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Community and Q&A

Choosing between ducted and ductless heating

Chrisroche | Posted in Energy Efficiency and Durability on

Hi All,

I am currently building a new energy efficient home in the New Hampshire climate zone 4b and am having some difficulty in deciding on the best route for heating the home. I plan on heating with electricity (powered by solar) and like the idea of a ducted heat pump due to its ability to provide consistent heat throughout (also better for resale). However, I have read that efficiency tends to drop with ducted systems. Additionally, it can be difficult to size a ducted system with efficiency in mind, unlike having several ductless mini splits. If I went with a ducted system, the ducts would be within the basement (conditioned space), my assumption is that I would not have issues of heat loss efficiency through the ducts, as the heat would just escape into the home.

Some information about the house I am building.

It is a 1,700 sq ft ranch with an additional 1,700 sq ft conditioned walkout basement. Roughly 2/3 of the basement is below grade. I had a manual J calculation performed by a local professional and the results were as follows.

16,000 BTU’s for the upstairs
8,500 BTU’s for the basement.
For a total combined heat calculation of 24,500 BTU.

For reference
The home has 12″ double stud walls with r-40 dense pack cellulose, r-7/8 tripple pane intus windows and r-60 attic. The basement has 2 inches of xps rigid foam on exterior and 3 inches of closed cell spray foam on the interior side. The exterior plywood was taped at all seams, including the sill plate and was then wraped with air sealing house wrap. The interior side of the exterior walls and ceiling was also wrapped with a smart membrane air barrier, so I should effectively have two air barriers with the home (interior and exterior).

I have attached a .PDF which has the house plans and insulating notes on what I just mentioned about the home.

I have narrowed down to two main options for heating and this is where my confusion starts. My HVAC contractor is not used to energy efficient buildings and does not seem to have the expertise in this matter. I am trying to get as close as possible to the Manual J, but also understand that it is only an estimation and could be wrong, so maybe I should plan on slightly over-sizing.

1- Ducted
Bryant – Evolution Series 2-ton 280ANV036000 Which has an expected maximum output of 18,000 BTU’s at -5F. At 40F it has a range of 14,000 BTU – 30,000 BTU, which to me indicates it is the sweet spot for not cycling too often.

Here is the link

This system has an HSPF of 13. and a COP of 2.2 at 5F running at maximum capacity.

Since this system would not deliver the full 24,500 BTU when its -15 degrees outside, which happens a few times a year, I would likely have to supplement with electric baseboard which I can do, but is not ideal, although correct me if I am wrong.

The 3-ton version has a maximum capacity of 28,000 BTU’s at -5 degrees F. At 40 degrees F it has a range of 25,000 BTU’s to 46,000 BTU’s. It seems to me the 3-ton version would meet all demand, but would likely cause lots of cycling and losses in efficiency.
It has an HSPF of 12.5 and COP of 2.24 at -5.

2 – Ductless mini-split

A single mini split in the family room, which would draft warm air into adjacent office and bathroom.
——-Mitsubishi MUZ-FH09NA Which has an expected maximum output of 10,900 BTU at 47F and a minimum of 1,600 BTU, At 5F it has roughly the same output with a COP of 2.32 at max capacity (3.13 COP at Min capacity) .

A 2 head unit multi mini split with a head in master bedroom and a head that heats entire basement below. (which will be on the walkout side of home)
——-Mitsubishi MXZ-3C30NAHZ2. Which has an expected maximum output of 28,600 BTUs at 47F and a minimum of 11,400 BTUs. At 5F it has roughly the same output, with a cop of 1.75 at max capacity (2.29 COP at Min capacity)

So here is where I stand, I cant seem to decide which approach is better or if I even have either option dialed in to where they should be. Thank you again and I apologize for the long post!

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  1. GBA Editor
    Martin Holladay | | #1

    As far as I know, the Mitsubishi equipment will perform better at low outdoor temperatures than the Bryant heat pump, and that would probably be the deciding factor for me.

    That said, you might want to wait until Dana Dorsett sees your question, because there is a good chance that he has an opinion to share.

  2. user-1072251 | | #2

    Good specs! I won't get into sizing your units, but we've built a number of similar homes in that area - all with Mitsubishi Hyper heat non ducted units (and one ducted). It's critical to get the house very tight, so plan on a blower door test prior to insulating & drywall. Beyond that, we've had zero problems with these units and a lot of happy clients. I'd suggest a separate unit or two in the basement, rather than a ducted, but either should work.

  3. Chrisroche | | #3

    By the way Bob, I wish I had run into you sooner, I have practically GC'd this project since my builder is not as experienced in these building techniques. I did a quick google of your name and it looks like you might be just a few towns over from where we are building in Gilford!

  4. Chrisroche | | #4

    Thanks Bob and Martin for the replies. I plan to eventually do a blower door test, but was thinking of waiting till after sheetrock as I am concerned the siga ceiling membrane will not support the r-60 insulation well. The only other problem with waiting is that I really need to run the exterior hvac plumbing now, before we spray foam the basement. So I don't know if a blower door test makes sense for us at this stage.

    I agree the Mitsubishi will definitely perform best based on performance numbers. It also looks like it will handle the cold weather better. Just this week we saw almost -10F temperatures for several consecutive days. I am just nervous about putting wall mounted units that may have uneven distribution of heat into a home and what that means for comfort. I worry about the event of having to sell the home for any unexpected reason. If my actual energy usage is only 10-20% higher with a ducted system, I think that would be the better option, but I would need it to be able to handle all our cold weather in NH, with maybe a few days on supplemental resistance backup.

  5. Expert Member
    Dana Dorsett | | #5

    For the record, NH is either zone 5A or 6A, not 4B.

    Without the 99% design temperature and indoor design temperature it's hard to tell if the Manual-J is reasonable, or inflated. The actual 99% outside design temp for Guilford is in negative mid- single-digits, not double digits, and with the thermal mass of a foot of 3.5-4lb cellulose in the walls the fact that it hits negative double digits doesn't really matter- the load "seen" by the heating system will never reach the load of a stead-state -15F.

    What was the presumptive infiltration/ventilation load?

    The load/area ratio of 24,500BTU/hr divided by 1700 square feet is a fairly high 14.4 BTU per square foot for a house with R40 walls and U0.13 2 windows, and simple rectangular designe, even at an outdoor design temp -10F.

    I would have anticipated a design load in the 15,000 BTU/hr range, not the 25,000 BTU/hr range, but if they were presuming ASHRAE 6.2.2 levels of ventilation without benefit of HRV it's at least remotely conceivable that it's that high.

  6. Chrisroche | | #6

    Hi Dana, thank you for the climate zone correction and reply. I have not yet seen the actual manual j report, only a few details, but I'll try and find out more information for you on the infiltration rate used. It may take a few days to hear back. For reference, I'll be mechanically ventilating with the Panasonic intelibalance 100 Erv. It's just two of us living there, so we may have it set to the lower speed at 50cfm.

    I hope to get an excellent blower door rating, but time will tell. The guys over at performance building supply in Portland Maine had told me that many of the builders that work with the specs I'm building to (double stud walls, intus windows, Siga tapes etc often see BTUs at 5-10 per square foot.So I too am a bit concerned my report came out high.

  7. Expert Member
    Dana Dorsett | | #7

    Running a basic I=B=R whole-house heat load calc on the place.:

    You have about 150 square feet of U0.13 window + sliding door, and assuming a 75F temperature difference (-5F outside, 70F inside) your window losses are:

    150' x U0.13 x 75F= ~1500 BTU/hr.

    The you have about 1900' of ~U0.017 attic for

    1900' x U0.017 x 75F= ~2400 BTU/hr

    Your gross wall area is about 2000', less 150' of window & door leaves 1850' of ~U0.025 wall.

    1850 x U0.025 x 75F= ~3500 BTU/hr.

    Ok add it up and you're up to ~7500 BTU/hr for the first floor conducted load. With an HRV and a passive-house tight structure you MIGHT have 2500 BTU/hr of ventilation & infiltration in a gale force wind, call it ~10,000 BTU/hr. Where is the additional 6000 BTU/hr of load coming from to come up with 16,000 BTU/hr? (I'm guessing they presumed non-HRV ventilation at the full ASHRAE 62.2 rate.)

    The basement losses will be something like half that depending on how the walk-out side (?) of the basement is insulated, call it 5000 BTU/hr and you're up to a whole-house total of 15,000 BTU/hr before you start discounting for the 24/7 plug loads and mammalian bodies.

    One would have to pore over the full Manual-J to figure out where the extra 10K of load is coming from. There may also be a separate document showing how the U-factor of the non-standard wall assembly was calculated. It's unlikely that the thermal mass effects of the cellulose was factored in, but that's kind of "in the noise" of the error bars.

  8. Chrisroche | | #8

    Wow Dana this is extremely helpful. I really appreciate your time! It sounds like I need to talk with my HVAC contractor to make sure they did a thorough job with the report. It appears from these basic calculations that we would be within the window of heating with a ducted Bryant 2 ton or an even smaller mini split option. Since the house is long and narrow at 68 feet, a single 9k btu mini split may have trouble reaching both ends of the first floor. My inexperience in heat distribution does make me worry here. So I would assume I would need another unit in the master bedroom, possibly a multi split that could also handle the basement as well. I am beginning to think going the minisplit option is best.

  9. user-1072251 | | #9

    Chris - the issue with doing the blower door test after insulating is that by covering up the framing, you may have lost the ability to fix many of the problems. In a house like you are building, there are far fewer cold spots since each room loses heat much slower, so there are fewer problems with "distribusion of heat". The homes behave very differently and can be counterituitive. In terms of installing the ceiling membrane (Majpel?) prior to insulation, we do that all the time - install the membrane, strap with 1x3 under the membrane and install the insulation later; we have no issues with support.

  10. user-1137156 | | #10

    Hi Chris,
    I agree with Dana! The "Manual J" report you have is very questionable at best. I'm curious as to why you didn't mention the Fujitsu XLTH multi-splits as an option? The Fujitsu duct ed heads allow for longer ducts, than the Mitsushi line. Using the duct ed heads on a mini or multi split does result in slightly higher electrical consumption but does not reduce the "capacity". I'd look at an AOU24RLXFZH with 3 indoor heads, a slim duct ed 12Kbtu head (located in the basement ceiling) for the bedrooms, a 9kBTU wall mount for the upstairs living area and another 9K BTU wall unit in the basement. Are you planning a PV array on that nice South facing roof?

  11. user-1072251 | | #11

    By the way, your climate zone is probably (6). Not to be confused with climate specific "planting zones" which have a different numbering system.

  12. STEPHEN SHEEHY | | #12

    Chris: Our new house in Maine has similar insulation specs and double stud wall. Ours is on a slab, with about 1700 square feet of conditioned space. You can certainly heat the great room with a single minisplit. We have a single Fujitsu 12RLS3h that heats a space about the same size. We have a 9RLS3h in the master suite which also keeps the space comfy, although we also have heat in the bathroom floor for one of us.

    If the extra bathroom is just for the occasional guest, you might just put in electric heat.

    For the office, why not change the door location and put in in the wall it shares with the great room? Our spare bedroom/office is off the great room. We installed an electric heater, but never need it since we keep the door open. Even With the door closed, guests seem comfortable even without using the supplemental heat.

    While on the subject of doors, I strongly recommend three foot wide doors throughout. I also suggest pocket doors which avoid doors intruding into the room or, especially, the hallway.

  13. Chrisroche | | #13

    Stephen and Bob, thank you both for the suggestions!

    Jerry, I had not considered going the multi split route because I never considered the option of going both ducted and ductless with a multi split. Thank you for the suggestion! Looking at the math there are some clear advantages for the Fujitsu in terms of shoulder season cycling. At 46F the Bryant has a minimum output of 14,000 BTU's. The Fujitsu has a minimum output of 6,200 I have not done the calculations, but during the shoulder seasons (sept,oct,nov,april,may) when the temperatures average 40-60s, I will want to be as close as possible to that 6,200 figure. So that leaves me with two options and eliminates the Bryant as well as going both a single split and multi due to having too much minimum output. .

    Considering a total load of 15,000 BTU's with 10,000 upstairs.

    Option 1.
    A single 10,500 mitsubishi mini split MUZ-FH09NA for the upstairs with the hope that it will heat all 65 linear feet of the home. with a minimum output of 1,600 BTU.
    Another single 10,500 mini for the downstairs with a minimum output of 1,600 BTU.

    Total combined min BTU of 3,200 at 47F
    Total Combined max BTU of 21,000 at 47F
    COP at max capacity at 5F of 2.32
    COP at min capacity at 47F of 4.26
    Input power at max capacity 1.35kW. 2 units would be running so combined input power of 2.7kW.

    Option 2.
    A multi split Fujitsu AOU24RLXFZH with 3 heads.
    25,000 rated capacity at 47F
    Minimum rated capacity of 6,200 BTU at 47F
    COP at max capacity at 5F of 2.09
    COP at min capacity at 47F of 4.0
    Input power at max capacity 3.57kW

    Heads divided as follows.
    One 7k ASU7RLF1 in the great room (754 sq ft).
    One 9k slim duct ARU9RLF ducted into the 3 bedrooms. (Roughly 900 sq ft if you include hallways and rooms off of bedrooms.) Or 7k if thats overdoing it.
    One 9k compact cassett AUU9RLF in the basement.

    So what I have taken from this is that the multi split has a COP range of 4.26 - 2.32 (minimum usage in warm weather - max usage in cold weather) and input power of .16kW - 2.7kW ((.08 - 1.35)x2) and the multi split has a COP range of 4.0 - 2.09 and an input power range of .5kW - 3.57kW.

    The single splits have a minimum capacity of 3,200 BTU and the Multi has 6,200 BTU.

    Clearly the single mini splits are more efficient, but the hard answer is at what cost (comfort) I am thinking the multisplit is the best option even though it is likely to be oversized, I am just hoping it wont cycle too often in shoulder seasons.

  14. brp_nh | | #14

    Backing up a bit to previous comments, everyone should keep in mind the house is a 1700 sq ft ranch with an additional 1700 sq ft conditioned basement, for a total of 3400 sq ft of conditioned space. I'm not an expert on this stuff, but the original manual J of 24,500 BTU sounds reasonable. Please correct me if I'm wrong.

  15. Expert Member
    Dana Dorsett | | #15

    "I need to talk with my HVAC contractor to make sure they did a thorough job with the report."

    So it was an HVAC contractor, and NOT a registered engineer or architect who ran the Manual-J?

    If yes that explains EVERYTHING!!

    (Sadly... it probably does.)

    HVAC contractors make their money on the installing & servicing the equipment and doing it well. About 19 out of 20 of them will grossly overestimate the actual heating loads (consciously or otherwise) out of anxiety about under-specifying the equipment so that they don't get the 5AM call from the irate freezing client on the coldest day of the year. Some don't even bother look at anything in the plans other than the dimensions and assume code-max U-factors on everything, and code max air leakage. At least these folks weren't THAT bad. But it' still a ~65% upsizing from the probable-reality. With modulating equipment (particularly ducted heat pumps with low turn-down ratios) that's way too much, cutting into both efficiency & comfort.

    A qualified certified P.E. or architect capable of doing the U-factor math on the non-standard wall construction would probably charge less than a grand for the full wall analysis + room-by-room Manual-J.

    A single compressor AOU24RLXFZH solution might be a bit overkill for a ~15K load and with the high minimum-modulated output of the compressor it won't modulate much. Get some real load numbers.

    A 65 foot long interior isn't well served by a single mini-split. A pair of half Mitsubishi FH06 half-ton mini-splits in the higher load areas or a FH06 + FH09 would probably cover it though. They modulate down to about 1600 BTU/hr each, so even a pair of them has about the same modulation range on the low end as a single Fujitsu head. Even three of them would have a minimum modulation level lower than a multi-split, and you'd have the option of turning one or more of them completely off during low-load periods. With a multi-split and a long house you can also run into maximum refrigerant line issues if they all have to run back to one compressor location.

  16. user-1137156 | | #16

    I think you are needlessly worried about cyclic operation. Before the age of inverters all systems operated by cycling between full capacity and no output. If you have a 6200 BTU unit supplying a heat loss of 3100 BTU the unit will run 50% of the time,at full capacity and efficiency, and use, on average, about 1/2 the energy that it would if supplying 6200 BTU. What is undesirable is "short cycling" , where the unit never gets to run long enough to properly stabilize. You should have more than enough thermal mass in each area to avoid short cycles with the typical +/-1 degree set points. However, If the cycles appear too short a switch to an external thermostat usually resolves the issue.

  17. woodguy00 | | #17

    Would it work to fur down the 9’ ceiling in the long hallway to 8’ and use the space for a single ducted Fujitsu with ducts servicing each bedroom and the family room space? No wall units that way but is it too much ducting?

  18. user-1137156 | | #18

    Your suggestion could work but it runs into the limited selection of XLTH models. There is no XLTH that supports ONLY a single duct ed indoor unit. The single indoor XLTH units are either wall mount or floor mount, this is unfortunate but reality. If a single head duct ed head were available either your way or putting the mini duct unit in the basement ceiling could solve the closed door heat distribution puzzle. The basement location allows the "cheat" of using the entire basement as a return plenum,

  19. Chrisroche | | #19

    Hi Dana,

    I think your assumption is correct, the mostly likely answer is that high air infiltration numbers were used. (I hopefully did not spend 6k on siga wraps and tapes for nothing). Working on getting updated figures, but at this point it looks like I would go with either three mitsubishi 9k mini splits, or a 22k fujitsu multi. Both options are the minimum sizing without cold spots and still hit the high figures given by the contractor, so it may not matter that the HVAC contractors estimates were high in my case.

    Slightly leaning towards multi split option as it cuts down on exterior compressors/maintenance, although I am drawn to the efficiency of the Mitsubishi's, the only remaining obstacle is that I have seen is some pessimism towards using a one of the heads as a fujitsu slim duct to ventilate 3 bedrooms, with ducts below in the basement. The issue seems to be that these ducts were designed for single room heating and have a low static pressure of .36, versus traditional whole house ducting which typically ranges from .5 upward. I have heard that this may not be enough static pressure to properly distribute the heat to three ducts via a plenum in the basement, unless designed very carefully. Also, mini splits work by throwing the heat, which may be reduced to very little air movement by the times its split 3 ways.

    In my case, it would be a 3 way split off the plenum that would travel roughly 6-7 feet in both opposite directions to hit bedrooms on either side of the middle bedroom. So it is possible that this limited amount of ducting may not be an issue for the low static pressure.

    Also, just a tip for anyone reading this. I found a terrific resource that compares many of the current cold weather heat pump options in terms of efficiency and output at different temperatures. It allows you to filter to exactly what you are looking for and is in excel format.

    Northeast Energy Efficiency Partnerships

  20. user-1137156 | | #20

    I'm mentally wrestling with a similar situation. My house is a bit bigger and I'm in a warmer region but I also want to use to use cold climate heat pump(s) as my sole heat source. Regrettably these things do break and I sure don't want to be completely without heat. So I'm inclined to opt for two systems, thinking I can live on one if one does crap out. I've done a bit of research into duct design and I'm sure you'll have no difficulty if you stick with 6" galvanized ducts. There are other options among "off brand" systems, I'm seriously considering using Midea from:

  21. Chrisroche | | #21

    Thanks for the advice. I spoke with my fujitsu distribution engineer and he agreed with you that with the short run (7 ft ducts in either direction) and using metal ducting we should be fine. In my previous post I noticed a mistake in my thinking regarding using multiple mini splits, the main thing is that all 3 units will rarely run at the same time, (input power at minimum capacity is less than half that of the multi) so the performance rating of singles truly will be much higher.

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