Posted By Lbear on 10 May 2014 03:37 PM
There is no way that 500'+ of line will be at the optimum efficiency level stated on the unit. Maybe some HVAC techs can speak up about this but I wouldn't be surprised to see a 30% + reduction in efficiency in longer run lines.

The Daikin installer who just installed 2 units on a commercial job I was at told me that a mere 25' extra length will reduce output.

Something doesn't make sense here. If the Mitsubishi engineers state that max length is 65' on a mini split, how then can Daikin claim 1,000'+ ?

You'll have to quibble with the Daiken engineers. As you can see, these are the official Daiken factory installation manuals. I say it's a bit presumptuous of you to question their manual, that is unless you are a well experienced HVAC or fluid flow engineer. The manual does state you need to upsize the piping with longer runs.

Be careful who you listen to. There are a lot of dunces out there in the HVAC field. This installation manual for Mitsubishi http://www.mitsubishipro.com/media/318879/pumy-nhmu-bs_install_bg79u872h02_1-08.pdf shows on page 6 illustrations where the max length to farthest unit is 80 m, or 262 ft. and the total length of piping is 120 m, or 394 ft.

So, now feel free to quibble with the Mitsubishi engineers as well as the Daiken engineers!

Mitsubishi Mini Mr.Slim

I think the problem is that you are quoting the NON-mini split manual. The above is the Mitsubishi Mr. Slim Mini manual and it clearly states on Page#5 that it is a 65 - 100 foot limit on the lines.

Here is another model of the Mr. Slim and on page #2 it also states a 65-100 foot max on the lines:

Mr. Slim Specs

Posted By Lbear on 10 May 2014 11:29 PM
Mitsubishi Mini Mr.Slim

I think the problem is that you are quoting the NON-mini split manual. The above is the Mitsubishi Mr. Slim Mini manual and it clearly states on Page#5 that it is a 65 - 100 foot limit on the lines.

Here is another model of the Mr. Slim and on page #2 it also states a 65-100 foot max on the lines:

Mr. Slim Specs

The first document is 1989 version and those units aren't available anymore. So throw that one out.

The second document is for a single indoor unit system, not multiple indoor units.

The larger the outdoor unit, the longer the runs can be. Don't get hung up about run lengths until you've got the needed capacity nailed down somewhat. These are little units you're referring to mostly. From what I remember of your house plan you're going to be looking at least 36 to 48 k Btu capacity, unless you're planning on having mulitple outdoor units around the house, but that won't be as economical as a single larger unit. From all these manuals it should be clear your options are for distances from a max of about 100 ft to almost 4000 ft. You'll find a system in that range somewhere that will fit your needs quite fine. One of the Mitsubishi MXZ manuals I looked at shows 82 ft max for each indoor unit with 230 ft max for the entire system.

I don't know what you mean by saying I'm quoting NON-mini split manuals. All the manuals I refer to are for split systems. All such systems with separate outdoor and indoor units are referred to mini splits or multi-split systems connected from one to eight indoor units depending on design.

These rated piping lengths of Mitsubishi compared to Daiken is enough to make me question if the Daiken isn't the better system. One of the Daiken units goes up to 96k Btuh capacity with double compressor. I'll bet it'll efficiently handle any load from 18k to 96k. The one great beauty of these systems is you can hang on indoor units with a total capacity of 125 to 150% of the outdoor unit capacity. That gives you a lot of flexibility in zone by zone capacity.

Maybe that's why my lines fit in a 2-1/2" space- my 'big' unit is a 12K; the small one is a 9K. My lines are 3/8" and 1/4" tubing. According to the calculations, the one 12K unit will handle the entire house except in extreme conditions. My line runs are 15' and 35'.

I was going to go with a multi-head system, with the units hidden in a drop ceiling, but the efficiency is much lower, and you can't get the same low temperature capability, so I went with two smaller stand-alone systems. I don't like the appearance of the wall units, but, like anything else, after a couple weeks you don't notice them anymore. Maybe I'll keep a framed copy of a power bill to show guests if they ask what that thing on the wall is!

We had temps in the low 90's this week, and our attic is not yet insulated, but the 12K unit easily kept the entire house at 74 degrees, running very slowly and quietly. In the master bathroom, 3 rooms away (with the doors open) it was 2-3 degrees warmer than the room with the unit. I can't imagine what R-49 insulation is going to do.

So if the 1st floor (open design) requires 24k BTU, is it better to install two 1-ton mini split units each with its own air handler OR one 2-ton mini split unit that shares 2 air handlers?

I was told that having two separate outdoor compressor units is the better setup vs. having two air handlers sharing a single compressor unit. Better in terms of efficiency and design.

I was told that having two separate outdoor compressor units is the better setup vs. having two air handlers sharing a single compressor unit. Better in terms of efficiency and design.

Is that the same for inverter driven compressors?

•••We had temps in the low 90's this week, and our attic is not yet insulated, but the 12K unit easily kept the entire house at 74 degrees, running very slowly and quietly. In the master bathroom, 3 rooms away (with the doors open) it was 2-3 degrees warmer than the room with the unit. I can't imagine what R-49 insulation is going to do.•••

At that rate, it would seem that an R49 ceiling would be a poor ROI, right? ;-)

I would say that the ease of cooling has more to do with the ICF walls then the mini.

Posted By ICFHybrid on 12 May 2014 01:57 AM

I was told that having two separate outdoor compressor units is the better setup vs. having two air handlers sharing a single compressor unit. Better in terms of efficiency and design.

Is that the same for inverter driven compressors?

I would say there is no simple answer to the question. If I were looking at it I would be considering the COP at various loads between a single unit vs. 2 or 3 units. A factor to consider is the percentile range of temperatures. For example, the design temp for cooling might be 100F. But that is the temp that is expected to exist only about 3% of the cooling season. Look at the hour temp readings for the locale. Even though design may be 100F, maybe 85% of the time the temp is only 85F or below and the cooling load may be only about 60% of the design load. What is the COP of a larger unit when it's running at 60% load compared to 2 out of 3 separate units running at near 100% load?

In AC mode the COP is dependent on outdoor temp. So for a system of a given size when the outdoor temp is lower, which means the load is lower, the COP goes up. This may well mean that the larger system running at a lower load will be more efficient than fewer small systems running, each at a higher relative load. In other words, a 36,000 Btuh unit may be more efficient, or just as efficient, running with a 24,000 Btuh load than to have 2 out of 3 12,000 Btuh units running at 12,000 Btuh output each.

Like I say, there is no simple answer, especially for a variable speed compressor unit like the Daiken. Unquestionably the situation will be different if the compressors are single speed. But how different I have no idea. And since I have no need to know at this point I'm not going to spend the effort to figure it out!

Posted By Lbear on 11 May 2014 08:50 PM
So if the 1st floor (open design) requires 24k BTU, is it better to install two 1-ton mini split units each with its own air handler OR one 2-ton mini split unit that shares 2 air handlers?

I was told that having two separate outdoor compressor units is the better setup vs. having two air handlers sharing a single compressor unit. Better in terms of efficiency and design.

In addition to my comment above, you have to consider the cost of equipment. In some cases there is very little difference between units of two capacities. I've been told that in the case of the unit I have the only difference between the 3 ton unit and the 4 ton unit is the main controller board. Very interesting. Supposedly, if I needed to, or wanted to, I could have my unit upgraded from 3 tons to 4 simply by changing the main board, plus maybe freon charge differences and some other adjustments.

I don't pretend to know jack about HVAC, however for you guru's I'll let you figure it out.

2 story house plus basement, each floor is roughly 900 sq.ft.

1 Daiken inverter unit on the outside 3 ton
Basement 7 Btuh head with 12' run
Main 12 Btuh head with 18' run
2nd 12 Btuh head with 42' run

ICF walls to the roof
Flat roof with 6" closed cell spray foam to u/s of deck plus 3" poly on top - roughly rated as R51/R52
5" EPS below the slab
Large Fiberglass windows with typical low E and some other coating on the south side where in direct sunlight

I'll post the blower door test once it's completed

Anyways, the system works fine, not over working, I say it's more for humidity control then cooling purposes

Posted By Chris Johnson on 13 May 2014 08:27 AM

ICF walls to the roof
Flat roof with 6" closed cell spray foam to u/s of deck plus 3" poly on top - roughly rated as R51/R52
5" EPS below the slab
Large Fiberglass windows with typical low E and some other coating on the south side where in direct sunlight

I'll post the blower door test once it's completed

Anyways, the system works fine, not over working, I say it's more for humidity control then cooling purposes

What climate zone?

Toronto, ON