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I am going to try a DIY mini-split!

P

pvdude

Solar Addict
Joined
Feb 8, 2021
Messages
654
Location
Florida
High temperatures here have been running around 101˚f/38˚c.
The solar power equipment is in a small concrete block shed.
A few years ago I put in a 14k BTU "Through the wall" AC unit, powered by the inverters @ 240vac.
(The previous owner had already made the wall opening and installed a smaller AC unit).

Anyway, at about 02:00 this morning, I realized that there is no redundancy, and subsequently discovered the wall AC model I have is no longer available.

Past experience has made me aware that there is very little/no standardization for wall unit measurements, so the chances of finding a replacement wall unit that fits the wall sleeve is low, especially if I need one quickly.

Thus, the Department of Redundancy Department requires a second AC unit be installed.
(I'll shut off the old wall unit, save it for emergencies)

After reading the forum, I understand that it is possible to buy a mini-split powered by 240vac, and w/ pre-charged plumbing, so I don't have to deal w/ having someone else pull a vacuum and charge the system.

Currently seeking information on something around 12-16K BTU, from a brand name I recognize (Mitsubishi, Fujitsu, not sure which other brands to look for)
 
They all come precharged with refrigerant. The regular types just require you to vacuum the lines and pressure test with nitrogen.

Last time I checked it was cheaper to buy a regular unit and buy a vacuum pump and manifold gauge set than it was to buy a mrcool or similar with quick connects , and you have the tools for life so if you need to add more mini splits later you're not stuck in always buying the quick connect types.
 
pvdude said:
So is it just the line sets that may or may not be pre-charged?
Click to expand...
Yes.

Btw, I use a $540 9k unit from Amazon for my solar shed. Working very well so far. Uses about 400w, I have it set at 80 degrees and it usually maintains at 77.

Has a WiFi app so I can control it remotely and the Wi-Fi app also tells me the temp in my solar shed
 
1201 said:
Yes.
Click to expand...
Great, thanks!
I have all the gear ,(vacuum pumps, gauges for many refrigerants, nitrogen purge system, etc)
BUT
I only have an EPA 609 license, and thus can't buy or work with R-410A.
So unless the precharge compressor and inside coil include a bit of extra refrigerant for filling the lineset, I'll have to go w/ a precharged lineset
version.
 
pvdude said:
Great, thanks!
I have all the gear ,(vacuum pumps, gauges for many refrigerants, nitrogen purge system, etc)
BUT
I only have an EPA 609 license, and thus can't buy or work with R-410A.
So unless the precharge compressor and inside coil include a bit of extra refrigerant for filling the lineset, I'll have to go w/ a precharged lineset
version.
Click to expand...
They usually have enough refrigerant for the line they ship with. The latest one I bought had a 13ft line .

The first three I bought didn't come with lines but the manual said they had enough charge for 25ft of lines
 
pvdude said:
So unless the precharge compressor and inside coil include a bit of extra refrigerant for filling the lineset, I'll have to go w/ a precharged lineset
version.
Click to expand...
They all come fully charged inside the outdoor unit for typical lineset lengths. Enough to fill like 16-25ft of lineset.

Only for longer lines do you have to add any.

Edit: Oh I should read the whole thread before just duplicating the previous comment lol.
 
pvdude said:
Great, thanks!
I have all the gear ,(vacuum pumps, gauges for many refrigerants, nitrogen purge system, etc)
BUT
I only have an EPA 609 license, and thus can't buy or work with R-410A.
So unless the precharge compressor and inside coil include a bit of extra refrigerant for filling the lineset, I'll have to go w/ a precharged lineset
version.
Click to expand...

If you buy R-410A from a site like ability refrigerants, you can check the box that states that you will be re-selling the refrigerant to someone who is EPA certified. That way you can purchase the refrigerant for the EPA 608 certified guy you found to do the actual work. 😉
Or you can get your 608 online from Skillcat in a few evenings of study. (used to be free, not anymore)
 
How do I pick the mini-split BTU size?
The existing "through the wall" Emerson is rated @ 14k BTU, for cooling 700 sq. ft.

I don't know how much of the "14,000 BTU" Emerson rating is from the marketing department.

The building it cools is about 400 sq. ft.

The Schneider solar electronics put a large amount of heat into the room.

If I turn the Emerson on max around 06:00-07:00, it will keep the room in the high 70's all day.

If I wait until things heat up: 10-11AM, the Emerson will eventually get the room into the low 80's.

I'm buying a Mitsubishi mini-split for this room, either a 12k or 15k BTU might do, but which one?
 
pvdude said:
How do I pick the mini-split BTU size?
The existing "through the wall" Emerson is rated @ 14k BTU, for cooling 700 sq. ft.

I don't know how much of the "14,000 BTU" Emerson rating is from the marketing department.

The building it cools is about 400 sq. ft.

The Schneider solar electronics put a large amount of heat into the room.

If I turn the Emerson on max around 06:00-07:00, it will keep the room in the high 70's all day.

If I wait until things heat up: 10-11AM, the Emerson will eventually get the room into the low 80's.

I'm buying a Mitsubishi mini-split for this room, either a 12k or 15k BTU might do, but which one?
Click to expand...
Get the larger one don't they have a very good turn down ratio
 
Don’t over size too much. You won’t get good dehumidification if it short cycles. I have a 24k btu unit that effectively cools the entire first floor of my home.
 
pvdude said:
How do I pick the mini-split BTU size?
The existing "through the wall" Emerson is rated @ 14k BTU, for cooling 700 sq. ft.

I don't know how much of the "14,000 BTU" Emerson rating is from the marketing department.

The building it cools is about 400 sq. ft.

The Schneider solar electronics put a large amount of heat into the room.

If I turn the Emerson on max around 06:00-07:00, it will keep the room in the high 70's all day.

If I wait until things heat up: 10-11AM, the Emerson will eventually get the room into the low 80's.

I'm buying a Mitsubishi mini-split for this room, either a 12k or 15k BTU might do, but which one?
Click to expand...
I’m going through that now. The Outback Inverter puts a lot of heat out and the garage gets to 105f in the summer prior to the Inverter being installed.

IMO, if 14k barely works now, go with the 15k BTU. I have been told a smaller BTU mini split AC will cool better, but being a numbers person I don’t see how 12 k BTU will work better than a 14 k BTU.

For me, I like the idea of hooking panels to a hybrid air conditioner to bring down the cost, and I think my garage is sized for an 18k BTU, so my choices are a Chinese alibaba 18 k import, or a 12k OR 24k BTU from EG4.

I think I will go with the 24k BTU EG4
——-
What’s even harder is trying to figure how much energy it will draw to do calculations for kWh usage.

For my house, want to get a 4 ton ducted inverter AC heat pump. I think I’m the first person to ever install it and want to know how much I will save. I thought that the Bosch BOVA 20 seer model would have some stats published, but I can’t find a spec sheet on line. You tube and google are full of “you’ll save,” “it’ll sip energy,” or someone promises to follow up with bills but doesn’t.

When I do the calculation off SEER rating, I get 20%, not the mythical it’ll pay for itself in three years, but will pay for itself in decades.
 
It's also a matter of how well the area is insulated. For example, I use a 9,000BTU mini-split for my 7 x 14 cargo trailer (100sq ft) and it works OK but I only have 1" (R5) insulation all around. If you have modern (last 30 yrs) home insulation such as R20-25'ish then I would expect the 'guidance' numbers to be more accurate.
 
OffGridInTheCity said:
how well the area is insulated
Click to expand...
That is one of the reasons for added cooling. There is only 4" of batt fiberglass in the ceiling, R-13?. Concrete block walls, no insulation.
I did put 2" foam board insulation on the doors, sealed cracks and seams. Roof is corrugated steel, covered w/ PV panels, 4" airflow under the panels, so there is convection. But generally, a crappy heat sink!
Screenshot 2024-06-23 at 12.45.49.png
 
pvdude said:
I'm buying a Mitsubishi mini-split for this room, either a 12k or 15k BTU might do, but which one?
Click to expand...
I would recommend against the Mitsubishi. I've had to repair more Mitsubishi minisplits than any other brand. Personally, I prefer units made by Midea. They are sold under lots of different brand names in the US. Worldwide, they are the largest maker of minisplits.
 
Brett V said:
Don’t over size too much. You won’t get good dehumidification if it short cycles. I have a 24k btu unit that effectively cools the entire first floor of my home.
Click to expand...
A typical, old-school hvac unit is either producing its max/rated BTU's or it's off. There's no in-between. Yes, if you oversize, the unit will cool quickly, turn-off quickly, not allowing the unit enough run-time to properly dehumidify (i.e., short-cycling).

However, this is not the case with an inverter equipped mini-split. The beauty of these units is their ability to vary their BTU output (and current draw). This allows even an oversized unit to properly cool, dehumidify and still operate efficiently, all at the same time. It simply reduces BTU output so it doesn't short-cycle, allowing it time to properly dehumidify.

For what it's worth, our garage only needs 12k BTU's to cool/heat properly. However, during the winter some mini-split heat pumps can lose 50% or more of their heating capability when temps drop down to 0-10f. This forced us to purchase an 18k mini-split in order to properly heat our garage during the winter when temps sometime drop to 0f. During the summer, our 18k unit spends most of its time producing only a fraction of its rated BTU output (along with using only a fraction of its rated current). It never short-cycles and cools/dehumidifies perfectly even though it's technically oversized.
 
Last edited:
OTRwSolar said:
some mini-split heat pumps can lose 50% or more of their heating capability when temps drop down to 0-10f
Click to expand...
I was considering a "cool only" unit (lower cost & complexity),
and continuing to use resistance element type electric space heaters for the cold days here.

(Last Winter I did actually get to see frost on the roof, a couple of times, before sunrise!)

But they use 1500 watts each, and are a fire hazard.
So the "heat pump" mini split is what I'm going to try.
 
In Tampa the distributors will sell you a 12,000BTU lowish SEER mini split for $350 in assorted tier 2 brands. Screw everything together open the valves and it works. 18,000 BTU is about $550.Pretty much throwaway prices.
 
Last edited:
plympton said:
You mean for $10/month I can sit on my ass and get HVAC certified (totally serious)? That's awesome!
Click to expand...
Well, EPA 608 only means you're certified to handle refrigerants responsibly, but that's worth a wage bump to some HVAC techs already in the field so I've heard. I doubt that this is a foot in the door for a total noob, but maybe? Or are you referring to some of the other courses available at Skillcat? I think we have some actual HVAC techs here, maybe they will speak up.
 
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