Multizone ductless minisplit heat pump
anonymoususer
| Posted in Mechanicals on
Hi, hvac installers and mechanical engineers! In the Comments section of Martin Holladay’s recent article on heat pumps, a kind contributor warned that ductless splits which are multizone (as opposed to single) tend to cycle on/off because they’re unable to modulate down to low speed. Is this less of a problem when there are not very many heads (yet still greater than 1) being served by each compressor ? For instance, which will cycle on/off more: a) one 40K compressor serving 8 indoor heads versus b) two 20k compressors, each serving 4 indoor heads ? Thank you
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The turndown ratio is baked into the system. So while one-to-one systems often have a turndown ratio of 5 or 6, multisplit systems are generally more like 2 or 3, and that's regardless of how many heads are running from one compressor. That said, if there are a bunch of heads, there is more likelihood that one is calling for heat or cool at any time so potentially, the compressor will be turning on and off more frequently.
Thank you, Peter. I do notice that for all Mitsubishi compressors whose model # starts with MXZ (2.7 ton up to 4 ton), the turndown ratio is anywhere from 2.0 to 3.2. For my house, I feel forced to purchase multiple compressors and connect only 2 or 3 heads to each one because the floor plan / layout is very spreadout, convoluted, lots of bumpouts. Running lines from, say the exterior south wall of the house to a head that is on or near an interior north would involve drilling through lots of joists and fireblocks and might not meet piping length restrictions imposed by manufacturer (altho admittedly, i have not measured yet). For this reason, Im skeptical I will be able to use, say, a single compressor with 8 heads; rather, I will probably need compressor A on the exterior south side of the house with 3 indoor heads, plus compressor B on the exterior north face with 3 indoor heads, plus compressor C on the exterior east face with 2 heads. Tough to figure out and tough to find around here a mech engineer who wants to take on the job of figuring this all out. I already paid a huge sum to one who scratched his head & walked away perplexed. Thank you for your explanation
Consider reading the questions on this forum about miny and multi splits. I think you will find the unhappy owners have two things in common. One is multi split and the other is hugely oversized.
Walta
Thank u, Walta. I have observed precisely what u point out. Unfortunately, I feel between a rock and hard place :-(
I'm a happy multisplit owner, though I acknowledge the challenges in designing them - they are definitely a compromise solution, though one that can make sense sometimes, especially when retrofitting old houses.
Yes, the lower turndown ratio (aka, higher minimum output) is a given, though even in this models vary, so look at the specs closely. Whether this is a problem depends a lot on how the system is designed and laid out in the house.
My guidelines for designing a multisplit would include:
- Ensure that all zones covered by one outdoor unit have a similar load profile, IE, similar use patterns, same side of the house, same floor of the house, to the best of your ability. If all of the units are calling for heat at the same time, then the minimums are less of an issue.
- Slightly overcommit the outdoor unit, 110-120%, with indoor units, that reduces the amount of time that the outdoor unit will try to be below its minimum.
- Use a good room-by-room Manual J calculation to be precise on sizing for each load area. This is always good advice, but is particularly important if you are sizing precisely for a multisplit.
- Use thermostats with remote temperature sensing. Good advice for any minisplit, but particularly helpful for multisplits because they are often helping each other heat common spaces between the units, and this keeps them cycling together, rather than randomly attempting to cycle. You want either most/all of them running, or all off, ideally.
- Use units that can turn down "off" units further - this isn't easy to determine from the specs, but for example the Mitsubishi Smart Multi outdoor units can restrict flow much further than the older Cxx series, which helps prevent overheating in low-load rooms like bedrooms.
- Use slim ducted units wherever possible to combine low-load rooms like bedrooms or bathrooms.
Sam, I cannot thank u enough for these very helpful guidelines--thank you! When you suggest that I "Slightly overcommit the outdoor unit, 110-120%, with indoor units" do you mean the following as an example: Outdoor compressor has nominal capacity 20K but the some of all, say, 3 indoor heads is slightly greater than this: like maybe 9K (head A) plus 9K (head B) plus 6K (head C) ? Note that in the example I thought up, the nominal capacities of indoors heads sum to 24K. Would it be preferable in this scenario to spec an outdoor compressor with nominal capacity of 20K or 24K ? Thank you
Yes, you want the outdoor unit to be slightly overcommitted, so the sum of the indoor heads slightly higher than the outdoor capacity.
Make sure you are checking the actual submittals and manuals for the components - the actual min and max capacities may be quite different from the "nameplate" value, especially for heating. What you are shooting for is getting the sum of the minimum of your indoor heads above the minimum of your outdoor unit, or as close as you can. The manuals have guidance on how overcommitted you can make the outdoor unit, but it can be up to 125% for Mitsubishi.
Unfortunately there is no good way to have 8 zones. Doesn't matter single 8 zone or 2x 4zone setup. The isn't just just modulation, the problem is the smallest heads are usually about 3x oversized for the load, so it will never run right.
The other big drawback of a multizone is that there is some refrigerant flow in all indoor heads during heat calls. The problem is any of this bypassed refrigerant that is not actually used for heat is actually wasted energy so COP tanks if only a few heads are calling for heat.
The newer Mistubishi SM series using the SM specific heads have their own EEV and apparently can turn down more. I have not worked with one of these yet so can't comment on how well it will work or if refrigerant bypass is still an issue.
Generally, if you want a multi split to work well, you want only a couple of large zones on them and you want those zones always supplying heat. This does work well but it generally means ducted units so a bit more work to install.
Thank you, Akos. I do see that the smallest heads have 1/2 ton nominal capacity, which is more than some of the rooms we need to heat. Unfortunately, Mitsubishi's SM model compressors are only available in 3 ton and larger. Because my house is so disjointed and spread out, 3 ton is too much for any one region of the house. I just dont see how we could get piping from one 3 ton compressor to each of the rooms where we'd like to put a head. I will think about it more, but right now it seems our only option may be to buy several 20K compressors and connect only 2 or 3 indoor heads to each. Ducted units seem out of the question, since 2 licensed mechanical engineers have already looked at our layout and were unable to figure out a ducted system that meets our heating requirements based on the manual J that I meticulously completed in wrighsoft. Thank you for your explanation
The only reason I can see for multi splits is exterior space or visuals for compressors. I have 3 individual units that I use for AC almost exclusively and one more would be required to use them for heat. There is no reason in my application to go with a multi split. I could probably get down to 3 for a heat/Ac system[no ceilings, no place to run extensive ductwork]
I am not sure why they keep forcing these into designs
Especially now that ducted splits are in the high performance arena
I spec'd out a Daikin 4mxl recently for my brothers house. I was pleasantly surprised how good the turndown is, and how low the min capacity doesn't go up on the larger 4 unit version versus the smaller 2 and 3 head variants. With only one head running (you have to install at least 2 but a low min running only one) it's min capacity is only 3500btu/hr, and with enough heads it's max is 54,000btu/hr (depending on OAT).
https://www.daikinac.com/content/assets/DOC/SubmittalDataSheets/Multi-Split/4MXL36TVJU-Submittal-Sheet.pdf
THank you--I will look into this. Mitsubishi seems to have best representation in my rural area, but perhaps there is a Daikin installer somewhere nearby as well. I looked at the sheet u sent--thank you
Can you post your floor plan. Do you have a basement or crawl? Is the basement finished?
Wall mount in every room is generally a bad idea.
Thank you, Akos. Im attaching floor plan for each of the 3 levels: finished basment, ground, attic. Pls note that the rooms labeled as "basement lounge" & "ground living east" are in reality, bedrooms. Certain adult family members are not willing to keep bedroom doors ajar at night, nor to have any sort of opening in their bedroom wall--not even small & high up--that would open it to a common area. So I see no solution other than to place a head in each of the 6 bedrooms. Smallest head mitsubishi offers is nominal half ton. On floor plan, the different colors show the rooms to be heated by a single head. The 4 blue rectangles at ground level show potential compressor placement. A licensed mech engineer who works for my local mitsubishi diamond dealer, plus a separate independent licensed mech engineer, were not able to figure out how to duct this house. We did purchase ceiling infrared panels for small rooms such as each of the 4 bathrooms, plus each hallway. We have not installed them yet but, if they keep us warm, we may also buy for other small rooms such as laundry and vestibule. I think it would be hard to place ductwork because the basement level has VERY low ceilings. Attic level has cathedral ceilings; no attic space that an air handler would fit in. I was thinking 4 compressors? Each one would serve the handler in the room closest to it at ground, basement, and attic level...? Thank you
That is quite a bonkers layout. How is it heated now? Just electric baseboard?
I think you might have to head the direction you are headed, I had a similar but less difficult conundrum, and ended up with 2 compressors, one on the E and W side of the house, and 5 heads (will be 6 when I redo my basement). Bathrooms are heated with suntouch in floor radiant on a schedule, which does well and is comfortable. Halls are heated by their adjoining rooms, and laundry/storage rooms are heated with small electric baseboards set very low (58F right now).
One thing to consider is if you can try to keep the heads on a similar floor, so if you could do the three attic rooms on one compressor you'd be better off, otherwise you'll be overcooling the basement in the summer, and overheating the attic in the winter. That might mean a few longer linesets, but simpler operation. For places with very different loads, like the basement, you might still want to do a 1:1. Remember that you can make a rack and stack two 1:1s on top of each other in the same space as a normal larger multisplit, with the same amount of lineset run. I'd prefer that over a situation where you have, say, a 9k in the attic and an 18k in the basement which would be a recipe for poor performance.
I'd also recommend doing this in phases. Figure out the easiest spots to do first, get the units in and running, and then see how things are feeling in the other spaces before then adding another multi or a few 1:1s to fill in the gaps.
Thank you, Sam. We only plan to use the heat pumps for heating, never cooling. For some reason, this house stays comfortably cool in summer--visitors, vendors, contractors and everyone in our family cannot figure out why, but even attic level is pleasantly cool in July, especially if windows are slightly open. Our family in particular is generally sensitive to cool temperatures (not sure how we ended up at the Massachusetts-Vermont border, but here we are!), so we are not worried about cooling needs.
This winter we are fully gutted and have been heating with our ancient oil furnace and its wrongly-sized, asbestos enrobed ductwork. She is huge and on her last legs, but she has kept us warm this bitter winter (at a cost of FIFTEEN gallons of oil per day!). We also have a 2020 EPA-certified log-burning insert in one of our 2 chimneys. Thls heats a small portion of the house quite well also, and the wood is free to us, since my brother's main line of business is harvesting cordwood and delivering to local residences.
I appreciate your suggestion of keeping each floor on a separate compressor. Let me look again at our manual J and layout and try to figure out if this would be feasible. For sure it would involve notching multiple joists, but im not sure how big the notches would need to be (I need to look at lineset diameters).
If we install, say, two compressors now, is it real cumbersome to install a third compressor in the future, assuming there is continuous exterior insulation in place? We are hoping very much to get CEI this summer on roof & walls (bought rigid rockwool panels, now seeking contractor to install them) and cavity insulation sometime later this year or next.
It has been a drawn out ordeal but we hope things will take off soon !
If you have ductwork in place already, why can’t new ductwork be run in the same locations as the existing ductwork? Is it because the existing ductwork makes the basement with low ceiling unusable? You are improving the envelope significantly, so the heat load should be much lower, and the undersized ductwork could potentially end up being right-sized. You won’t know unless you do a manual D. If you do multiple 1:1 ducted units, the ductwork can be smaller than one large ducted unit. For instance, three 1 ton units can have smaller ductwork than one 3 ton unit. Plus, you will still have *some* heat if one of the heat pumps stops working.
Edit to add that I would suggest removing any chimneys that won’t be used in the future. It will be easier to insulate and air seal. Plus, if they are inside the house, you will have a little more space and remove an obstruction that you mentioned.
Thank you, Adrienne. Our existing ductwork is wrongly sized based on the cfm's showing for each room on manual J. When I plug those into ductulator app, it shows I should have different size duct than existing. And yes, the existing ductwork in basement is a constant "headbanger" to us tall people. We hope to keep both chimneys because in our community, winter outages that last several days on end are frequent. We get free seasoned cordwood because that is my brother's line of business, so no excuse for not burning wood. We use the log-burning insert to supplement existing furnace. Also, it's a lifesaver each time the power goes out in our whole neighborbood in ebruary and the power company takes forever to come to our neck of the woods to fix it.
Got it. I misunderstood you. For some reason I thought that your existing ducts were undersized for your existing furnace. I didn’t realize that you had already calculated them to be undersized with the new heat loads. Oddly, there is no rely button under your reply, so I replied here instead.
That is ugly but doable. Which way do your floor joists run? Any big pocked beams in the basement?
I would put a larger ducted unit in the basement mechanical room with a supply trunk along the hallway ceiling. From this trunk run takeoffs through the main floor joist space to feed both basement and 1st floor.
For the 2nd floor, you want a slim ducted unit in the hallway ceiling at the end of the hall after the stairs. From there a couple of simple duct runs to each bedroom and bath. Bulkhead in the corner of the hallway ceiling to enclose the ducts.
Wall mount in the living east room and floor heat or baseboard heaters for the bath and hallway.
I know it looks like a big job and the problem is typical HVAC folks are not up for this type of work, you'll want a GC that knows how to work around a finished house.
If you do actually look at it cost in detail, the BOM cost including ducting is less but does require more labor. The work is more disruptive but you'll end up with a much better performing system that will be cheaper to run and most importantly way less maintaince (wallmounts are not fun to clean and with 8 heads, you have 8 potential problems).
Thank you, Akos, for your helpful response. We are not worried about disruption at this time because the house is 100% gutted on all floors: no insulation nor drywall in any stud / rafter cavity. No continuous exterior insulation. Our only R value during this bitter New England winter (border zone 5 vs 6; MA-VT stateline) has been the R1 we get from our wall / roof sheating and our brick foundation. We have been living in it all along, and our very ancient oil furnace, with its wrongly-sized, asbestos-enrobed ductwork, has been doing a great (albeit wasteful) job at stopping hypothermia from setting in and pipes from bursting (knock wood). The log-burning insert in one of our 2 chimneys has also helped, although it only heats a small portion of the house.
In response to your questions, all joists run north to south with actual dimensions of 2 x 8; 2 x 8.5; or 2 x 9 (depending on which part of the house). None have large holes in them. In the past, we considered running ductwork east to west (ie, perpendicular to joists) along the ceiling of the basement hallway, as you suggest. The reason we scrapped the idea is because the distance from floor to ceiling in the basement is only 80 inches (I realize this is probably "illegal headspace" but the house was this way before we moved in). Most males & females over age 15 in this family are very tall.
For the attic level, to clarify: your suggestion is to install ducted unit in hallway just south of stairs, then run ducts to each of the 3 bedrooms on this level ? If yes, it seems like running ducts to the attic bedroom east and attic bedroom south might be feasible. Running ductwork from handler to attic bedroom north will be more challenging due to layout, and presence of chimney. But might be feasible if the ducts are fairly narrow and if flexible duct is an option.
You suggest wall mount for living room east. Would this be single zone ductless ? How would you suggest heating the attic office: single zone ductless vs multizone ductless (same compressor as living room east)?
You say that wallmounts are cumbersome to clean: have you also found this with the floor mounts and the "ez fit" ceiling cassettes? These last 2 are the head configurations we were considering.
Thank you for your help and thoughtful input.
With a gut job the answer is simple, run the ducts, anything else is a hack.
I'm in the land of low basements and the way to deal with ductwork is to run it along an outside wall. This keeps it away from any foot traffic and any supply takeoffs to rooms can now run along the floor joists. If done properly, you don't even notice once the trunk gets boxed in.
See roughly marked up version. Green is the supply trunk along the outside wall. Blue are supply feeds to basement. Red to main floor. The return runs up beside the staircase to the main floor.
Yes it will block some of the basement windows. Since it is set back a bit, you will still get light. You can also turn the trunk on edge and run it along the wall bellow the window and it won't block anything.
For the 2nd floor, that is what I meant. I would do all in flex. Simplest for complicated runs, just oversize a bit and keep the ducts out of attic space.
Missed the attic office. That wall mount there as well. No point in trying to tie it into the rest of the house plus the heat load there will be very different as it is exposed on the bottom. You can put the living east and attic office on a 2 zone multi split.
By wall mount, I mean standard ductless unit. Ceiling cassette and floor mount are also possible. I would say floor mount is the easiest to maintain but because it is not common, HVAC tech won't know how. Ceiling cassette is the hardest as there are a lot of moving bits in a very tight space.
Thank you, Akos. This markup is helpful. Is it acceptable to have a single return for the entire ground floor? I thought I had to have a duct returning to the handler from each room where there is a supply...? What about the return for attic level: would that run next to the supply ducts going to each attic level bedroom ? Assuming attic space is inside thermal envelope, is it okay to have ductwork there?
Around me a central return is pretty much universal. Usually in a hallway so it can service the entire floor.
For any rooms with doors you need a return air path. This can be either dedicated ducting, jumper ducts to hallway or door undercuts. Typical here is door undercuts, if you want better for sound isolation say in the main bedroom, you can do jumper ducts. I see very little benefit to dedicated return ducting.
For the attic rooms, since the ducted unit is already in the hallway, you can have a short return duct with a filter grill right on the ceiling behind the unit. For the rooms, same idea, either door undercuts or jumper ducts. If your attic is conditioned (that is a LOT of spray foam for that in cold climate), you can run ducting through there. You can also put the air handler into the attic space if you don't want it to take up ceiling height, make sure you think about future serviceability and ease of filter replacement.
This is a vertical mount unit, but you can take the exact same setup and mount it on the ceiling:
https://terrylove.com/forums/index.php?threads/ducted-mini-split.84418/
I agree with Akos with the walls and ceiling open find /make the space for the duct work.
The other thing that has not been said is you have a big problem with the exterior wall. The fact is the old timers did a piss poor job of flashing the windows and door. Lots of water has been getting in forever but it did not matter because there was more than enough heat in the wall to evaporate the water and plenty of air movement to carrier it away long before anything could get moldy and rot. If you are going to have modern insulation and air barriers you simply must have modern water barriers and flashing. My guess is if you look closely, you will find water stains around most of the windows and doors.
Walta
Thank you, Walta. One of the reasons we gutted is so it would be easier to install new flanged windows, doors, hvac, and to upgrade our elec scv from 125 to 400 amp. We bought new windows and doors for every opening but have not installed them yet; they are in cellophane wrap propped next to each opening where are supposed to go. We want to get the windows installed as "outies" because we bought 2.5 inch thick rigid rockwool for continuous wall insulation. But contractors in our rural area are mostly unfamiliar with building a "box" in the rough opening so the window can get installed proud of the existing sheathing. This is why windows have not been installed yet. We are praying to find a contractor this year to do the work.