Minisplit questions
-I’m in zone 4, midwest US, and am about to buy a ductless split system for my main structure and the addition I’m finishing now. I am concerned as three of the five rooms I am putting them in are pretty oversized (~2600k btu load cooling, using the 6k heads) and the rep is saying I should go with the H2i. He said if I never need the low temp operations, the heater was worth the money.
I decided not to use one ducted unit for these three zones because the install cost for the ducting alone cost more than the extra two heads and line sets, let alone I figured it would be less efficient since the unit and ducts would be in the unconditioned attic. One of the main hurdles I have to deal with is that 2/3 of my home is slab on grade the other 1/3 has a basement, so ducting is rough.
1. Due to the fact these are oversized so much, and nearly exactly the right size for peak loads in the other two rooms, does it make sense to go with their H2i (hyper heating) for better low temperature operations? Or is this an added $1000 expense that would be wasted?
2. Also has anyone mounted a mini above 10′ in the air? what if there is an obstruction beneath it a few feet below?
3. I am also unable to find operating charts for these units, but it looks like the 42k and 48k compressors are able to modulate to the same minimum amount. Does anyone have these manuals or numbers handy?
If anyone has any other input, please feel free to chime in. I need to get these ordered soon so I can finish running wire and pipe.
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The whole house heat load relative to the minimum output of the H2i multi-split is the primary thing to look at. An FH06 head can modulate down to about 1600 BTU/hr but the compressors only modulate down to 6-7000 BTU/hr. If your average whole house load at +47F is less than 7000 BTU/hr it will do a lot of cycling in a zone 4 climate.
Your instincts to avoid putting ducts and mini-duct cassettes in the attic, punching holes in the pressure & thermal boundary is a good one.
For a zone 4 climate you don't really need the extended temperature capacity charts. The submittal sheets give you the nominal output, which it is guaranteed to deliver down to +17F or lower. The 99% outside design temps in zone 4 are usually at most a few degrees cooler than that, and the submittal sheets show the nominal outputs of various configurations.
https://meus.mylinkdrive.com/files/MXZ-5C42NAHZ_Submittal.pdf
https://meus.mylinkdrive.com/files/MXZ-8C48NAHZ_Submittal.pdf
https://meus.mylinkdrive.com/files/MXZ_Multi-zone_Indoor_Outdoor_Combination_Table.pdf
https://meus.mylinkdrive.com/files/M-Series_Multi-Zone_MXZ-8C48NA_MXZ-4C36-5C42-8C48NAHZ.pdf
http://www.ecomfort.com/manuals/8b59fdef29543849683a74f56e56deda.pdf
Dana,
Thank you for the reply. I'm supposed to drop the order today. After messing with my heat loss numbers I realized I was giving myself a lot more credit for insulation on the north side of my house so the values are closer to 3k btu now.
My outdoor design temp I used for my calculations was 8 F. I found this somewhere on the internet, it must be true right?
Also they are saying that I can mount a unit 10' off the floor and about 1' off a shelf framed into the wall. This would be mounted on a kneewall. I was concerned it might obstruct air flow.
The manuals typically give 4" or 6" as a minimum clearance at the top, but it varies by vendor & model. If you mount an interior head 10' from the floor there is going to be a significant stratification offset between the setpoint and the average room temperature down where mere humans live. It's better to put it no higher than 7' if you can. In the engineering manual for the FH series heads they say 85mm (3-3/8") minimum clearance to a ceiling (or under a shelf), but experience would suggest that if you have more headroom you should take it. A foot below a shelf should be fine. Side clearances to flush surfaces (like a bookshelf with the front edges of the shelf aren't proud of the wall surface) hardly matter, but you don't want to stuff it in the corner of a room, since it'll be more prone to short-cycling. The FH spec calls out 122.5mm (4-7/8") clearance on the left side, and 132.5mm (5-1/4") from the right side. See the diagram on page 3:
https://meus.mylinkdrive.com/files/MSZ-FH09-18NA_Install_JG79A805H03_English_02-15.pdf
Find a nearby city on this list to estimate your 99% temperature bin:
http://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
or this list:
https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf
(be sure to look at the 99% column, not the 99.6% column.)
I'm sure it hit's 8F (and colder) sometimes, but that's an unlikely 99% design temp for US climate zone 4A/4B. It would be pretty typical for a zone 5A/5B location. Zone 4A NYC's 99% bin is +15F, compared to zone 5A Boston's +12F.
At the cold edge of zone 4A in St. Louis, MO the 99% bin is +8F, so it IS possible, but not common.
The H2i series can deliver the full "rated" output even at +5F for most models, whereas mini-splits not designed for cold climates have a dramatic fall off in capacity going from +17F (where it's tested) down to +5F. With other mini-splits it's wise to look at the extended temperature capacity if the load at +8F is very close to the nominal or rated capacity at +17F. If the nominal heating capacity is 25% or greater than the +8F load, don't bother looking it up (unless you have to for making the code inspectors happy).
Thank you again Dana, I've been pushing my wife to try to get it in a better spot. Unfortunately not only is it about 10' in the air, it is above a shelf, not below it. So I was worried it would short cycle in this little nook. The room is 16' x 26' and the split would be in one of the corners facing the shorter measurement.
Another one of the splits being contemplated in a similar arrangement, but it has a good 20' plus of room to 2/3 sides of it.
I was really hoping not to stratify that much hence having the extra heads. If ducting wasn't so expensive I could mount a ducted unit in a closet and make soffits for the supply and returns. I'm sure the difference in the ducted unit and ducts would add another few thousand.
I'm going to piggy back on to this question as I'm trying to plan my system. Right now if I were to buy two separate systems, one 9k btu with condenser, and another 4 zone system at 4 heads with 36k.
If I did this I would save myself over $1000 just in the branch boxes that I would need alone plus another $600 wrapped up in the larger condenser.
- Would a 5 head system on a 42,000 btu condenser end up being more efficient than two separate units? My gut feeling is a single system since one compressor could probably more easily modulate with more zones and loading being used. Contrary to this though the units listed individually have higher SEER ratings. The 5 zone system is rated at 19 seer, while the 4 zone is at 19.2 and the single zone is 30.5.
- I planned on doing a home run type condensate drain system since all the units are on interior walls. Will this pose a problem for bending line sets? there will have to be a 90 degree bend inside a 2x4 wall. I planned on running the drains through the studs as needed to the drain system. I may also have a second drain for one unit to prevent a 40' run of condensate pipe.
Thanks a lot for all of the help everyone. I'm hoping to get this bad boy ordered soon so I can start running lines for it.
The single head single compressor units have much more range at the lower modulation range than multi-splits. It's often cheaper and more efficient to go with multiple separate mini-splits (especially when you start needing branch boxes) but not always. The total combined minimum modulation of multiple compressors will often be higher than that of a multi-split though:
A single FH09 modulates down to 1600 BTU/hr @47F, but five of them would mean a min-mod of 8000 BTU/hr, which is more than the ~7K out of their multi-splits.
But with multiple single units you can just turn individual units off when the loads are low enough that some zones are fine being passively heated by adjacent zones, etc.
Dana,
Thanks as always. I'm going to play around a bit with having possibly three systems then too. The only downfall I can see is the extra breaker space. Since I'm replacing a 20kw floor heat system and a 5 ton air conditioning system though, the space would not be filled unless I have more than 3 separate condensers.
The three systems would probably be an 18k unit for the living/kitchen, which has a 17,900 load under worst conditions. The second would be a 9k unit for the master suite which I came up with about 8800 btu. The final zone would be three 6k units that would supply either two bedrooms and an office, or possibly just the two bedrooms with the third unit in the basement. The bedrooms and office only have loads around 3k btu each.
My whole house loads came in around 42k heat (18k cooling) including basement. This did not include solar gains as my windows have pretty low SHGC so I thought this would be minimal in the winter and summer. My load calc was technically not a manual J but based on Siegenthaler's book on Modern Hydronics.
Again your thoughts and information are invaluable. Thank you for your help. I'd hate to spend $8k plus and still have this system uncomfortable or inefficient after all the money I've spent insulating and sealing.
Now I'm going to have to decide if the Mitsubishi units are worth the extra cost as some of the other manufacturers seem to be better pricing with 3 units.
An 18K minisplit can typically deliver more than 20,000 BTU/hr @ +8F. Depending on the minimum-modulation you might want to look at a 15K unit.
The FH15NA can still deliver 18K @ +5F:
http://meus1.mylinkdrive.com/files/MSZ-FH15NA_MUZ-FH15NA_Submittal.pdf
The FH18 delivers over 20K @ +5F:
http://www.theheatpumpstore.com/wp-content/uploads/MSZ-FH18NA_MUZ-FH18NA_Submittal.pdf
But it appears the minimum output of either is still ~5K, so you don't gain anything on the low-output range end, within this series anyway. The Fujitsu -15RLS3 (or 15RLS3H) modulates down to 3K out, with comparable output to the FH18 at low temp (it still delivers 15,000 BTU/hr @ -15F)
https://portal.fujitsugeneral.com/files/catalog/files/15RLS3H6.pdf
Dana,
Thank you for all of the insight, you are very helpful. One of my major concerns has always been that the cooling loads for my area tend to be much lower than the heating requirements so sizing a unit seems to be hard. The other problem is that with so many options the literature can be hard to find minimum and maximum values for all options.
How important is it to actually hit the design day? I can probably count on one hand how many days I've seen sub 8F in Missouri in the last 8 years. We tend to sit in the low 20's sometimes lower on very cold days. With my 47,800 btu/hr wood stove too, it seems that it would probably be prudent to work with design values at higher temperatures so I don't completely oversize the system. This is my biggest concern since it will obviously cost more to buy and operate in the long run.
The Captcha on this website is getting out of hand. I can sign in fine, but it takes me about 30 tries to reply to a post or create one.
Mike,
Thanks for your feedback on the CAPTCHA problem. We value this type of feedback -- it helps us improve the web site. I've passed your comment along to the technical staff at Taunton.
From a code compliance perspective in most locations you have to be able to hit 68F at the 99th percentile temperature bin for the location in normally occupied rooms (not necessarily the basement or laundry room, etc, but for sure bedrooms/living room/kitchen) using thermostatically controlled equipment such as the mini-split or central heating system (which is NOT the wood stove.)
Backstopping a slightly undersized mini-split with a few hundred watts of resistance baseboard on a thermostat counts, and you don't actually have to turn the baseboard on- it just has to be there, and be functional.
Mind you, most heat load calculation methods overshoot measured reality by quite a bit. How aggressive was your Manual-J?
A decade or so ago I was in St. Louis when the daily HIGH barely broke +8F for a couple days running- it really does happen, but as you point out, it's rare.
Dana,
I'm not super worried about the code compliance part. I have a 20KW electric boiler for the "heated" floors in my house. The inspectors failed to realize they never insulated the slab somehow so I'm all but abandoning it for mini splits. I also have ducted AC in the original structure, all with the R4 crap ducting, which is also being abandoned. They way oversized the unit before I moved in so it was useless anyways. I had 5 tons of cooling in a 2000 sq. ft, slab on grade home, zone 4.
I did use the 8 degree design day for my loading, but it is very hard for me to be 100% confident in the values. One reason is I truly didn't do a manual J because I've only taken in heat/cooling losses based on area and R values. I did not take into account solar heating. If I say my uninsulated slab is R0 (R0.0001 for calculations so you don't divide by 0) I end up with astronomical heat loss (over 200,000 btu/hr) which I know is incorrect because our 47.8 kbtu/hr woodstove has no problem heating most of the house with just a floor fan near it to 75 F +.
With all of that said, I had 18,000 btu of cooling running in the main house this summer and had relatively no problem keeping it below 76 until the days we exceeded mid 90's.
I have found some differences between choices on different websites which is making this even harder now.