Safety note: Refrigerant and electrical work have legal and safety requirements. Per the EPA, connecting or disconnecting pre-charged refrigerant lines on a mini-split requires Section 608 technician certification. Follow the installation manual, the National Electrical Code and your local code, and use a licensed electrician where your jurisdiction requires one. This is general information, not professional advice.
MRCOOL’s DIY 5th Gen line uses R-454B refrigerant and pre-charged line sets. Here is how the single-zone sizes compare, taken from MRCOOL’s own documents. We haven’t tested these units.
On this page
Side by side
| Size (Btu/h) | SEER2 | EER2 | HSPF2 (Region IV) | HSPF2 (Region V) | Line sizes | Factory charge |
|---|---|---|---|---|---|---|
| 9,000 | 23.6 | 14 | 11.5 | 8.6 | see manual | see manual |
| 12,000 (115 V) | 23.5 | 12.5 | 10.2 | 7.6 | 1/4 in + 3/8 in | 30.34 oz |
| 18,000 | 22.5 | 13.7 | 11.5 | 8.5 | 1/4 in + 1/2 in | 44.80 oz |
| 24,000 | 22.7 | 12.7 | 9.5 | 7.1 | 3/8 in + 5/8 in | 67.37 oz |
| 36,000 | 18 | 8.5 | 8.5 | 6.5 | see manual | 109.70 oz |
Source: MRCOOL DIY 5th Gen brochure and the 12k 115 V condenser support page, checked Oct 12, 2026. Confirm against the spec sheet for the exact model number you buy.
What stands out
- Efficiency falls with size. SEER2 runs from 23.6 (9k) to 18 (36k); HSPF2 Region V from 8.6 (9k) to 6.5 (36k).
- The 9k and 18k have the highest heating ratings in this line (HSPF2 Region IV 11.5).
- Line sizes can change with capacity, so you can’t assume line sets or couplers are interchangeable between sizes—check the manual for your exact model and line set.
The 12k (115 V) in detail
From MRCOOL’s support page and brochure (support page, brochure):
- Minimum circuit ampacity 18 A; maximum fuse 20 A.
- Ships with a 25 ft line set; maximum line length 82 ft.
- Heating operation: MRCOOL’s operating-range page lists −13°F to 75°F or −22°F to 75°F for 5th Gen units, depending on the model. Check the manual for your model number.
- Outdoor sound level 54 dBA.
- Suggested area 350–500 sq ft (the brochure says up to 550).
Choosing a size
Pick the size from a load calculation, then compare efficiency within the sizes that fit. See what size mini-split do I need. If two sizes both fit, the smaller one usually has better ratings here and cycles less at low loads.
Running cost by size, worked
Using each size’s Region V HSPF2 and an example 20 million Btu of seasonal heat at the U.S. average 18.31¢/kWh (EIA, July 2026):
| Size | Region V HSPF2 | kWh | Cost |
|---|---|---|---|
| 9k | 8.6 | 2,326 | $426 |
| 12k (115 V) | 7.6 | 2,632 | $482 |
| 18k | 8.5 | 2,353 | $431 |
| 24k | 7.1 | 2,817 | $516 |
| 36k | 6.5 | 3,077 | $563 |
This compares ratings only. In practice each size suits a different load, so you won’t be choosing between a 9k and a 36k for the same room. Run your own numbers in the operating cost calculator.
Factory charge and line length
The factory charge (30.34 oz for the 12k up to 109.70 oz for the 36k) is sized for the included line set. For the 12k, MRCOOL lists a 25 ft line set and an 82 ft maximum line length (MRCOOL). Line sizes must match the unit: see line set length. The 5th Gen uses R-454B; see R-410A vs R-454B and R-32 for what that means for installation.
Before you buy any size
- Get a room-by-room ACCA Manual J load calculation. See what size mini-split.
- Check the voltage of the exact model: the 12k we cover here is 115 V; others may need 208/230 V. See electrical requirements.
- Check heating capacity at your design temperature in MRCOOL’s performance data, not just the nominal size.
- Plan who will make the refrigerant connections; EPA says it requires Section 608 certification. See EPA 608.
- Check rebate rules before buying if you hope to use one; DIY installs often don’t qualify.
FAQ
Which refrigerant does the 5th Gen use?
R-454B, according to MRCOOL’s brochure. See R-410A vs R-454B and R-32.
Does the 12k need 230 V?
MRCOOL’s support page covers a 115 V version of the 12k condenser. Check the model number, because voltage varies by model.