The furnace is running. You can hear it. The house is not getting warm, and the thermostat is still asking for heat it is never going to get.
That combination almost always means the heat is being made and not delivered. A furnace that produces no heat at all stops and shows a fault. A furnace that runs continuously is working; something between it and your rooms is the constraint.
Which is good news, because the constraint is usually cheap and sometimes nothing at all.
What Should You Check Before Anything Else?
The filter, and check it by looking rather than by remembering when you changed it.
Slide it out. Hold it up to a light. If you cannot see light through it, the furnace is trying to breathe through a blanket.
This is the most common cause of exactly this symptom and it is the one people dismiss, because a filter does not look like a heating component. A blocked filter starves the blower, so less air crosses the heat exchanger, so less warm air reaches the rooms while the burner runs continuously trying to satisfy a thermostat it cannot reach.
There is no useful universal interval for changing them, because a thin filter in a dusty house with two dogs is a different thing from a thick pleated one in a tight house. Looking at it takes ten seconds and settles it.
Why Would Closing Vents in Unused Rooms Make It Worse?
Because a ducted system is a closed circuit, and closing part of it does not redirect the air so much as resist it.
The instinct is sensible and it is wrong on most systems. Shutting registers in two spare bedrooms raises the pressure in the ductwork, the blower moves less air overall, and the rooms you were trying to help get less rather than more.
It also changes how much air crosses the heat exchanger, which is the thing the appliance was designed around.
So the first thing to undo, if somebody has done it, is the vent closing. Open everything, run it for a day, and see what changes. It costs nothing and it is frequently the answer.
The same goes for furniture over a floor register and a rug over a return.
Is It the Ductwork Rather Than the Furnace?
Often, and in the houses around here that is worth taking seriously.
A great many forced air systems in this area were put into buildings that were never designed for ducts, which means runs threaded through closets, along cellar ceilings and up through chases that were available rather than ideal.
Three things follow. Long runs lose heat along the way, especially through an unheated cellar or attic. Undersized or crushed sections throttle the whole system. And a disconnected joint in a cellar quietly heats the cellar instead of the house.
The tell for the last one is a cellar that is noticeably warm while the house is not, which people often read as the furnace being generous.
If a whole floor rather than a room is cold, that is its own problem, and the top floor case is covered separately because it has causes a filter will not explain.
What If It Starts and Stops Every Few Minutes Instead?
That is a different fault with a different meaning, and it is worth separating.
Running constantly points at delivery. Starting and stopping repeatedly, which is called short cycling, points at the furnace shutting itself down and trying again.
The common reasons are overheating because airflow is restricted, which brings you back to the filter and the registers, a thermostat that is reading badly or sited in a draft, or a safety control doing its job.
Short cycling is the one to call about rather than investigate, because the appliance is telling you it has stopped itself for a reason. A furnace that cuts out on a safety control and restarts is protecting something.
Could the Thermostat Be the Whole Problem?
Sometimes, and it is quick to rule in or out.
A thermostat in direct sun, above a lamp, on an exterior wall or in a hallway with a draft is reading a microclimate rather than the house. It will either satisfy early while the rooms stay cold, or ask forever while its own spot stays cool.
The other version is a thermostat that is simply set to run the fan continuously, which makes a working system sound like a struggling one. Worth checking the fan setting before anything else, because it explains the noise without explaining any cold.
A thermostat in a different unit from the cold rooms is a bigger problem than a thermostat in the wrong corner, and in a two or three family it is the usual reason one floor never catches up.
When Is It Worth Calling Rather Than Checking?
Four cases, and three of them are not about comfort.
Any smell of burning that does not clear in the first minutes of the season, any sooting around the appliance, or any sign of scorching.
Short cycling, as above.
A furnace that runs constantly after the filter is clean, every register is open and the thermostat is in a sensible place. At that point you have eliminated the answers that cost nothing and what remains is the system.
And anything at all that makes you wonder about combustion. The right response to that question is a working carbon monoxide detector in the house and a technician checking the appliance, in that order, rather than reading about it.
Our furnaces page covers the repair side, the heating page covers the rest of what we do, the maintenance page covers the annual check where most of this is caught before February, and the no heat page is written for the call where nothing is working at all. If the house is a rental, the requirement is written per room and runs through the season.
Questions We Hear
Is It Cheaper to Leave It Running Than to Cycle It?
The question comes up every winter and it is the wrong lever to pull, because a system running constantly without reaching the thermostat is not a setting, it is a symptom. A correctly working furnace cycles. If yours no longer does, the saving to chase is in fixing why, not in choosing between two bad patterns.
The Air Coming Out Is Warm but Not Hot. Is That Normal?
Air from a working forced air system is warm rather than hot, and noticeably cooler than a radiator, so this alone is not a fault. What matters is the volume: plenty of warm air heats a room, and a trickle of hotter air does not. If the air feels weak at every register, you are back to the filter and the ductwork.
It Was Fine Last Winter
That is usually the filter or something that changed in the house rather than the appliance, and it is worth thinking back over the year. A filter upgraded to a denser grade, a room closed off, furniture moved over a return, or work done in the cellar near a duct run all produce exactly this and none of them feels like a heating change at the time.
How Long Should It Take to Warm the House Up?
Longer than most people expect from cold, and the useful measure is whether it gets there at all rather than how fast. A system that raises the house steadily and then cycles is working. One that climbs a couple of degrees and then stalls for hours is the one described in this article.
Can I Put a Thicker Filter In for Better Air?
It is a reasonable thing to want and it is not a change without a tradeoff, because a denser filter resists airflow more and some systems do not have the capacity to spare. If you want better filtration it is worth matching it to what the system can move rather than fitting the highest grade on the shelf, and that is a question worth asking with the appliance in front of us.
Something in Your Own Building Not Right?
Tell us what it is doing and we will tell you what it usually means. Call (617) 549-5734 or send it through the shop.
