Why are some rooms in my house hotter than others?

Some rooms run hotter because they get less conditioned air than the rest of the house, gain more heat than the rest of the house, or both. ENERGY STAR lists rooms that are difficult to heat and cool as one of the signs of a poorly performing duct system,^[1] and the ducts are the first place to look. Sun, insulation and the way air gets back to the system matter just as much.

The ducts feeding that room

In a typical house, about 20 to 30 percent of the air that moves through the ducts is lost to leaks, holes and poorly connected ducts.^[1] The room at the end of the longest or leakiest run is the one that suffers. ENERGY STAR's warning signs include ducts that run through an attic, crawlspace or garage, and tangled or kinked flexible ducts.^[1] Sealing and insulating ducts can help with rooms that are too hot in summer or too cold in winter.^[2]

Damage plays a part. Dominion Energy's advice is to periodically check your ductwork for leaks or tears and repair fallen or crushed ductwork,^[7] and at the rooms themselves, the joints where registers meet floors, walls and ceilings are common places to find leaks and disconnected ductwork.^[1] A flex duct that someone stepped on in the attic, or a boot that pulled away from the register, can leave a room with a trickle of air while the system runs normally everywhere else.

If you go into the attic or crawlspace to look, start early on hot days, because attics heat up as the day moves on; walk only on joists or truss chords, not on drywall or insulation; and wear an OSHA-approved particulate respirator or double-strap dust mask.^[8] In an older home, look but do not touch old duct wrap, tape or register boots. Asbestos-containing materials such as insulation and register boots should be disturbed or removed only by specially trained and equipped contractors,^[9] and you generally cannot tell whether a material contains asbestos by looking at it.^[10] The duct sealing answer has more on working safely up there.

No way for the air to get back

Air has to return to the system as well as leave it. Many systems have a supply register in every room but only one central return.^[5] When a door is closed, air in that room cannot reach the return and is forced out through whatever gaps it finds, while unconditioned air is pulled in elsewhere.^[5] The fix is a return in each room, or a vent in the door or above the door frame.^[5] Even with doors open, ENERGY STAR notes that many systems have air return ducts that are too small.^[3]

Simple habits help here. Dominion Energy's advice is to leave interior doors open, do not close vents, and do not block returns,^[7] and to check air filters monthly and change them when they are dirty.^[7] Keep furniture clear of supply registers and return grilles.^[5]

The room gains more heat

Top-floor rooms under an attic, rooms over a garage, rooms with large sunny windows and finished attic rooms often pick up more heat than the rest of the house.

Insulation is often part of it. ENERGY STAR says adding insulation to the attic helps block heat loss in winter and heat gain in summer, and the easiest place to add insulation is usually the attic.^[4] It also says some rooms being too hot or cold can come from inadequate air sealing or insufficient insulation,^[4] while humidity problems, excessive dust, or rooms that never seem to get comfortable can point to leaky or poorly insulated ductwork.^[4] An energy audit finds both.

The system was never sized room by room

Sizing matters: an accurate load analysis helps prevent the problems that come with an oversized or undersized system.^[6] Size is only half of it, though. Virginia Cooperative Extension's guide to choosing a system includes designing a ductwork system to deliver the correct amount of conditioned air to each space,^[6] and says load calculations are usually done for each zone following ACCA Manual J or a similar method.^[6] One of the questions it says a contractor should ask you is "Are there rooms that are too hot or too cold?"^[6]

What a contractor does next

If the checks above do not even things out, an HVAC contractor should inspect the whole duct system, including the attic and crawlspace, evaluate the balance between supply and return air, and repair damaged and disconnected ducts.^[3] Measuring the airflow at each register shows which rooms are short.

The bigger fixes are more return capacity, a rebuilt duct run to the problem rooms, or zoning, which splits the house into zones with their own controls. If a zoned system is used, the loads for each zone should be calculated,^[6] and ENERGY STAR suggests a programmable thermostat for each zone in a home with multiple heating and cooling zones.^[4] Ask the contractor which fix suits your house.

Stop and call a pro if

• The duct to the room is disconnected, crushed or collapsed somewhere you cannot reach, such as inside a wall or ceiling.
• The fix means a new return, a new duct run or zoning. Ask your building department whether the work needs a permit; see whether you need a permit to replace ductwork in Virginia.
• The whole house is warm, not just some rooms, or the system runs all day without catching up. That is a capacity or equipment problem; see why a new HVAC system might not keep up.
• You see ice on the refrigerant lines, water around the indoor unit, or the outdoor unit is not running. Those are repair calls.
• You find old duct wrap, tape or insulation that might contain asbestos.

Related

To seal the ducts you can reach yourself, see how to seal leaky air ducts. If the problem is a muggy, heavy feel rather than heat, read why a house feels stuffy. Browse heating and cooling services, find HVAC businesses in Blacksburg, or read more heating and cooling answers.

When you want someone to find the cause, describe the job once and licensed local HVAC businesses quote it.