Heat Pump Turning On and Off Constantly? Causes & Fixes

A heat pump that runs for four minutes, shuts off, then fires back up four minutes later is telling you something. It's called short cycling, and it's one of the most common calls we take across Federal Way, Kent, Puyallup, Tacoma and Olympia. Sometimes the fix is free and takes ten minutes. Sometimes it's a $300 part. And sometimes the honest answer is that the system was too big for the house the day it was installed, which no repair fully cures.

Here's how to tell which one you're looking at.

What short cycling actually means

A heat pump is designed to run long and slow. Long runtimes are how it moves heat efficiently, keeps room temperatures even, and pulls moisture out of the air in summer. In our mild marine climate, a healthy system in shoulder-season weather typically cycles two to three times an hour, with each run lasting roughly 10 to 20 minutes.

Short cycling is when that collapses to three, four, five minute bursts, over and over. The compressor takes its biggest current draw and its biggest thermal hit at every startup, so a system doing 12 starts an hour is aging itself far faster than one doing three.

What is not short cycling

Before you panic, rule out the two normal behaviors that look alarming:

• Defrost cycles. In cold, damp Puget Sound weather the outdoor unit periodically reverses for 5 to 15 minutes to melt frost off the coil. The outdoor fan stops, steam pours off the unit, and then heating resumes. That's engineering, not failure. More on cold-weather behavior in do heat pumps work in Washington winters.
• A variable-speed system modulating. Higher-tier equipment ramps up and down rather than slamming on and off. If the outdoor unit is changing speed but rarely fully stopping, that's the system doing exactly what you paid for.

The eight real causes, cheapest first

1. A clogged air filter

This is the single most common cause we find, and it costs a few dollars. A restricted filter starves the indoor coil of airflow. In heating, the coil overheats and the system trips out on a high-limit switch. In cooling, the coil frosts and the pressure switch kills the compressor. Either way, the system stops, cools off, and restarts. Change the filter, check it monthly for a season, and see if the pattern disappears.

2. Blocked airflow inside the house

Closed registers, a blocked return grille, furniture over a floor vent, or crushed flex duct in the crawl space all do the same thing a dirty filter does. If half the vents in the house are closed because one bedroom was too cold, open them.

3. A smothered outdoor unit

The outdoor coil needs to breathe. Blackberry vines, cedar debris, a stack of firewood, drifted leaves, or a fence built 8 inches away all choke it. Give the unit at least 18 to 24 inches of clear space on all sides and several feet overhead, and rinse the coil fins gently with a garden hose from the inside out.

4. Thermostat placement or wiring

A thermostat mounted in direct sun, above a supply register, on an exterior wall, or in a drafty hallway reads temperatures the rest of the house never sees. It satisfies fast, shuts down fast, then drifts and restarts. Wireless sensors, a relocated stat, or a corrected anticipator setting fixes it. Loose low-voltage wire at the stat or the air handler board causes the same erratic starting and stopping.

5. Low refrigerant charge

Refrigerant doesn't get consumed, so low charge means a leak. Low charge drives the system into pressure or temperature limits, and the safety controls do their job by shutting the compressor down. Symptoms often pair with weak heat, a coil that ices up, or hissing. This needs a licensed tech with gauges, a leak search, and a proper weighed-in charge. Topping off without finding the leak is throwing money into the air.

6. A failing capacitor, contactor or relay

A weak run capacitor can't hold the compressor or fan motor in operation, so it starts, struggles, and drops out on internal overload. Pitted contactor points do something similar, chattering the compressor on and off. These are among the most satisfying repairs in this trade because they're inexpensive, fast, and they genuinely solve the complaint.

7. Defrost control or sensor faults

A bad defrost board, a failed outdoor thermistor, or a stuck reversing valve can send the system into and out of defrost far too often. From the hallway it looks like the heat pump can't make up its mind. If cycling started with the cold weather and the outdoor coil frosts and clears repeatedly, this is a likely suspect.

8. Oversized equipment

The one nobody wants to hear. A heat pump sized two tons too large satisfies the thermostat before the house ever settles, then shuts off, then starts again. It will short cycle forever because the load and the equipment don't match. This is common in homes where somebody sized by square footage or simply matched whatever was there before instead of running a load calculation. Duct fixes, thermostat relocation and airflow corrections reduce it, but they don't erase it.

Free checks to run before you call

1. Thermostat set to HEAT or COOL, fan on AUTO, not ON, and not on EM HEAT in winter.
2. Replace the air filter, even if it looks acceptable.
3. Open every supply register and clear every return grille.
4. Clear 2 feet around the outdoor unit and gently rinse the coil.
5. Reset the breaker once. If it trips again, stop and call — repeated tripping is an electrical fault, not a nuisance.
6. Watch the clock. Note how long the system runs and how long it rests. Those two numbers are the most useful thing you can hand a technician.

If the cycling survives all six, it's a real fault. Heat pump repair is what we do, and diagnosing cycling faults is mostly a matter of measuring pressures, amps and temperature splits rather than guessing at parts.

What the repair typically costs

Capacitors, contactors and thermostat work sit at the bottom of those ranges. Defrost boards, blower motors and refrigerant leak repairs sit higher. You get a price before the wrench comes out, not after.

When repair stops being the right answer

We'd rather fix your system than sell you one. The math we use:

• Under 10 years old: repair it, almost always. Parts are available and the remaining life justifies the spend.
• 10 to 12 years old: repair if the fix costs less than half of replacement.
• 12 to 15+ years old, or running R-22 refrigerant: price replacement. R-22 is expensive and shrinking in supply, and a compressor failure on a system that age is a bad bet.

Replacement runs $10,000 to $16,000 in standard markets, $11,500 to $19,200 in affluent and luxury markets, with a full ducted new install at $12,000 to $18,000 standard. Every Varsity install carries a 10-year parts and labor warranty, and most are finished in one day. For a fuller price breakdown, see how much a heat pump costs in Washington.

One honest note on incentives: qualifying heat pump installations can earn up to $4,400 toward a qualifying heat pump when it replaces an existing electric or fossil-fuel system, and the exact amount depends on the system you're replacing, the equipment you install, and which utility bills your address. Programs change — we confirm your current eligibility and handle the paperwork. From April 2, 2026 PSE rebates require a PSE-certified installer. Varsity is PSE Certified.

Sizing is why we run a load calculation on every estimate

If oversizing turns out to be the root cause of your cycling, the fix happens at replacement time, not repair time. Washington's energy code puts winter design temperatures across our service area anywhere from 17°F in Olympia to 29°F in Tacoma. That 12-degree spread changes the required capacity, and no rule of thumb catches it. Every Varsity estimate includes a real Manual J load calculation at no extra cost, and where the load is genuinely small we'll say so, even if it means selling a smaller system.

If your heat pump also can't hold temperature, not just cycling but genuinely losing the fight, start with heat pump not heating instead.

Get it diagnosed

Short cycling isn't an emergency, but it isn't harmless either. Every extra start is wear you paid for. Call (253) 785-4622, Monday through Friday 8 to 6, or request an estimate online. We're owner-operated out of Federal Way, we pull permits on every install, and we answer the phone after the check clears.