Line chart: modeled indoor temperature of a 1970s, 1990s and newer house overnight with the heat off on a 32°F Dulles night, a test of when to turn on the heat

Trying to Hold Off on Turning On the Heat? We Tested How Cold Your House Gets Overnight

It’s the first chilly week of October. The house feels a little cool at bedtime, the thermostat still says “off,” and you’re wondering whether you can make it through one more night before you turn on the heat for the season. How cold will it really get by morning?

We tested it, virtually. This is a computer simulation, not a measurement in a real home: we built an hour-by-hour heat model of three typical Northern Virginia houses and ran each one through three real fall nights recorded at Washington Dulles International Airport. Here’s what happened with the heat off from 10 PM to 7 AM.

How we ran the overnight test

Each house is a two-story, 2,000 sq ft detached home with a 1,000 sq ft footprint, 8-foot ceilings and windows on about 15% of its walls. Only the construction changes between the three houses:

The insulation and window values are the most common options for Virginia homes of each era in NREL’s ResStock housing database. The air-leakage values come from the same database for the older houses and from Virginia’s current code limit for the newer one. ACH50 is a blower-door number: how many times the house’s air would leak out in an hour under a strong test pressure. Lower means tighter.

The model tracks two things hour by hour: how fast heat escapes through walls, attic, windows and leaks, and how much heat is stored in the air, drywall, framing and furniture. Air leakage changes with the indoor-outdoor temperature difference and the real wind speed at Dulles that night. We counted 1,500 Btu per hour of heat from the fridge, electronics and sleeping people, and no sunshine, since it’s dark. The full list of assumptions is in the sources section at the end.

Results: how cold each house got with the heat off

All nine runs start at 70°F at 10 PM. “Not reached” means the house never got that cold by 7 AM.

On the coldest night the gap between houses is easy to see. The 1970s house dropped below 65°F about 12:20 AM and below 60°F just after 4 AM. The 1990s house crossed 60°F around 6 AM. The newer house never got below 62°F. In all three, the first hour was the steepest, because the warm air cools quickly before the slower-to-cool walls, floors and furniture start giving back their stored heat.

The second chart shows how much the weather matters. On the mild night, every house lost less than 4°F, and the newer house barely lost 2°F. On the cool night, the drop roughly doubled. On the 32°F night, the 1970s house lost 13.5°F, nearly twice as much as the newer house.

Why some houses cool faster than others

If you’re trying to decide when to turn on the heat, it helps to know what drives the overnight drop. There are two big factors.

A house loses heat in proportion to the difference between inside and outside. A 54°F night pulls heat out slowly; a 32°F night pulls about two and a half times as hard. That’s why the drop isn’t a fixed number of degrees per hour. It depends on the night.

The age of the house decides how easily that heat gets out. In our model, the biggest single difference between the 1970s and newer houses is air leakage: the older house trades its warm air for cold outdoor air about four times as fast. Better windows and more attic insulation add to the gap.

What this means for your home

  • Mild nights (lows in the 50s): most houses lose only a few degrees overnight. Whether to turn on the heat is a comfort choice, not a problem.
  • Cool nights (lows in the 40s): expect to wake up somewhere in the low-to-mid 60s. Many people decide to turn on the heat here, often on a low setting.
  • The first night in the low 30s: an older house can fall into the 50s by morning. This is the point where we’d turn on the heat, even if it’s set low.
  • Pipes: none of these fall nights came close to freezing indoors, but pipes in crawl spaces, garages or exterior walls are much colder than the living room. Don’t use the living-room temperature as a pipe-safety reading.

In Northern Virginia, that first freezing night comes earlier than many people expect. Over the last 10 years, Dulles recorded its first fall low of 32°F or colder between October 21 and November 10, so late October is when the question of when to turn on the heat gets real.

When your house could cool faster or slower

  • Wind: two of our three nights were calm. On a windy night, a leaky house loses heat noticeably faster.
  • Size and shape: a townhouse with heated neighbors on both sides cools much more slowly. A rambler with a big attic or a house over a vented crawl space can cool faster.
  • Basements and slabs: we used one simple floor-loss value. A finished basement can hold the house warmer; an uninsulated crawl space can do the opposite.
  • Sun and windows: overnight there’s no sun, but a house that soaked up afternoon sun starts the night with more stored heat than our 70°F starting point.
  • Renovations: a 1970s house with new windows, attic insulation and air sealing can behave more like our 1990s or newer house.

Before you turn on the heat for the first time

The first time you turn on the heat each fall, the furnace has been sitting idle for months. A burning-dust smell for the first few minutes is common. A smell that doesn’t fade, a burner that clicks but won’t light, or a blower that runs cold air are signs to have it checked. Our furnace repair and heat pump repair pages cover the most common first-start problems, and a $99 heating tune-up before the first cold night catches most of them early.

Make sure your carbon monoxide alarms work before you turn on the heat with a gas furnace. Test them and replace batteries if needed.

Heating repair in Northern Virginia

The first cold nights of October are when we see the most furnaces that won’t start after a long summer off. Woodland handles heating repair across Prince William, Fairfax, Loudoun and Fauquier counties, plus Arlington and Alexandria. We’re based in Woodbridge, so Woodbridge, Manassas, Dumfries and Lorton are usually same-day calls, and we’re in Fairfax, Centreville, Springfield, Arlington and Alexandria every week. See our heating repair page or call (703) 335-1009.

Sources and methodology

Methodology: we used a simplified hourly heat-balance model rather than a full EnergyPlus simulation, so we could run real Dulles weather through the same house quickly and change one thing at a time (home age or weather).

Assumptions: 2,000 sq ft two-story detached house, 1,000 sq ft footprint, 15% window-to-wall ratio, 8-ft ceilings; foundation losses to 50°F ground; 1,500 Btu/h of internal heat; no solar gain; heat stored in air and furnishings (1 Btu/°F per sq ft) and in slower building mass (6 Btu/°F per sq ft).

Sanity check: the model’s design heat loss at 14°F outdoors is about 54,000 Btu/h for the 1970s house, 37,000 for the 1990s house and 24,000 for the newer house, in the normal range for a house this size. These are modeled estimates, not measurements of real homes.