Why is my new mini-split not heating or cooling well?

When a new mini-split underperforms, the cause is usually one of three things: how it is set, how it was designed for the room, or how it was installed. The first you can check in a few minutes with the remote. The other two are a call to the installer, and on a new system that visit is often covered by the installer's labor warranty.

Start with the remote

The controller is the first thing to check. For cooling, Trane says to make sure the unit is set to Cool with the temperature several degrees below the room temperature.^[2] For heating, Lennox says to make sure it is set to Heat with the temperature 3 to 5 degrees above the current room temperature.^[1] Look for a timer or sleep setting too, because a timer or schedule can override what you ask for.^[1] If the unit does not respond to any button, the remote may need new batteries.^[1]

How you run it matters as much as the set point:

• Avoid Auto mode. ENERGY STAR advises setting a heat pump to Heat or Cool rather than Auto, so it does not switch modes on unseasonably warm or cold days.^[5]
• Hold a steady temperature. Unlike a furnace, heat pumps do not save energy by being turned down when you are away or asleep.^[5] A big setback can leave the room cold for a while when you come back.
• Mind the fan. ENERGY STAR suggests the Auto fan setting, or the lowest fan speed that spreads the air far enough.^[5] A fan on its lowest setting may not reach the far side of a large room.

Normal winter behaviour that looks like a fault

A mini-split in heat mode will sometimes blow cool air or pause for a while, and that is usually the defrost cycle. Lennox says frost can start forming on the outdoor coil when it is below about 47°F outside,^[1] and the system temporarily sends heat to that coil to clear it. Defrost generally takes 3 to 15 minutes, and you may see steam rising from the outdoor unit.^[1] It is worth a service call if a defrost lasts longer than 15 minutes or comes back more than once within an hour.^[1]

Airflow

Even a new unit needs clear air paths. Clogged filters reduce airflow, so the system works harder while delivering less.^[1] For a ductless system that is not keeping a room warm or cold enough, Trane's fix is to check the filters.^[4] Trane also advises keeping bookcases, picture frames and other furniture away from the indoor unit.^[4] Outside, Lennox recommends at least 24 inches of open space on every side^[1] of the outdoor unit. Construction debris, a new shrub or a pile of leaves can be enough to hurt performance. Filter care is covered in how often a mini-split should be cleaned.

Design problems: size and placement

If the settings are right and the filters are clean, look at whether the system fits the space. An undersized unit runs longer and harder and struggles to hold a steady temperature, while an oversized one short-cycles, which causes temperature swings and humidity problems.^[6] A room that feels cool but clammy can be a sign of the second.

Placement can look like undersizing. Heat rises and cool air sinks, and heat struggles to move through an open doorway and cannot get through a closed one.^[5] One head in a living room usually will not keep the bedrooms down the hall comfortable, however big it is.

Cold snaps test capacity too. ENERGY STAR certification tests cold-climate heat pumps down to 5°F,^[5] and ENERGY STAR notes that pairing the heat pump with a backup heat source heats the home most efficiently when it is colder than that.^[5]

ENERGY STAR says your contractor should confirm the size with a "Manual J" calculation.^[5] If you suspect a sizing or placement problem, ask the installer to show you the load calculation for the room.

Installation faults

Some problems come from the install itself. Trane says a mini-split should not be low on refrigerant unless there is a leak or it was charged incorrectly at installation.^[3] The signs it lists include weak cooling or heating, a whistling or hissing sound near either unit, higher energy bills, and a frozen coil on an indoor unit.^[3] If a coil is frozen, turn that indoor unit off and call for repair.^[3] A technician finding a leak usually examines the flare fittings where the refrigerant lines join the indoor and outdoor units.^[3] Getting the charge right is not a detail: properly charging an air conditioner or heat pump helps reduce energy use and the risk of system failure.^[7]

Electrical and control faults happen too. If a breaker trips again right after one reset, leave it off and request service.^[1] A faulty sensor is less obvious: a sensor that reads 5°F warmer than the room can tell the system no more heat is needed.^[1]

When to call a licensed pro

Call the installer first. If your mini-split is newly installed, Trane says to call your installer back, and many HVAC companies offer a one-year labor warranty at no extra cost.^[3] Call right away for a hissing sound, ice on the indoor coil, water dripping from the indoor unit, a breaker that keeps tripping, an error code that returns, or a defrost cycle that will not end. Leaking water has its own causes: see why a mini-split leaks water inside.

Refrigerant and electrical repairs are licensed work. The EPA requires Section 608 technician certification for anyone who could reasonably be expected to break into the refrigerant circuit while servicing or repairing equipment.^[8] Under Virginia law, no individual may work as a tradesman without a licence,^[9] and the Board for Contractors requires a master tradesman licence as a condition of licensure for HVAC contractors.^[10] If the installer will not come back, check any new company's licence before it opens the system, and read what installing a mini-split involves to know what a proper install should have included.

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If your system needs a second opinion, describe the problem once and licensed local HVAC businesses reply.