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True Trade Path

Diagnostic guide

AC Running Constantly

AC that never cycles off may be undersized for the heat load, low on refrigerant, fighting poor insulation, or set to fan On instead of Auto.

System
Cooling
Causes covered
7
You can check
3 without a technician

Stop and read this first

  • Welded contactor runs unit continuously — high bill and wear; shut breaker if needed.

Work through these in order

Ordered from most to least likely. Working top down means you rule out the cheap, common causes before paying anyone to look at the expensive, rare ones.

  1. Extreme heat beyond design conditions

    CommonYou can check
    How to check
    Runs all day on record highs but house cool-ish.
    The fix
    Normal for marginal days; verify supply air temp.

  2. Dirty filter or blocked return

    CommonYou can check
    How to check
    Weak supply; long run times.
    The fix
    Replace filter; open returns.

  3. Low refrigerant charge

    CommonNeeds a pro
    How to check
    Lukewarm supply; ice on line possible.
    The fix
    Leak search and recharge ($250 to $900).

  4. Undersized equipment

    CommonNeeds a pro
    How to check
    Never reaches setpoint summer.
    The fix
    Manual J and possible replacement.

  5. Thermostat fan set to On

    CommonYou can check
    How to check
    Blower runs 24/7 even when not cooling.
    The fix
    Set fan to Auto.

  6. Poor insulation or air leaks

    PossibleNeeds a pro
    How to check
    Heat gain exceeds capacity.
    The fix
    Audit insulation and seal ($500 to $5,000).

  7. Stuck contactor welded shut

    UncommonNeeds a pro
    How to check
    Outdoor runs with thermostat off.
    The fix
    Replace contactor immediately.

Direct answer

AC that never stops may be extreme heat, dirty filter, low charge, undersized unit, or fan On — verify supply air temperature before replacing equipment.

Severity and urgency

Fix within weeks

High bills and wear; charge issues kill compressor.

Cost of ignoring it

$100–$300/month extra electric; compressor failure $1,500+.

How a pro will diagnose it

Check delta-T, pressures, filter, load ($99–$179).

Prevention and maintenance

  • Filter monthly
  • Seal ducts
  • Fan Auto
  • Shade windows
  • Maintain charge

For trade owners

Pricing this job as a contractor?

Homeowners see national ranges on this page. If you are building the estimate, use the same scope assumptions — then price to burdened labor, overhead recovery and a real margin.

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Find a licensed pro

When you are ready to hire for ac running constantly, start with licence verification — not with whoever answered first.

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Common questions

Could extreme heat beyond design conditions be causing this?
Runs all day on record highs but house cool-ish. Normal for marginal days; verify supply air temp.
Could dirty filter or blocked return be causing this?
Weak supply; long run times. Replace filter; open returns.
Could low refrigerant charge be causing this?
Lukewarm supply; ice on line possible. Leak search and recharge ($250 to $900).
Could undersized equipment be causing this?
Never reaches setpoint summer. Manual J and possible replacement.
What is the most common cause of AC running constantly?
Filter and fan Auto.
Can I fix AC running constantly myself?
Some checks are DIY-safe (shutoffs, filters, simple adjustments). Licensed HVAC technician work is required when gas, sewer, refrigerant, or main lines are involved.
How much does professional repair for AC running constantly cost?
Leak fix $250–$900; new unit $5,000–$12,000.
How urgent is AC running constantly?
Warm air all day — soon.
Should I shut off water or power for AC running constantly?
Shut off the local supply or breaker when leaks, flooding, sparks, or gas odor are present. If unsure, stop using the system and call a pro.
Will AC running constantly damage my home if I wait?
Moisture-related issues can rot framing and grow mold within 48 hours. Mechanical issues often worsen — seized pumps, cracked heat exchangers, or compressor damage cost far more when delayed.
What should I tell the HVAC technician on the phone?
Describe when it started, what changed (weather, remodel, new appliance), what you already tried, and whether multiple fixtures or zones are affected.
Does homeowners insurance cover AC running constantly?
Sudden pipe bursts are often covered; slow leaks, maintenance neglect, and sewer backups frequently are not. Document with photos and read your policy endorsements.
How do I know the quote for AC running constantly is fair?
Get two written quotes with scope, parts, warranty, and permit line items. Compare to regional mid-range pricing — not the cheapest emergency rate.
How can I prevent AC running constantly from coming back?
Follow the prevention steps on this page: maintenance schedules, avoiding grease and wipes in drains, filter changes, and annual tune-ups where applicable.

Once you know what it is

Step one

Know what it should cost

Get the range before you call, so the first number you hear is not the only number you have.

Step two

Decide repair or replace

On equipment near end of life, the repair is usually the first of several.

Step three

Check the quote

Compare against the typical range and confirm the terms before you sign.

HVAC System Replacement cost guideRepair or replace calculatorEstimate repair costAnalyze a quote

What the fix costs

Tools for this decision

Other symptoms