Heat Pump Not Cooling Properly Ontario 2026: Stuck Reversing Valves, Low Refrigerant, and the 6 Most Common Causes

The Symptom Pattern

The complaint is specific and worth separating from a dead-on-arrival system. The compressor runs (a low hum coming from the outdoor unit), the outdoor fan spins, and the indoor blower pushes air out of the registers. Nothing looks obviously wrong. But the air coming out of the registers is only a few degrees cooler than the air in the room, the thermostat setpoint never gets satisfied, and the indoor temperature drifts up through the afternoon despite the system running almost continuously. In the worst version of the problem, the supply air is actually warmer than the return air.[8]

The first diagnostic step is to quantify what the system is doing. Put a digital thermometer in the closest supply register to the indoor unit and another in the return-air grille. Let the system run in cooling mode for at least 15 minutes on a warm day (ideally above 22 degrees Celsius outside), then read both thermometers. The difference between the two is the supply-return temperature split, and it is the single most useful number in heat pump cooling diagnostics.[1]

The 6 Most Common Causes in Ontario 2026

1. Stuck Reversing Valve

This is the failure unique to heat pumps. The reversing valve, also called a 4-way valve, sits inside the outdoor unit and routes refrigerant in one direction during heating and the opposite direction during cooling. The valve has a solenoid that shifts an internal slide when the thermostat changes mode.[3]Three failure modes are common. First, mechanical: the slide sticks due to wear, debris, or extended idle periods, and does not shift fully when commanded. Second, electrical: the solenoid coil fails open or the wiring to it breaks, so no shift command reaches the valve. Third, leak: the valve body develops an internal refrigerant leak between the heating and cooling ports, so some refrigerant flows the wrong direction regardless of commanded mode.

The diagnostic tell is acoustic. When the thermostat is switched from heat to cool or cool to heat, a healthy reversing valve produces a distinct metallic click from the outdoor unit within a few seconds. No click (or a very faint one when the rest of the system is running) strongly suggests the valve is not shifting.[4]A stuck reversing valve is not a DIY repair. Replacement involves brazing the valve into the refrigerant circuit, recovering the existing refrigerant charge, evacuating the system to vacuum, and recharging. TSSA requires a licenced Refrigeration and Air Conditioning Mechanic.[6]Ontario 2026 pricing: $600 to $1,400 all in.

2. Low Refrigerant Charge

A heat pump is a sealed refrigerant system. If it is low on charge, refrigerant has leaked out somewhere (the evaporator coil, the condenser coil, a service valve seal, or a brazed joint). Low charge reduces the system's ability to absorb heat indoors and reject it outdoors, so the supply-return split shrinks. The reduced split is the telltale.[7]A split in the 5 to 10 degree Fahrenheit range on a warm day with clean coils and a clean filter points squarely at refrigerant charge.

Under Canadian environmental regulations and TSSA refrigerant-handling rules, the correct repair is to leak-test the system (electronic detector, UV dye trace, or pressurized nitrogen), repair the leak, evacuate, and recharge to the manufacturer's weighed-in spec.[5]Any quote that says top up without leak testing is declining the actual repair and venting refrigerant to atmosphere. Ontario 2026 pricing: $500 to $1,800 depending on where the leak is and how accessible it is. A service valve seal is a simple swap; an evaporator coil leak is a coil replacement.

3. Dirty Outdoor Condenser Coil

In cooling mode the outdoor coil is the condenser, which rejects the heat the indoor coil absorbed. If it is packed with dust, cottonwood fluff, dryer lint, or vegetation, it cannot reject heat efficiently, the refrigerant stays hot, and the system's cooling capacity drops. This is the same failure as on a central air conditioner and the diagnostic is the same: look at the coil. Reduced airflow through the fins, visible buildup between them, or a warm-to-the-touch coil body during operation all point to it.[1]

This is a DIY check and often a DIY fix. The companion guide on AC condenser coil cleaning covers the step-by-step approach (garden hose, fin comb, gentle spray from the inside out). Professional cleaning runs $150 to $350 in Ontario 2026 and is worth it if the coil is deeply packed or has bent fins.

4. Dirty Indoor Evaporator Coil

In cooling mode the indoor coil is the evaporator, where refrigerant absorbs heat from the house air. A coil coated with dust and debris (usually because the air filter was neglected for years) cannot absorb heat effectively. Unlike the condenser coil, the evaporator is not accessible from outside the cabinet in most Ontario installations; the technician has to remove the coil access panel. That makes this a pro repair. A clogged filter usually rides along with this problem: the filter is the first line of defence for the coil.[4]Replace the filter on a 1 to 3 month schedule for a 1-inch MERV 8 to 11 filter, longer for higher-MERV media filters.

5. Oversized Heat Pump

This is a sizing problem, not a failure. An oversized heat pump (one with more cooling capacity than the house needs on a design-day load) short-cycles in cooling mode: it runs for a few minutes, satisfies the setpoint temperature, and shuts off before it has had time to dehumidify the indoor air. The house feels humid and clammy, and the supply air at the register is cold but only for short bursts.[2]

Oversized equipment is usually the result of an installer who skipped the CSA F280 (or equivalent Manual J) heat-loss and heat-gain calculation and sized off square footage. If the unit is new and the complaint has been there since day one, the diagnosis points at sizing rather than failure. Short-term remediation is a variable-speed indoor fan or a two-stage compressor control; long-term remediation is correct equipment sizing on the next replacement. A companion guide on heat pump short cycling covers this in detail.

6. Thermostat Wiring or Mode Configuration

Heat pumps use the thermostat's O or B terminal to tell the reversing valve which mode to be in. Manufacturers split on convention. Most brands (Carrier, Trane, Lennox, and most ductless systems) energize O during cooling. Some brands (older Ruud and Rheem) energize B during heating. A thermostat installed with the wrong convention set in the installer menu will energize the reversing valve in the wrong mode, leaving the heat pump trying to heat when the homeowner calls for cooling.[4]

The symptoms mirror a stuck reversing valve: supply air warmer than return air, or essentially no cooling. The difference is that the click from the outdoor unit is still audible when the thermostat changes mode; it just clicks in the wrong direction. Fix is a configuration change in the thermostat's installer menu or a wire swap between the O and B terminals on the heat pump's control board. DIY for a comfortable homeowner with the manufacturer manuals; $150 to $250 for a technician call.

At-Home Diagnostics in Order

1. Measure the supply-return split. Digital thermometer at the nearest supply register and the return grille, after 15 minutes of cooling mode runtime. The number tells you how severe the problem is.
2. Check the filter. Pull it, hold it up to a light. If light does not pass through, replace it and re-measure the split after 30 minutes.
3. Look at the outdoor unit. Fan spinning smoothly, no debris against the cabinet, coil fins visible and not packed, no ice or frost on the refrigerant lines.
4. Listen for the reversing valve click. Stand near the outdoor unit, have a helper change the thermostat mode from Cool to Heat and back to Cool. A healthy valve produces a distinct metallic click in both directions within a few seconds.
5. Confirm the thermostat mode. Set to Cool, not Emergency Heat or Heat. Confirm fan is set to Auto, not On (On keeps the blower running continuously even when the compressor is idle, which can feel like no cooling on warm days).

If the filter, coil, fan, and thermostat mode are all confirmed good and the split is still poor, the problem is in the sealed refrigerant system (charge, reversing valve, or compressor). That is a pro call.

Why Heat Pumps Are Harder to Diagnose Than Plain AC

A central air conditioner runs in one direction: it cools. A heat pump runs in both directions, sharing most of the same hardware, and adds a reversing valve to switch modes. That extra mode doubles the failure surface. A valve that works fine in heating can leak internally in cooling. A thermostat wiring mistake that is invisible in one mode produces wrong behaviour in the other. A homeowner comfortable with traditional AC troubleshooting needs to extend that thinking to include the mode-switching layer.[3]

Red Flags on a Tech Quote

The heat pump service market in Ontario has a wide range of competence and honesty. Three specific patterns on a quote should be treated as red flags.

• A quote to replace the whole outdoor unit without first testing refrigerant charge and reversing valve operation. The correct sequence is diagnose the specific failure, price the repair against replacement, and let the homeowner choose. A jump straight to replacement is a commission pattern, not a diagnostic conclusion.
• A quote that does not measure the supply-return split before and after the repair. The split is the most basic performance measurement on a cooling system. A technician who does not measure it is not verifying that their repair actually worked. Ask for the measurement in writing on the service invoice.
• A quote to top up refrigerant without leak testing. This is both regulatory non-compliance (venting refrigerant is a regulatory violation in Canada under the Ozone-Depleting Substances and Halocarbon Alternatives Regulations) and bad engineering practice. Charge does not drop on a sealed system without a leak. Any technician proposing a top up without a leak test is scheduling the next service call.[5]

A credible quote names the specific failed component, explains the diagnosis in a sentence or two, shows the before and after measurements, and separates parts and labour. If any of those are missing, get a second opinion. Related: our Ontario 2026 guide on HVAC quote structure and what to look for on the paperwork.

When to Repair and When to Replace

A reversing valve replacement on a 6-year-old heat pump is a clear repair. A reversing valve replacement on a 13-year-old heat pump past its expected useful life is usually a replacement conversation because another major failure is likely within a couple of years. A low-charge leak repair on a unit with a leaking evaporator coil is often a replacement because coil replacement pushes past $1,500, and on an older unit the coil leak is a symptom of broader system aging. The decision framework in the companion Ontario 2026 HVAC repair-versus-replace guide applies here as well: the $5,000 rule, refrigerant type, warranty status, and rebate eligibility.

Frequently Asked Questions

Related Guides

• Heat Pump Auxiliary Heat Running Too Much Ontario 2026
• Heat Pump Short Cycling in Heating Mode Ontario 2026
• AC Condenser Coil Cleaning Ontario 2026