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How to fix hot and cold spots
Hot and cold spots after an AC installation usually come from airflow, not the unit: leaky or undersized ducts, closed or blocked registers, a missing return, afternoon sun on one side of the house, or an oversized system that short-cycles. Most are found by measuring airflow room by room.
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- The six usual causes
- Diagnose it in order
- What you can fix without a contractor
- What needs a contractor, and what it costs
- Why it is worse in the Valley
- Room by room: what usually causes it
- When sizing is the cause
- Zoning, mini-splits and other bigger fixes
- What to tell the contractor
- Three myths about uneven rooms
- If the installation is new
The six usual causes
| Cause | What you notice | Who fixes it |
|---|---|---|
| Duct leakage into the attic | Far rooms weak; attic dusty around ducts | Contractor: seal and retest |
| Undersized or crushed duct to one room | One register barely blows | Contractor: replace the run |
| No return path from a closed room | Room stuffy with the door shut, fine with it open | Contractor: transfer grille or return |
| Afternoon sun on west or south rooms | Hot from 3 pm, fine in the morning | You: shades; contractor: airflow balancing |
| Oversized system | Short runs, clammy air, uneven rooms | Contractor: settings, staging, or sizing review |
| Thermostat in the coolest room | Thermostat satisfied while other rooms are hot | Contractor: relocate or add a remote sensor |
Why installation matters this much
ENERGY STAR says improper installation can reduce system efficiency by up to 30 percent, and that "Sealing and insulating ducts can improve the efficiency of your heating and cooling system by as much as 20 percent." Uneven rooms are often the first sign of a duct or airflow problem.
Diagnose it in order
- Write down the pattern. Which rooms, what time of day, door open or shut. A morning-fine, afternoon-hot room points to sun.
- Check every register and return. Open supplies, clear furniture off returns, and check no register is closed.
- Replace the filter. A clogged filter cuts airflow everywhere. ENERGY STAR: "At a minimum, change the filter every 3 months."
- Run the fan for an hour and feel each register. A weak one points to its duct.
- Check the attic, if safe, for ducts that are disconnected, crushed or lying on hot insulation.
- Call the installer with your notes if the problem is still there.
What you can fix without a contractor
- Open every supply register; clear returns.
- Replace the filter with the size the system takes.
- Close blinds on west windows in the afternoon.
- Keep interior doors open, or ask about a transfer grille for rooms you keep shut.
- Move lamps and electronics away from the thermostat.
Do not close lots of vents
Closing one or two registers can push air elsewhere. Closing many raises duct pressure, can reduce total airflow and can make the coil too cold. Ask the contractor first.
What needs a contractor, and what it costs
- Airflow measurement per register, and balancing dampers.
- Duct leakage testing and sealing.
- A new return or transfer grille.
- Replacing an undersized or crushed duct run.
- Zoning, or one mini-split head for a single stubborn room.
Why it is worse in the Valley
Attic ducts sit in the hottest air in the house. On Valley afternoons that air is far hotter than outdoors, so a leaky or poorly insulated duct loses more of its cooling before it reaches the far room.
Woodland Hills averages 35.6 days a year at 100 °F or above on NOAA normals; LAX 0.2. See coast vs Valley cooling.
Room by room: what usually causes it
| Room | Common cause | First thing to check |
|---|---|---|
| Far bedroom at the end of the ducts | Long run losing air or heat in the attic | Airflow at the register; duct condition |
| West-facing living room | Afternoon sun load | Shades; register airflow at 4 pm |
| Upstairs | Heat rises; roof sun; long runs | Return air upstairs; balancing |
| Room with the door always shut | No return path | Does it improve with the door open? |
| Converted garage or addition | Never designed into the duct system | Whether it has a supply and return at all |
| The room with the thermostat | Feels fine while others suffer | Thermostat location |
When sizing is the cause
An oversized system cools the air near the thermostat quickly and shuts off before the far rooms catch up. The result is short runs, clammy air and uneven rooms.
An undersized one runs constantly on the hottest days and still falls behind. Both trace back to sizing; ask for the load calculation. See oversized AC problems.
Zoning, mini-splits and other bigger fixes
Zoning a ducted system
Motorised dampers and more than one thermostat let upstairs and downstairs run separately. It works best when the ducts and equipment can handle the change; ask the installer.
A mini-split for one room
A single-zone head cools one stubborn room without touching the ducts, published at $4,000–$7,500 in LA.
What to tell the contractor
A contractor finds the cause faster with a clear description. Before you call, write down:
- which rooms, and how far off they are from the thermostat setting;
- what time of day, and whether doors were open or shut;
- whether it started with the new system or was there before;
- the filter you use and when you last changed it.
Three myths about uneven rooms
Myth: a bigger unit fixes it
A bigger unit short-cycles and often makes it worse. The fix is usually airflow.
Myth: closing vents saves money
Closing many raises duct pressure and can reduce total airflow. Close one or two at most.
Myth: it is the new unit's fault
A new unit on old leaky ducts carries the old ducts' problems. The Title 24 duct test should have caught leakage.
Myth: nothing can fix one hot room
Balancing, a return, a duct repair or a single mini-split head usually can.
If the installation is new
Call the installing contractor first. The Title 24 duct test should have been passed and registered: ask for the results. Uneven rooms on a brand-new system are a labor-warranty question.
- Ask for the airflow and duct leakage readings.
- Ask whether the system was sized by load calculation, and for the result.
- Put the complaint in writing, with dates.
See what to inspect after installation and equipment vs labor warranties.
Sources
- ENERGY STAR, "Heat & Cool Efficiently" (retrieved 2026-10-18)
- Energy Code Ace (CEC-supported), Single-Family Compliance Manual ch. 9, § 150.2(b)1E–F, 2022 code text; confirm against the 2025 code in force since 1 January 2026 (retrieved 2026-09-29) — verify before relying on it
- NOAA NCEI, U.S. Climate Normals 1991–2020, annual/seasonal (stations USW00023174, USW00093134, USW00023152, USC00049785) (retrieved 2026-09-29)
- Reliable Heating and Air (Woodland Hills), AC installation cost in LA and Ventura County (retrieved 2026-09-29)
- Cold Cloud (Glendale), central air cost in Los Angeles (retrieved 2026-09-29)
- E & A Mechanical (Tujunga), heat pump installation cost in Los Angeles (retrieved 2026-09-29)
- HVACProjectCost.com, Los Angeles HVAC replacement cost (retrieved 2026-09-29)
Questions
Should I close vents in rooms I do not use?
Closing a few can push air elsewhere, but closing many raises duct pressure and can reduce total airflow. Ask the contractor before closing more than one or two.
Will a bigger AC fix a hot room?
Rarely. A hot room is usually an airflow or sun problem. A bigger unit can make the rest of the house colder and the problem room no better, and it short-cycles.
Why is my upstairs hotter than downstairs?
Heat rises, the roof adds sun load, and upstairs rooms are often at the end of the longest ducts. Airflow balancing, return air upstairs or zoning are the usual fixes.
Can a new AC cause hot spots?
A new unit on old leaky ducts, or one sized wrongly, can. The installer should have sealed and tested the ducts under Title 24; ask for the results.
Is a mini-split a good fix for one hot room?
Often. A single-zone mini-split is published at $4,000–$7,500 in LA and cools one room without touching the ducts.
Do smart vents fix uneven rooms?
They can redirect some air, but they do not fix leaky or undersized ducts, and closing many vents has the same pressure problem as closing them by hand.
How do I know if my ducts leak?
A duct leakage test measures it. Signs include dusty insulation around attic ducts, weak far registers and high bills. A new AC permit includes a duct test.
Does the thermostat location matter?
Yes. A thermostat in the coolest or warmest room runs the whole house to that room. A remote sensor or relocation can help.
Can I fix hot spots by turning the thermostat lower?
It makes the cool rooms colder and costs more, and the hot room often stays hot. Fix the airflow instead.
Does attic insulation help?
Often. Better attic insulation reduces heat gain into the rooms below and around the ducts. Ask whether ducts are insulated too.
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