have had problems with comfort on multi splits in rooms with low loads.
Option #4: Bedrooms each on their own head and compressor. Mitsubishi makes a 6,000 BTU head/compressor combo that has great turndown ratios. I’m very concerned about humidity control in this setup. The rest of the house would be ceiling cassettes on a multi split. Either two or three heads.
Going with ducted avoids the extra cost of having to have the ceiling drywall replaced to remove the old duct vents. We’re in a very high cost of living area so cost is a concern.
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Replies
Number 1 is the clear best option in my opinion. Avoid ductless units if you can. Why replace the ductwork? Are some rooms noticeably warmer than others during the summer?
My suspicion is that the ductwork was done shoddily and incorrectly, but I don't have enough knowledge to say one way or the other.
The rooms are within one or two degrees of each other in the summer when the old A/C is running full blast. Ducts are quite loud as well.
#1 or #2.
Although my eye caught on this:
" Basement is drywalled in so moving ducting into the basement isn’t an option."
Drywall is incredibly cheap. HVAC work is expensive. I wouldn't foreclose this option simply because it means moving some drywall.
Thanks for calling this out! I hid a bit of information for simplicity sake but it would be removing drywall & electric and also impact our ability to fully finish the basement in the future (potentially).
Muilt split tends to equal unhappy home owner best to avoid those options.
If you use the duct work in the attic you are likely to lose 25-50% of your heating energy if you condition the attic you come out slightly better by using losing 20-40% of you heat.
I know it is a bitter pill but redoing the basement would likely be the low cost option over the next 30 years and the highest cost option if you sell the house in the next 5 years.
Walta
Where are these numbers coming from?
Not saying they are wrong just curious if you have the study
Leave the basement drywall. It's not just removing drywall and boxing in new ductwork....it's spotting and cutting in registers into finished floors, finding a spot for a return which has substantial size, it's a whole new duct system and working around everything that's already there...wiring, plumbing, drains...trust me this is not a good option. Mitsubishi compact ducted (ceiling concealed) air handlers work very well when installed correctly, we've installed them for many years. I have one in my home, a 1960's colonial 2100 sf. From a design standpoint the correct thing to do is consolidate the smaller loads such as bedrooms and baths on a compact ducted unit, and handle the larger loads with ductless zones. Yes, new attic ductwork and needs to be meticulously air sealed and insulated R-8. Ductless is more efficient and gives you independent control over each zone. Ceiling cassettes (Mitsubishi MLZ series) are very nice, they do add cost. Stay away from the four way cassettes they're a lot more difficult to maintain. The MLZ (comes in 6k, 9k, 12k, 18k) is designed to be maintained from below. If they're installed, be sure they're properly insulated above. We build air-tight boxes from rigid foam around them. Used to use rockwool, that works also. We have hundreds of multi-split systems out there with absolutely zero comfort issues. Dehumidification issues arise when companies oversize drastically which is all too common. I have a multi-split, 3.5 ton hyper heating branch box system. They're amazing (when applied correctly!). Sounds like two one-way cassettes and a compact ducted (PEAD series) would be perfect. Three zone branch box, 3 ton system (42,000 btu at 5 deg F). That's a three day job for a crew that knows their business and it should look damn pretty when done. Like anything else, if it's designed and installed correctly you'll be ecstatic. If it's not, you'll be disappointed.
Thank you so much for the thoughtful comment. Do you have suggestions of what I should ask installers about how they do their duct work? Do you have any thoughts on the MZ-FS06NAH on a single compressor for small loads?
Yes, the numbers are my estimates.
The duct loss leakage is based on my recollection of published articles.
The conditioned attic heat loss is common sense because the roof line has 20-50 % more surface area than the ceiling plus the fact that insulating the roofline all but forces one to lower the R value because an R60 roofline insulation would be unaffordable if not impossible.
If anyone thinks my estimates are out of line, please feel free to express your opinion.
Walta
The MUZ-FS06NA is a great system! Excellent turn-down, very efficient, it does have very low moisture removal ability, but sufficient for small areas to keep humidity controlled. I have one in my office. The issue is cost. Putting a single zone system in each bedroom is a huge expense, and you end up looking at a lot more outdoor units. Some people don't care, some people do not want to see them at all. Not to mention all the extra circuits required, you may need to add a sub panel. Designing these systems is all about balancing your priorities. First and foremost the system must do the job it's designed to do and must do it efficiently. Then there's cost to consider, and of course aesthetic preferences. The best design for you depends on your preferences, there are multiple ways to design this. You could use all single zones, one multi, or a multi with singles. I see a comment that multi-splits result in unhappy customers. Look....I have hundreds and of them out there as whole-house systems, all with very happy customers. Some we've installed KWH meters on so customer can monitor energy usage (solar customers like this). We don't advertise, no facebook page, and the webpage at the moment is a pathetic splash page. Our customers love their systems and the workmanship, and keep us busy with referrals all year. We install every day, even during winter. No advertising, and heat pumps is all we do every day (ERV's also to support our customers). Are multi splits always the best solution? No, definitely not, nothing is, but they do a fantastic job when applied correctly and would save you money in this example. It seems a lot of people here dislike them, must be there have been a LOT of negative experiences that they've heard of and I can understand their viewpoint. We've had to remove, re-design, and re-install multi split systems installed by others and it's disturbing to see some of what goes on out there. Be very selective with your installer, ask to see similar installs in your area. As far as questions to ask about duct work, they should have the ability to design to Manual D, but for these low load applications with 4 or 5 rooms, home runs from a plenum (lined, air sealed with mastic, and wrapped) work very well. Also allows direct heating/cooling in the bathrooms where you would not install a ductless unit. Air handler directly above return filter grille, 12" return. All companies have their preferences on how they configure, the importance is that it's properly sized, sealed and insulated. Attic work sucks. The biggest risk is that you have a crew that rushes and doesn't take the time with the details. They want out of there. I've crawled around in attics for years, being in 18" of cellulose in July is no fun....but too often an air handler for those small loads is what will serve the customer best. As the owner, it's imperative to me that the system performs well and the customer is happy and gets what they've paid for. You need a company that's not just quick to sell you something. Lots of sales people out there that know how to sell but not necessarily provide multiple design options to suit your preference. Nothing intentional or malicious about it, just the way it is. People are creatures of habit, they get comfortable with an approach and stick with it...like the old wall unit in every room approach. It's all they know, it's what they do, and sometimes it's a great solution but should not be presented as the only/best solution. Will that approach work? Sure, but some people hate wall units, and a $4,000 system for a bedroom can be deemed excessive. Also adds to maintenance cost. When you have 6 units that need the indoor coil and fan cleaned at 150.00 each...well that adds up. It's true that most do not need this every year, (some do, like restaurants or units with heavy cooling use, some should be twice a year!) but most residential units are good for 3 yrs or more before they need that service. Point is you have to understand your own goals and communicate them effectively to make sure you end up with a solution you're happy with and there are no surprises.
Thanks again at Mitsuman! Unfortunately pretty much, every company (except for maybe one) I've had quotes from so far, all well reviewed and part of Mitsu's Diamond Dealer and they've all quoted me without first doing a Manual J and they've all been "head in every room" quotes.
I think, although I could be wrong, given our semi-open floor plan, we don't need a head in every room if we're going the mini/multi-split route. Also they're all way oversized, assuming my Manual J is correct.
So I'm kind of stuck in a state where I don't know what's best or appropriate, so that's why I'm reaching out here and trying to advocate for myself! I also think I have a good handle on the tradeoffs: efficiency, cost, aesthetics, comfort and I'd rank them comfort, cost, efficiency, aesthetics although my better half might bump up aesthetics.
Re: MUZ-FS06NA I have a quote that is only a $2,000 difference between a mini-split in each bedroom and a multi-split with a head in each bedroom which seems like a good tradeoff to me. I've had my panel upgraded and have plenty of space in it. Also the location where the outdoor compressors would go is an unused side of the house so aesthetics don't matter much.
I do like the idea of just having a small ducted portion for the small loads and then doing the rest with 2?3? ceiling mini-splits
“they've all quoted me without first doing a Manual J and they've all been "head in every room" quotes.”
It seems to me that expecting a contractor bidding on your job to invest 4 hours into a manual J calulation for your home is unrealistic. Someone may stick a few numbers into a program and get a print out but it would not likely be worth the paper it is print on.
If you want useable numbers I think you should pay someone not bidding the job to run them like energy vanguard.
Walta
Well that's unfortunate to hear. From my experience your load calc looks to be in the ballpark. Can't speak to exactly how accurate it is without getting the pertinent data and running one but it seems the purpose of this was for you to use to check against the quotes you received. I wish there were more consumers like you! I use Elite RHVAC for residential loads. Like any program it's only as good as the inputs you give it.
I looked at the layout. That southwest end is set up nice for an airhandler. Given comfort and cost are 1 and 2 with possible aesthetics in there, it's a really good fit. You get direct heat in the bathrooms and foyer. Provides really balanced conditioning with good coverate. It saves cost over all the single zone systems and aesthetically, no wall units. I like the Airtec MV diffusers. They have nice butterflies if you want to tune, and they come in 1, 2, 3, and 4 way square and a round 360 (and an antique style that doesn't really diffuse the air). We use mostly the smaller size in these applications (about 8"x8").
The other part of the house is actually set up pretty well for another airhandler but there are also other options.
The central part of the house you could cover with one unit, although the trusses are not really going in right direction for a one-way ceiling cassette. You could use a 4-way cassette centrally located. We've re-framed and done this, it's not always ideal as the framing can interfere with the connections at the unit but it's certainly doable. Requires some care because you're cutting framing that's attached to drywall. Not all HVAC installers are skilled carpenters. Some HVAC companies hire carpenters to do this part for them. It doesn't really take long, 2-3 hours to frame it out, suspend the unit and get it piped. The Den and Kitchen would be a good fit for the little MLZ-KY06NA. They're really nice, a downsized version of the MLZ-KP09, 12, and 18. If you did this, it would be a multi for the ductless (they don't make a 6K outdoor unit that can run a 6K ceiling cassette). Would be something like 6,6,9 or 12 on a MXZ-3C24nahz, and a SUZ-KA15NAHZ, PEAD-A15AA8. Sorry I'm in a rush, wife's bday I gotta go to dinner but anyway that's just off the cuff. The other option is just run another airhandler for that side of the house...so you have a Living area zone and a bedroom zone. There are other options also...it's a lot easier when face to face, in the space!
Thank you so much, it's been really really really helpful to me to get the perspective of a professional. Much appreciated.
You're most welcome.