Upstairs Too Hot, Downstairs Too Cold: How to Fix It

By Ray Bennett · Updated 2 October 2026 · 6 min read · Checked against the sources at the end

Upstairs Too Hot, Downstairs Too Cold: How to Fix It

Short answer

It looks like two problems and it is one. Warm air rises, one thermostat downstairs decides for both floors, and the returns are nearly always on the wrong floor. That is why the same house is unbearable upstairs in July and freezing downstairs in January.

Most two storey houses do this, and most people describe it as two separate complaints in two separate seasons.

It is one thing. Warm air rises and cool air sinks, so the top floor collects heat all summer and the bottom floor loses it all winter. On top of that, the machine is being told what to do by a thermostat that only ever stands on one of the two floors.

Before you start

What to carry: a cheap thermometer, or two if you have them, and a torch for the attic. An hour, spread over a day.

Measure it before you fix it. Put a thermometer on a table on each floor, away from vents and windows, and read them both in the afternoon and again in the early morning. A gap of two or three degrees is ordinary in any house. Six or more is what this page is about.

Gas or electric makes no difference. A gas furnace, an electric one and a heat pump all share the same ductwork and the same thermostat, so the pattern is identical.

The one fix that makes it worse, and everybody tries it. Do not close registers to force air upstairs. It feels obvious and it is the wrong lever. A duct system is sized for all of it to be open, and closing rooms off raises the pressure inside the ducts.

What that actually does. In summer it slows the air over the cold coil until the coil freezes solid. In winter it traps heat in the furnace until the limit switch cuts the burners out. Over years it is one of the things that cracks a heat exchanger.

The safe amount. Easing a few registers part closed is fine. Closing any of them fully is not: what a furnace that keeps cutting out is telling you.

Why one house does both

The same physics, twice a year, pointing opposite ways. In summer the top floor gains from the roof and collects the warm air from below. In winter it keeps the heat and the ground floor loses it.

Check 1

Understand which of the four is yours

The thermostat is on one floor and decides for both. It is usually downstairs. In summer it reaches 74 and shuts the system off while the bedrooms are still at 80. In winter the same thing happens in reverse.

The returns are on the wrong floor. This is the big one and the one nobody checks. Many two storey houses have every return grille downstairs. The system therefore pulls its air from the cool floor and has no way of dragging the hot air off the top one.

The ducts serving upstairs run through the attic. An attic can pass 130 degrees on a summer afternoon. Air that left the coil at 55 arrives at the bedroom vent much warmer, and any leak in that duct is throwing cooled air into the loft.

The roof itself. The top floor has a hot surface above it that the ground floor does not, and in summer that adds a lot of heat.

How to tell which. Hold a hand at an upstairs vent with the cooling running. Strong and cold means the delivery is fine and it is a balance or a return problem. Weak or lukewarm means the duct in the attic is the fault.

The free fixes, in the order they work

Check 2

Do these before anybody quotes you anything

Move the thermostat’s opinion, not the thermostat. Most modern thermostats accept a remote sensor that can live in an upstairs bedroom, and some average the two. That alone fixes a lot of houses.

Use the fan on a circulate setting. Running the blower between heating and cooling cycles mixes the floors and flattens the difference. Use the timed circulate option rather than leaving the fan on ON all summer, because a fan running constantly over a wet coil puts humidity back into the house: why the fan setting changes how damp the house feels.

Ease back the good floor, gently. In summer part close some downstairs registers, in winter part close some upstairs ones. You are taking air away from the floor that is already comfortable, and that is the only direction that works.

Open every upstairs door, and the doors of rooms you keep shut. A closed door with a supply vent and no return can only breathe through the gap under it.

Move the furniture off the returns. A sofa against the only return grille in the house is common and costs nothing to fix.

The handle almost nobody knows about

Where the ducts split, there is often a lever on each branch. It is meant to be moved twice a year and on most houses it has never been touched.

Check 3Free, if you have one

Look for a seasonal damper where the ducts split

Where to look. At the furnace or air handler, where the main duct splits into the branch going up and the branch going along. Look for a small lever or a wing nut on the side of the duct.

What it means. The lever is in line with the duct when the damper is open and across it when it is closed. Many of them are labelled summer and winter with an arrow.

How to use it. In summer open the upstairs branch and ease the downstairs one. In winter do the opposite. Move it a little, live with it a week, then move it again.

Mark where it started. A pencil line on the duct before you touch anything, so you can always put it back.

If there is no lever, dampers can be fitted into existing branches, which costs far less than zoning the whole system.

When it needs money spent

Check 4

What is actually worth paying for, and in what order

Seal and insulate the attic ducts first. DOE says sealing and insulating ducts in unconditioned spaces is usually very cost effective, and on a hot upstairs that is where the cooling is being lost.

Then the attic floor itself. Insulation and air sealing over the top floor ceiling helps in both seasons, unlike almost everything else on this list.

Then a return upstairs. Adding a return on the top floor gives the system a way to take the hot air out rather than just pushing cold air in. It is the fix that changes the most and it is rarely offered.

Then dampers, then zoning. A proper zoning system with motorised dampers and two thermostats is the full answer and it is also the expensive one.

A bigger system is not the answer. It cools the downstairs faster, shuts off sooner, and leaves upstairs exactly where it was: why oversizing makes comfort worse, not better.

Questions people ask

Should I just buy a window unit for the bedroom?

It is a legitimate answer and often the cheapest one, especially if the problem is only two rooms and only in July and August. It does nothing for the winter half of the same problem.

Why is it worse at night?

The roof and the attic hold the day’s heat for hours after sunset and give it back through the ceiling. That is also why bedroom doors being open matters more at night than at any other time.

My upstairs is also much more humid.

Same cause. Upstairs gets less of the cooled, dried air, and a system that shuts off early never runs long enough to dry anything out.

Can I fix it with a smart thermostat?

Partly. The useful bit is the remote sensor rather than the app, so check the model takes one and that it can average the floors rather than just switching between them.

Is a ceiling fan worth it?

Upstairs, yes, and it is cheap. It does not cool a room, it moves air over you, which is a different thing and works well in a bedroom: which way it should turn.

Everything is open and it is still six degrees out.

Then it is the return air and the attic ducts, in that order, and it is one of the few cases where spending money genuinely is the answer.

Sources: Lawrence Berkeley National Laboratory "Register Closing Effects on Forced Air Heating System Performance" (I.S. Walker, LBNL 54005, 2003), the U.S. Department of Energy Energy Saver Guide (2017) (ducts in attics, attic insulation, ceiling fans, oversized air conditioners), ENERGY STAR "Duct Sealing", Trane "What Is an HVAC Return Air Vent System?", Bryant's AC sizing guide (short cycling) and Florida Solar Energy Center report FSEC-GP-171-00 (summer attic temperatures)