MRCOOL DIY 5th Gen single-zone mini-splits (R-454B, DIY-xx-HP-WMAH / -C ...D25): quick sheet

Covers: MRCOOL DIY 5th Generation single-zone ductless heat pumps on R-454B with pre-charged Quick Connect line sets. Model pairs from MRCOOL's catalog: DIY-09-HP-WMAH-115D25-O + DIY-09-HP-C-115D25-O and DIY-12-HP-WMAH-115D25-O + DIY-12-HP-C-115D25-O (115 V); DIY-18-, DIY-24- and DIY-36-HP-WMAH-230D25-O with matching -C-230D25-O condensers (208/230 V). WMAH = wall-mount air handler, C = condenser, 115D/230D = voltage, 25 = 5th Gen R-454B. The same catalog lists 5th Gen Hyper Heat, Multi-Zone (DIY-MULTI3-18HP230D-O to DIY-MULTI6-55HP230D-O) and OuttaSight ceiling cassettes. Not interchangeable with 4th Gen R-410A parts or line sets (separate sheet). Search terms: DIY-12-HP-WMAH-115D25-O, DIY-24-HP-C-230D25-O, MRCOOL 5th Gen, MRCOOL R-454B.

Single sourceBrands: MrCool4 components

Inverter heat pump sold for owner installation: the line set is factory charged and connects with Quick Connect couplings, so no vacuum or charging is needed on a normal install. The 5th Gen adds R-454B (an A2L, mildly flammable refrigerant) with refrigerant leakage detection, a DIYPro interconnect cable and app control. Field calls: power and breaker sizing, interconnect wiring, Quick Connect couplings not fully seated, drain, filters and error codes on the indoor display.

Safety first

R-454B is an A2L (mildly flammable) refrigerant: no open flames near a suspected leak; refrigerant work is blocked and goes to an EPA 608-certified technician.
Inverter boards hold DC charge after power-off.
Do not mix 4th Gen and 5th Gen parts or line sets.

How it works

Power goes to the outdoor unit; the DIYPro cable powers and communicates with the indoor head.

The catalog lists refrigerant leakage detection as a 5th Gen feature. Treat any leak alert or leak code as a refrigerant issue for a certified technician; the exact response of the unit is in the 5th Gen manual.

Low-ambient cooling and DC inverter compressor and fans are standard per the catalog.

Components & tests

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Official manufacturer documentation

Always confirm values against the tech sheet for the exact model.

OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work. © 2026 OnCall Tech. Original content; not a reproduction of any manufacturer document.Generic part photos (not of this brand): Two ductless mini-split outdoor units on a house wall, one on a steel wall bracket, with the insulated line set running into the wall by Pedalito, CC0

Diagnostic guides for this appliance

Guide · Air Conditioner

PTAC not cooling (fan runs, air not cold)

Most PTAC no-cool calls are airflow (filter, coils), a wrong control setting or a failed compressor start component. Rule those out before suspecting the sealed system.

6 steps ~60 minmoderate

Guide · Air Conditioner

PTAC not heating (electric heat or heat pump)

PTACs heat with electric strip heat, a heat pump, or both. Heat pump models switch to strip heat in cold weather. Check settings, then the heater element and its limits, then the reversing valve.

5 steps ~60 minmoderate

Guide · Air Conditioner

PTAC error codes: how to read and clear them

Most PTAC brands store fault codes that can be shown with a button combination on the control (for example, on GE Zoneline (2019 technical service guide) you hold FAN and the red AUX button together while the unit is OFF, and AUX clears stored faults; other models use other sequences, so use the model's tech sheet). Read the active code, fix the cause, then clear it.

4 steps ~20 mineasy

Other air conditioners manuals

Manual · Air Conditioner

Carrier Infinity Greenspeed 24VNA6 air conditioner and 25VNA4 heat pump (variable speed): quick sheet

Carrier Infinity Greenspeed Intelligence variable-speed outdoor units: 24VNA6 air conditioner (up to 26 SEER) and 25VNA4 heat pump (up to 24 SEER, up to 13 HSPF), 2-5 ton. Nomenclature from the service manual: 24 = AC, 25 = HP; V = variable speed; N = Infinity series; A = Puron (R-410A); 6 / 4 = SEER tier; two digits = nominal capacity (e.g. 36 = 36,000 BTU/h); voltage digit 3 = 208/230-1; compressor ID 4 = variable-speed scroll, 5 = variable-speed rotary. Serial format: week + year + plant letter + sequence (e.g. 0118E00001). Other 24VNA families (24VNA0, 24VNA9) use different software and code lists.

5 components4 common failuresCarrier

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Trane XV20i / XV18 variable-speed outdoor units (4TTV0 AC, 4TWV0 heat pump, ComfortLink II): quick sheet

Trane variable-speed TruComfort / ComfortLink II communicating outdoor units: 4TTV0 air conditioners (XV20i family, e.g. 4TTV0024A1000C, 4TTV0036, 4TTV0048, 4TTV0060, 4TTV0061) and 4TWV0 heat pumps (Service Facts 4T-V0-SF). They use an Integrated Variable Speed Control (IVSC) board and a Communicating Display Assembly (CDA) in the outdoor unit. Distributor listings note several 4TTV0 revisions; confirm the full model on the nameplate before ordering parts. Single-stage XR14/XR16 units are on a separate sheet.

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Lennox XC21 air conditioner and XP21 heat pump (Dave Lennox Signature, iComfort-enabled): quick sheet

Lennox XC21 (air conditioner, service literature Corp. 1007-L2) and XP21 (heat pump, Corp. 1031-L7), 2, 3, 4 and 5 ton, R-410A, expansion valve indoor coils only. Both use an A175 main control (outdoor control part number 101798-xx) that works in communicating (iComfort) or conventional 24 V mode, plus a separate fan motor control. The XP21 literature describes a two-stage scroll compressor; the 2018 XC21 literature describes the compressor as one-stage while the same control section covers two-stage units, so confirm staging on the nameplate. For ML14XC1/EL16XC1 single-stage units see the separate sheet.

4 components3 common failuresLennox