Looking for and sizing for mini split AC

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  #1  
Old 04-03-15, 11:19 AM
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Looking for and sizing for mini split AC

I'm having a hard deteriming what size mini-split I need. Online calculators are all over the place with the estimates so I am hoping a human can help.

The room is a bit tough because of an extension with a cathedral ceiling, so I will try my best to explain the dimensions.

The house is a cape.

The extension is a family room on the south side of the living room/kitchen. The living room was originally an external wall, but now is open (with a 15 foot span and header that comes down about one foot) to the extension.

The living room has 4 windows and a door in the front (east) and the kitchen has one window in the back (west).

The extension has 1 window in front and 1 door in the back. It also has a row of 4 windows on the south side as well as a 6 foot half round above them. The ceiling is 16 feet at the top/center, and slopes down to standard 8 foot ceilings at the sides. It is on an unfinished basement, as is the living room.

The extension was built in 2005 and is properly insulated (2x4 walls and 2x10 ceiling rafters).

What is making this tough for me is that the sizing requirements I am getting from online calculators range from 15k BTUs to 24k BTUs.

Also, I have been using a 15k BTU window unit since it was built and it has always worked fine. This cools the family room and the living room together. Even thought it seems 15k is not enough for this space. Is it possible that, even thought the 15k worked, it was working harder than it had too?

But the window unit is over 15 years old (was in the living room when we bought the house), and loud, and heavy so it is time for a split.

But I am at a loss for the size. Can anyone help?

The image below is pretty accurate as to dimensions (each box equals 1 square foot); the family room is 14x24 and the living room similar size. The blue are windows. Although the left of the pic is south.

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  #2  
Old 04-03-15, 12:23 PM
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I don't know what on-line calculators you are using but sizing of air conditioning systems is based upon the heat gain of the structure, not the area or volume of the structure. The calculation is called a Manual J, manual as in book. It does take into consideration the volume of the space but also the construction and insulation as well as the size and orientation of windows and doors and also the expected outside temperatures.

My gut feeling is that IF you are totally comfortable with a 15,000 BTU window unit then that is the size of mini-split you should use. Be very careful not to oversize the unit as it will then cycle on temperature and not run long enough cycles to remove the humidity which will leave you feeling cold and clammy. With A/C a bit warm but comfortable humidity is far, far better than cold with excessive humidity.
 
  #3  
Old 04-03-15, 12:32 PM
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I based this off of size of the room and you saying 15k did the job. Id go with a 3 zone 24k (2ton) with 3- 8k wall mount evaporators. 18ft will be close to the spectrum of airflow from a ductless. This will give you much better comfort with even temperature.
 
  #4  
Old 04-03-15, 12:41 PM
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The only other things I am thinking are:

1) the room does get shade, in the morning. A large tree in the front gives it shade till about 11:00 am. If the tree ever comes down (it rips up my sidewalk every 6 years or so, so I may decide one day to get rid of it) I will lose a little shade.

2) the outside unit of this split will be on the north side of the house, while the inside unit will be on the south wall. That is about a 60 foot run (50 horizontal + 10 vertical). I will be sure to get a unit where the max run is ok with this (I have seen 100ft on some models). but still, I am assuming there is some loss with this.

Would it make sense to compensate with a slightly larger unit?
 
  #5  
Old 04-03-15, 12:49 PM
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airman, I appreciate the feedback, but I definitely dont want to put 3 blowers in the room.

Aesthetics aside, I really only have one convenient spot anyway--the south wall.

I should also have said that the family room has a ceiling fan that hands at about the 12ft level.

-----

I could do a multi-zone to take care of some bedrooms at the same time. I thought the way to go would be put the family room on a one-zone, and then get a multi for the bedrooms. Plus , I dont see any configurations that fit my needs if I used an 18k BTU, which I expected to use.

But, if I use a 15k, I see a 15+6+6+6.

The 15 for the family room and the 6s for each bedroom.

the problem with THIS is that 6 seems a bit low for a bedroom. Our bedrooms all have 10k units. But not because I know they need it, its just what I always bought (not knowing any better), and some were even left in the house when we moved in.

The rooms are all about 14x14 so the calculations say a 6k (or less) is fine. But two of the rooms are upstairs in the cape and they get HOT, so I wonder 6k would be right.

The problem is that if I put in a 6k mini split, it isnt so easy to change if I am wrong.
Any thoughts on this one?
 

Last edited by rmathome; 04-03-15 at 01:08 PM.
  #6  
Old 04-04-15, 08:29 AM
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Most of the literature I see starts out with 9,000 BTU for single zone units . Do not know about the multi-zone units ?

The newer , higher SEER MS units have inverter drive variable speed compressors . I plan to install one in our bed room , it is on order .

As the heating load drops , I read the compressor slows down . I interpret that to mean being somewhat over sized is not as big an issue . Plus , I live in a relatively dry climate . So I am not as concerned about dehumidification .

MS's are also available with ceiling cassette fan coil units that mount flush with the ceiling . But they seem to be more expensive than the wall mounted units .

God bless
Wyr
 
  #7  
Old 04-07-15, 03:25 PM
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Does anyone know what the effect is of a long distance between the indoor and outdoor units? I know it will need extra charge, but aside from that. what is the energy issue? Do you need a larger BTU unit to compensate?
 
  #8  
Old 04-07-15, 05:28 PM
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There is no significant heat loss from using a longer lineset, you just need to adjust the charge accordingly.

As for sizing the unit, do you plan to use it for heating as well? If so keep in mind in a cold climate it is alot easier to cool than to heat. Given a room temp of 22C, and a hot day at 40C, that's only an 18C difference for the AC to work. In the winter in NY, it is not uncommon to see temps of -10C. That is a 32C temp differential, plus you need to remember that as the outside temps drop, the unit efficiency also drops. Sorry for the metric examples, but you get my point.

Just based on the info you gave, I would not go with anything less than an 18,000btu. Of course, this is not going to heat or cool your bedrooms much with only one indoor head in the living room.
 
  #9  
Old 04-07-15, 08:29 PM
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Is this something you are going to install yourself ? If not.... call and get a professional to give you sizing and price too.
 
  #10  
Old 04-09-15, 06:06 AM
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You can do one wall mount, but the temps will be exactly what you have now. Im sure you have hot and cold spots with only one evaporator. 9K is the smallest evap you can get.
 
  #11  
Old 04-09-15, 07:08 PM
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I am not concerned with the heating ability of the unit. I have a pellet stove in the room and it does a great job. There is also a large cast iron radiator to supplement the stove.

I will be installing this myself, but will probably get someone to charge the line (unless I can do that myself as well).

So, I am thinking I will go with 18k but next question is where to install the units. In that attached pic I show the locations. The outside unit will be at location A. That is the only reasonable spot, so cant change.

For the inside unit(s), they would be either B, C, B and D, C and D.

Coming from A, the lines will run into the basement and then, from there, to the inside units.

If running to B, the lines will run along the basement ceiling and then go inside the wall as they go up to the unit that will be about 8 feet high on the wall. I would like to run inside the wall so that the lines are hidden. But that means a flared connection will be hidden in the sheetrock. Is this ok?

Or I can run from outside at A to inside the house, then go back outside at B, run up the wall outside and come back in 8 feet up. I wouldnt have to open sheetrock and wouldnt have a flared connection hidden in the wall, but would have the wires and lines running outside a little bit, and then right back in--as the lines would exit the house low and then re-enter 8 feet high.

If running to C, then I would enter the house high at A--into a crawlspace behind a kneewall. Then could run it right to the unit at C.

B would blow into the main living space better. C would blow only into the one room. Although I suspect the temps would equalize and both rooms would get coverage equally the same with either install.

But I could also add another unside unit at D. But I would rather not. I could though.

However, the single unit in my window now does not leave any cold spots or warm spots, at least not that I ever noticed.

So, would B or C make a difference? And if B, would you go up inside the sheetrocked wall?

B would be most similar to what I have with my window unit. The only difference would be that the split unit would be a few feet higher.Name:  house2.jpg
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  #12  
Old 04-10-15, 03:21 AM
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B or D will give you better distribution.

Flare connection in wall is not the best but will be ok as long as you are 100% sure it is not leaking at all before you close it in.

You will need to invest in some tools if you plan to charge this Yourself. You will need a vacuum pump, manifold set and a digital vacuum Guage at a minimum. That's around $350 for low end stuff. You should also have a charging scale to weigh in the charge too, which would be another $100 minimum.
 
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Old 04-10-15, 07:23 AM
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I currently am set up like B with a 15k window unit and it has always been fine. I guess there is no reason to switch--I should go with a 15k split. The only difference is that the split will be installed about 7 feet higher which should not matter much (if at all).

I will likely be doing a multi-zone next year for bedrooms, so maybe investing in the equipment wouldnt hurt either.
 
  #14  
Old 04-10-15, 08:47 AM
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I would still opt for the 18000 only because the cost difference is so small. You really should consider the heating capabilities of mini splits. They are super efficient and can save alot of money with heating cost, especially in the spring and fall.

I'm not sure what the regulations are like in the US, but you will likely need to take a course before you can buy the gas to charge the units.
 
  #15  
Old 04-10-15, 11:29 AM
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You will not be able to charge the system. You will have to have an EPA license.
 
  #16  
Old 04-10-15, 10:58 PM
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The MS we installed 2 seasons ago came pre-charged with sufficient refrigerant for about 15' of line set . The refrigerant was contained in the condenser unit .

The system must be pressure tested , vacuumed down and if all is well , the service valves opened to allow the refrigerant to flow through the system .

No additional refrigerant is needed if the distance is within spec . Mine uses R410a . I think they all do .

And , by the way , I do have an EPA card .

God bless
Wyr
 
  #17  
Old 04-13-15, 12:44 PM
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Ok, I think I am all set with only two question remaining.

I will be going with a single 18k unit.

1) I plan to put the flared connection behind the sheetrock after making sure that it does not leak. But, would it be better to run the lineset outside just to avoid having the flare not in the sheetrock? In other words: instead of coming straigh up from the basement (within the wall), go outside and run up the exterior wall, and then come back in behind the inside unit?

2) I can't find any good wiring diagrams-even in the Mitsubishi manuals. I know the outside unit will require a 12/3 because it is 20amp/220v. I also see that there is some kind of a shutoff with 50' of the unit. So, this would be installed on the side of the house--sort of a like a subpanel I guess. I have seen them on houses with minis and assuming I could get at Home Depot.

But for the inside unit, do I run another line from the breaker? Or do I now run a line from the outside unit? There must be communication between the two so I would expect that the inside unit would get its own power from the breaker , and would also have a low voltage line from the outside unit. Then I also read that the inside unit needs a shutoff within 50' of it as well. So, do I need a switch on the wall for my inside unit?
 
  #18  
Old 04-13-15, 01:20 PM
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I would run the line set outside and back in through the wall. Much easier to service if there is ever a problem. Might have a leak or replace the head unit, or maybe move the unit.

You need a fuseless disconnect out side by the outdoor unit, This is so you can disconnect the power to the unit while working on it outside. It should be right next to the outdoor unit. No switch on the inside wall.

You will run a power and communication line with the line set for the inside unit from the outside unit. It must be a 3 wire + ground. You manual will give you sizing for the wire, but generally 3 way lighting wire works fine.

All of these details will be in the installation manual of your unit when you get it.
 
  #19  
Old 04-13-15, 01:43 PM
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The flared fittings used on mini-splits have to be pulled very tight, I would not want it buried in the wall. They do make channel and cover to hide the lineset when on the outside of the house.

Yes, you need an electrical disconnect withing fifty feet and direct line-of-sight from the unit. Most of the time the disconnect is within a few feet of the unit. You also need a 120 volt, GFCI-protected receptacle within twenty-five feet of the unit for servicing. If exposed, all the code requirements of needing a bubble cover and weatherproof receptacle apply.

Some mini-splits use a combination power and communications cord between the inside and outside units to provide power to the inside unit. At least some Mitsubishi "Mr. Slim" models require a 120 volt source near the inside unit.

AND, although the numbers "110 and 220" are in common usage the voltages are much closer to 120 and 240.
 
  #20  
Old 04-14-15, 03:06 AM
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The mini split we installed 2 seasons ago called for 18/4 between the indoor & outdoor unit . I measured the voltage between the 2 units . The highest I read was either 32 V or 36 V . Can not remember . I used some 18/4 twisted/shielded cable from work .

Put the flare fittings on the outside if you possibly can .

Yes on the Rain Tite disconnect for the power .

Your install manual may be slightly different .

God bless
Wyr
 
  #21  
Old 04-14-15, 08:18 AM
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ok, through the wall and along the outside it is.

Thanks so much for all of the help. I'll be ordering the unit soon.
 
  #22  
Old 04-14-15, 10:33 AM
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Ok, so I found the AC I want:

Mitsubishi MZ-FE18NA M-Series - 18,000 BTU - Ductless Heat Pump System - Wall Mounted - 20.2 SEER - 10.3 HSPF

Good price and free shipping.

I was adding accessories and noticed that everything (lineset, shutoff box, drain tube, wiring) is so much more expensive than what I can get at Home Depot.

Any reason to not get all accessories at HD? The only thing I need to get with the AC is the lineset cover (lineset hide?) because I don't think HD will have that.

Even for the linesets, its nice that the website sells them together (3/8 and 5/8) and they are pre-insulated and flared. But I can certainly buy the copper rolls and insulate and flare myself.

Any valid reason to pay the premium that I might be missing?
 

Last edited by rmathome; 04-14-15 at 01:17 PM.
  #23  
Old 04-14-15, 01:41 PM
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The only thing to watch out for is that you buy a good grade of copper for the lineset. I wouldn't cheap out when it comes to that. Everything else though it shouldnt matter where you get it.
 
  #24  
Old 04-14-15, 02:27 PM
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Does that just mean Type L?
 
  #25  
Old 04-14-15, 02:45 PM
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Someone else would be better suited to answer that question as I am not sure how soft copper is rated. All I know is that all soft copper is not created equal.
 
  #26  
Old 04-14-15, 04:51 PM
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The copper must be type ACR. It is specially cleaned & dehydrated for air conditioning & refrigeration work. Other tubing could have residual oils which can damage the system.
 
  #27  
Old 04-14-15, 05:23 PM
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ok. I think I have that all now. But, of course, as I start to plan with more detail more questions come up.

As far as placement for the outside unit. I am pretty sure that right under a window is not a good idea for safety reasons. But, the place I wanted to put it has obstacles in the way.

I wanted it to the right of the chimney. But there is the incoming electric service, the basement window, and then a bedroom window all in a row.

I dont want to go left of the chimney because next year I plan an putting in another split, and that one will go left of the chimney.

Im thinking that it could go in front of the electric service --but just about two feet in front to give it clearance.

I could also go in the back of the house, but I'm not sure how loud the outside unit is and I have a very, very small yard.
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  #28  
Old 04-15-15, 03:51 AM
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They are not very loud , not as loud as conventional condenser units for central air systems .

I was told to use Mueller brand of copper refrigeration tubing . So that is what I used . A 50 foot roll for the small and the large lines . I have enough left for a 2nd , and maybe a 3rd MS .

Be sure and insulate both lines . I had some automotive heater hose on hand & used it for drain line .

Get flare nut wrenches & " crows feet " to fit your torque wrench . Vacuum pump and oil . Refrigeration gauges and hoses for R410a .

God bless
Wyr
 
  #29  
Old 04-15-15, 04:52 PM
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I think I read somewhere that the lineset should slope toward the compressor, sort of like a drain would.

But I am having a problem getting a nice straight run. Is it ok if my lineset comes into my basement at its ceiling, then goes down a few feet and runs along the wall, and then comes up to the air handler on the main floor?

In other words, can I totally disregard the sloping?
 
  #30  
Old 04-15-15, 05:40 PM
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You should try to avoid the up & down in the line set if at all possible. Runs like that create traps in the line set which can cause poor oil return to the compressor. It would be preferable to run the lines along the ceiling then up to the air handler.
 
  #31  
Old 04-16-15, 04:24 AM
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What about running the lineset outside vs inside? Lets say a total of 65 feet. Does it matter? Im wondering if there is any loss of efficiency.

I didnt think I would have such a problem with the location of the outside unit. The spots I want to put it that are out of the way are blocked by windows or electric.

I can put it behind a garden on the side, but I really want it off the ground by a foot or so for when it snows, but that would make the entire unit four feet tall.

I also dont want it right under or next to a window so that it doesnt become a step ladder for someone to get in through the window.

I could mount it on the wall next to a window I suppose, but not sure about vibration. Also, I have vinyl siding. Wouldnt the mounting brackets if secured nice and tight to the sheathing and studs, crush the siding?
 

Last edited by rmathome; 04-16-15 at 06:59 AM.
  #32  
Old 04-16-15, 05:00 PM
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Check the installation manual about line set length.
 
  #33  
Old 04-16-15, 05:56 PM
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The length will be fine. I am wondering if being outside as opposed to inside will matter for efficiency.
 
  #34  
Old 04-16-15, 06:59 PM
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Insulate both lines well & there won't be any significant efficiency loss.
 
  #35  
Old 04-17-15, 07:43 AM
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Ok, I may need to run along a different wall. If that is the case, I think I should consider a multi-zone.

I had planned on doing my living room for this summer, and then next year doing a 3-zone for my three bedrooms.

But maybe I should go with a 4-zone and do the living room and bedrooms on the same compressor.

If that is the case, I would still want to only hook up my living room for this year (just because time is always at a premium and if I plan on 4 rooms I know, realistically, I wont get to them all).

Is there a problem with having a mult-zone compressor and only using one zone?

Does it affect the charging? Is each zone charged separately? For two of the zones, I will likely be within the distance limit and wont need someone to charge those two lines...or does it not work that way?

Are there any benefits or drawbacks of this approach that maybe I'm not aware of?
 
  #36  
Old 04-17-15, 08:25 AM
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On VRF systems you have to use at least 50% capacity. If you don't the unit will not work correctly. I do not know on a conventional unit. Will you go with a 3 pipe or a two pipe system if you go zone? Also will you exceed the line set length on the zoned system?
 
  #37  
Old 04-17-15, 11:09 AM
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why not price a MUZ-GE24NA AND A MSZ-GE24NA. That will get you almost 3 tons. so you might be able to get some cool air into those beedrooms. It is rated at 22,500 BTU's but will put out 31,400 and the min is still 8,200 just like the model you are looking at.
 
  #38  
Old 04-17-15, 11:15 AM
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Two of the bedrooms are on the second floor. And the other is on the opposite end of the house.

Currently, my 15k does not reach them at all. It isnt a size issue, it is really a location air flow issue I believe.
 
  #39  
Old 04-17-15, 12:56 PM
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Perhaps you could post a few pictures of each side of your house so we could better help with placement. Based on your discriptions so far, I think you are considering a less than ideal install.
 
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