Heat Is On But the House Never Warms Up

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

Short answer

This is not the same complaint as a furnace that will not run. The air at the vents is hot, the thing runs for hours, and the house still sits below the number. Measure the temperature rise across the furnace first, because that one number says whether you are chasing the machine or chasing the house.

There is a version of a cold house where nothing has failed. The furnace lights, the air at the vents is hot, the blower runs for hours at a time, and the thermostat never reaches the number you set.

That is a race rather than a fault, and the first job is to work out which side of it is losing. One measurement settles it.

Before you start

What to carry: two thermometers, a torch and a new filter. Half an hour.

Gas or electric? It changes what hot means. Carrier puts the air from a gas furnace at roughly 120 to 140 degrees, and it feels hot on your hand. An electric furnace is a little cooler. A heat pump only manages about 85 to 100, Carrier says, and feels cold even when it is working perfectly, so if you have one of those this is the wrong page: what a heat pump should actually deliver.

Check the air is hot before anything else. If it is lukewarm, the problem is at the machine and this page is not it.

One fault hiding as a size problem, and one thing never to do. A furnace that keeps cutting out mid cycle is overheating, not undersized. If it runs for a few minutes, stops, and goes again a few minutes later, it is shutting itself off on its safety limit because it cannot get enough air through. That is a filter and airflow fault: time the burners and you will see it.

Never use the oven or a gas hob to heat a room. It gets suggested every cold snap and it puts combustion gases straight into the kitchen with nothing venting them. A plug in heater for one room, kept clear of anything that can burn, is the safer stopgap. An open flame for hours is not.

Is the furnace doing its job?

Two thermometers and a subtraction. This is the measurement a technician takes first and it costs you nothing.

Check 1

Measure the temperature rise

Let it run for ten minutes. Put one thermometer in the return grille and another in a supply vent close to the furnace, and give each a couple of minutes to settle.

Subtract the return from the supply. That is the temperature rise, and the plate inside the furnace door tells you the band yours should be in, usually about thirty degrees wide.

Inside the band. The furnace is making the heat it was built to make. Nothing on the machine will improve it, and the rest of this page is about the house.

Below the band. Too much air passing over too little flame. On a modern furnace that often means it is running on a low stage, or the gas pressure is low.

Above the band. The air cannot get away fast enough. Filter, closed registers, a crushed duct, or a blower that is not moving what it should: why the air gets weak. A furnace in this state will start shutting itself off before long.

A race it is losing

Heat going in against heat leaking out. When the second one wins, the thermostat can only keep asking.

If the rise is right, the furnace is doing what it says on its plate, and the house is losing heat faster than it is arriving.

That is not a repair, it is an accounting problem, and it has two sides. Either heat is being lost before it reaches the rooms, or the rooms are losing it once it gets there. The next section works through both, cheapest first.

What is stealing it

Check 2

Follow the heat out of the house

Ducts through an unheated space. A loft or a crawlspace is close to outdoor temperature in January, and ductwork running through it with thin or torn insulation gives away a large share of what it carries. A branch that has come off its fitting heats the roof instead of the bedroom.

The chimney damper. An open fireplace with the damper up is a hole in the roof. It pulls warm air out of the house continuously, and it is free to close.

The attic hatch and the recessed lights. Warm air rises and leaves through every gap in the ceiling. A cheap insulated cover on the hatch and a weather strip round it is an hour’s work.

Leaky return ducts. The return side is under suction, so a leak in a cold loft pulls freezing air into the system and the furnace spends its day heating the attic.

Insulation. Get into the loft with a torch and look at the depth, then look for the bits pulled back for wiring or squashed under storage.

Draughts rather than windows. The Department of Energy counts air leaks among the greatest sources of energy loss in a home, and sealing the ones round doors, letterboxes and floorboards is one of the quickest money-saving jobs it names.

The thermostat and the schedule

Check 3

What you are asking it to do

A deep overnight setback in a cold snap. Drop the house ten degrees overnight in the worst week of the year and a correctly sized furnace can take hours to claw it back, which looks exactly like a system that has stopped working. Reduce the setback when it is very cold, or start the recovery earlier.

Where the thermostat is. In a hallway, near a lamp, in a sunny spot or right beside a supply vent, it reads something the rest of the house is not feeling.

The fan set to ON. Between burner cycles the blower keeps pushing room temperature air out of the vents, which feels like a draught and convinces people the heating is failing. Set it to AUTO.

Hold, and lost schedules. A programmable thermostat left on hold at a low temperature, or one that lost its schedule after a power cut, can leave a house mysteriously cold every afternoon.

One room dragging the rest down. If the thermostat’s room is the cold one, everything else is being heated to suit it: why one room is never right.

Questions people ask

It was fine last winter and it is not this year.

Then something has changed, and a house does not lose insulation overnight. Start with the filter and the temperature rise, because a machine making less heat than last year will show up in that number.

It only fails on the very coldest days.

That is normal for a system sized to the usual winter rather than to the record. A few days a year at the edge is a design choice, not a fault.

Should I get a bigger furnace?

It is the most expensive answer and usually the wrong one. Sealing and insulating costs less, lasts longer and makes every future bill smaller, and an oversized furnace brings its own problems.

The air is hot but the rooms stay cold.

Then it is arriving and not staying. Ducts in cold spaces, an open chimney and a leaky ceiling are the three to work through in that order.

Is it cheaper to leave the heating on all day?

Not with a furnace. The Department of Energy says turning the thermostat down 7 to 10 degrees for 8 hours a day can save as much as 10 percent a year on heating and cooling. In a very cold spell a shallower setback is for comfort, because the house takes longer to warm back up.

Only the upstairs is cold.

That is a balance and insulation question rather than a furnace one, and the whole trail starts with whether that floor is getting its share of the air.

Sources: Goodman installation instructions IOG-2009E for the *MSS9 furnaces and the rise ranges in the Goodman *M9S80 and York TM9M manuals (temperature rise), Carrier "Heat Pump Not Heating? Troubleshooting Cold Air Issues" (supply air temperatures), the U.S. Department of Energy Energy Saver Guide (2017) (air sealing, ducts, fireplace dampers, thermostat setback) and CDC's warning against heating a home with a gas oven