Heating load is driven, more than anything else, by the temperature difference the house has to maintain across its envelope.
The design temperature
The 99% winter design dry-bulb is the outdoor temperature that local conditions stay above 99% of the time in winter. It is the condition a heating system is sized to meet.
The load is proportional to the difference between it and the indoor setpoint, conventionally 70°F.
| City | 99% design | ΔT to 70°F |
|---|---|---|
| Miami, FL | 47°F | 23 |
| Atlanta, GA | 23°F | 47 |
| St Louis, MO | 6°F | 64 |
| Chicago, IL | −2°F | 72 |
| Minneapolis, MN | −11°F | 81 |
A house in Minneapolis faces more than three times the temperature difference of the same house in Miami, and its heating load reflects that almost proportionally.
Why not the record low
A furnace sized for the coldest temperature ever recorded would be enormously oversized for every other hour of the winter, with the short cycling and temperature swings that follow.
The 99% point is the deliberate trade: on the handful of hours a decade that are colder, the furnace runs continuously and the house may drift a degree or two, which is exactly what it should do.
The rule of thumb by zone
As a starting point, in BTU per hour of output per square foot:
| Climate | BTU/h output per sq ft |
|---|---|
| Hot (Gulf, Southwest) | 25–30 |
| Mixed (mid-Atlantic, Southeast) | 30–40 |
| Cold (Midwest, Northeast) | 40–50 |
| Very cold (Upper Midwest, Northern Plains) | 50–60 |
Remember that furnaces are sold by input. Divide the output requirement by the AFUE to get the input size to shop for.
What the rule does not know
Envelope quality, which matters more for heating than for cooling because there is no solar gain to complicate it. A modern well-sealed house can need half what a 1950s house of the same size needs in the same city.
Foundation type. A conditioned basement, an unconditioned crawl and a slab behave differently.
Duct location. Ducts in an unheated crawl or attic lose heat continuously, and that loss is proportionally larger in a cold climate.
Wind exposure, which drives infiltration and is not captured by temperature alone.
Elevation and altitude derating
Furnaces require derating above roughly 2,000 feet, because thinner air supports less combustion. Manufacturers publish the adjustment.
That means a house in Denver needs a larger nominal input to deliver the same output, and a furnace installed at altitude without the correct derate will overfire.
The heat pump complication
Where a heat pump is involved, the picture changes: capacity falls as temperature falls, so a cold-climate design has to consider the balance point and the backup heat rather than a single output figure.
In a mild climate a heat pump covers the whole heating load easily, which is why the fuel decision and the climate zone are connected.
What to do with these numbers
Use them as a sanity check on a quote, not as a specification.
If a contractor proposes a 120,000 BTU input furnace for a 1,600 square foot house in a mixed climate, this arithmetic says the number needs explaining — and asking for that explanation is usually enough to produce a load calculation.
The cooling side does not follow the same map
A house with both a furnace and an air conditioner has two loads that peak in opposite seasons and do not scale together.
Minneapolis has an enormous heating load and a modest cooling one; Houston has the reverse. That is why the equipment sizes for the two functions are calculated separately, and why a blower that suits one may be wrong for the other.
Where the two are far apart, the airflow requirement of the cooling coil often governs the blower selection rather than the furnace's temperature rise, and that is a detail worth confirming on a quote.