On a slope a house is rarely one simple box. It steps up or down the hill in levels, often with the garage at street height, the furnace in the lowest space and a bedroom wing several half-flights above it. Warm air rises through the stairwells, the top level overheats, the bottom level stays cold, and the thermostat, usually somewhere in the middle, is satisfied while neither end is comfortable. On the hillside streets of Daly City, around Sign Hill in South San Francisco and on Brisbane’s steep streets this is a familiar complaint. The cause is usually airflow and physics, not a weak furnace.
Four forces that split a hillside house
The stack effect. Warm air is lighter than cool air. In a tall house it rises through open stairs and half-levels, leaks out through gaps near the top and pulls cold outside air in through gaps near the bottom. The greater the height from the lowest floor to the highest ceiling, the stronger that pull.
Long runs from a low furnace. With the furnace at the bottom of the house, the duct to the farthest upper bedroom is long, with more turns and more joints that can leak. The short run to the room right above the furnace gets its air easily; the long one gets what is left.
Nowhere for the air to go back. Many older multi-level homes have a single return grille on one level. Air delivered to a bedroom on another level has to find its way back through stairwells and under doors. Close the door and less air arrives, because the room cannot push out what it is being sent.
Different exposure on each side. The downhill side of the house stands tall, catches the wind and often sits over an open crawlspace or the garage. The uphill side may be partly below ground, cool and steady. A south-facing slope lets the sun do some of the heating; a north-facing one does not.
Read the pattern before changing anything
Place a simple thermometer on each level at about chest height, away from registers, windows and lamps. For three or four days, with the heat on its usual schedule, write down the readings in the morning and the evening. Then, with the system running, hold your hand at each register to judge how strongly it blows. The pattern usually points to the cause:
| Pattern | What it points to | Usual fix |
|---|---|---|
| Top level warm, bottom cold, every register blowing well | Stack effect, no return upstairs | Return upstairs, balancing, zoning |
| One upper room cold, the rest fine | Long or leaky run, crushed duct, closed damper | Duct inspection and repair |
| A room is cold only when its door is shut | No return path | Transfer grille or a short return duct |
| Lower level cold, floors cold | Ducts or floor over a cold garage or crawlspace | Seal and insulate ducts, insulate the floor |
| Windward rooms cold on windy days | Air leaking through the shell | Weatherstripping and sealing first |
| Uneven since a new furnace went in | Blower speed or ducts not matched to it | Airflow measurement, blower setting |
Fixes, from simple to structural
- Balance the dampers. Many branch ducts have a small damper handle near the main trunk. Partly closing the short runs near the furnace sends more air to the far rooms. In heating you throttle the upper level; in a rare warm spell the need flips, so mark both settings on the duct.
- Give every level a way back. A return grille on each level, or at least transfer grilles over bedroom doors, lets air circulate. A return on the top level also pulls the warm air that collects up there back to the furnace, to be sent down again.
- Seal and insulate ducts in unheated spaces. Leaks in a garage or crawlspace waste the heat before it climbs to the far rooms. Duct sealing is often the cheapest real improvement in a multi-level house.
- Rethink the thermostat location. A thermostat at the top of a stairwell reads the warmest air in the house. Moving it, or using a model that averages remote room sensors, lets it see the whole house. See thermostat installation.
- Run the fan to mix the levels. Running the blower continuously at low speed evens out temperatures between floors. On newer variable-speed blowers that costs little in electricity; on older single-speed motors it costs noticeably more.
- Resize the ducts that cannot keep up. A run that is simply too small for its room needs replacing, not balancing. That is ductwork modification work.
- Zone the levels. Motorized dampers and a thermostat on each level let each one call for heat on its own. The design has to account for the moment when only one small zone calls and the furnace still wants to move its full airflow. Zone control covers how that is handled.
- Treat the outlier separately. A converted lower level or a top-floor bedroom that never matches the rest may be better served by its own ductless unit than by forcing the main system to reach it. See mini-split installation.
What not to do
- Close half the registers on the warm level. It raises duct pressure, lowers airflow across the furnace and can make it cycle on its high-limit switch. One or two registers partly closed is fine; more is not.
- Buy a bigger furnace. More heat through the same ducts reaches the same rooms first, so the top gets warmer and the bottom barely changes.
- Ignore the shell. On a windy ridge, air leakage can outweigh anything the ducts do. Sealing the downhill and windward sides often makes the HVAC fixes work better.
A note on homes in the hills here
Hillside homes in Daly City face the ocean wind as well as the slope, and wind-driven leakage adds to the stack effect on the windward side. Many houses on these streets have the furnace in a ground-floor garage below the living space, so every level depends on ducts that start in an unheated room. In Brisbane, older and newer homes sit side by side on steep lots, and the newer ones are not always free of the problem: an open stair between levels moves heat upward in any age of house.
If the top floor also overheats in a warm spell, our pages on a hot upstairs and on uneven cooling cover the summer side of the same problem.