What Constant Running Actually Means
Set to 70. Sitting at 64. The furnace has been going for two hours and it's not gaining ground.
The instinct is that the furnace is too small or dying. Usually it's neither. A furnace that runs and runs is generally making heat just fine. The heat is going somewhere other than your living room, or the thermostat is measuring something other than your living room.
That distinction matters because of what happens next. "Your furnace is undersized" leads to a replacement quote. "Your return duct is disconnected in the crawlspace" leads to a couple hundred dollars of duct work and a house that finally holds temperature. Both diagnoses fit the same symptom, and only one of them is right for your house.
Also worth saying up front. On the coldest handful of nights each winter, a correctly sized furnace running almost continuously is normal and expected. Equipment is sized for a design temperature, not for the worst night in a decade. If this only happens when it's 5 degrees out, that's normal for a correctly sized system.
Start With the Cheap Stuff
Work these in order. Three of them are free.
The filter. A clogged filter chokes airflow. Less air across the heat exchanger means less heat delivered, and it can also trip the high limit switch, which cuts the burners while the blower keeps running. Pull it out and hold it to a light. If light doesn't pass through, replace it and give it a day.
Blocked or closed registers. Walk the house. Furniture pushed over a floor vent, a rug covering one, closet doors shut on a supply. People also close registers in unused rooms thinking it saves money. It mostly doesn't, and it raises system pressure in a way that hurts more than it helps. Open them.
Return air. Just as important and almost never checked. If your returns are blocked by a couch or the door to a room with a return is closed, the system can't move air. Look for the big grilles, usually in a hallway, and make sure nothing's in front of them.
Where the thermostat is. A thermostat on an exterior wall, in a drafty hallway, above a lamp, or in direct sun reads a temperature nobody in the house is experiencing. If it's reading 64 while your living room is comfortable, the problem is the location, not the furnace. Set a separate thermometer in the room you actually care about and compare for a day.
The fan setting. If the thermostat fan is set to ON rather than AUTO, the blower runs constantly between heating cycles and the air feels cold even when the furnace is working correctly. Easy to confuse with a heat problem, and we walked through it in furnace blowing cold air.
Where the Heat Is Going Instead
If the cheap list didn't do it, this is where the answer usually lives.
Your ductwork runs through spaces you don't heat. Crawlspaces, attics, unconditioned basements, wall cavities. When a duct joint separates or a run develops holes, you're paying to heat those spaces instead of your house. Disconnected returns are worse still, because the system starts pulling cold crawlspace air into the supply and delivering it to your rooms.
This is common in older east-side houses, especially where additions were built onto original structures and the ductwork was extended by whoever was cheapest at the time. We covered the full picture in what duct leakage is costing you, and the losses are larger than most people expect.
Signs pointing this direction: rooms that were always cold even when the system was new, a basement or crawlspace that's oddly warm, dust that never stops, and heating bills climbing year over year with no change in habits.
Undersized or crushed ductwork causes the same symptom in a different way. If somebody put in a bigger furnace years ago without touching the ducts, the ducts are now the bottleneck. The furnace makes plenty of heat and can't deliver it.
The House Itself
Sometimes the equipment and the ducts are both fine and the house is the problem.
Attic insulation settles and compresses over decades. A house that was well insulated in 1978 may have half of what it needs now. Heat rises, and the attic is where most of it leaves.
Air sealing matters as much as insulation and gets less attention. Gaps around rim joists, plumbing and wiring penetrations, chimney chases, and old recessed lights let heated air out and cold air in continuously. On a windy January night this gets dramatically worse, which is why some houses feel fine in mild cold and terrible when it's blowing.
Windows get blamed for this constantly and are usually not the biggest offender. Replacing windows is expensive and the payback is long. Air sealing an attic is cheap and the payback is fast. If somebody's leading with windows as the fix for a heating complaint, be skeptical.
If your house has an addition or a finished attic or basement that was never properly tied into the system, that space can drag the whole house down. That's a different fix, sometimes a dedicated zone or a ductless head rather than a bigger furnace.
When It Really Is Undersized
It does happen. Here's what makes it credible rather than a sales line.
A real undersizing conclusion comes from a load calculation. Somebody measures the house, accounts for square footage, ceiling heights, insulation levels, window area and type, orientation, and air leakage, and produces a required BTU number. Then that gets compared to what's installed.
What isn't a load calculation: "a house this size needs 100,000 BTU." Square footage rules of thumb are how houses get oversized in the first place, and oversized equipment causes its own set of problems, mostly short cycling, which we covered in what causes short cycling.
Undersizing is genuinely more likely if the house was added onto after the system went in, if the original equipment was replaced with something smaller to hit a price, or if the house was converted from another fuel type at some point. Those are real scenarios and they show up in older neighborhoods.
But if a company reaches undersized without checking your filter, walking your ducts, or running numbers, they didn't diagnose anything. Ask to see the calculation. A company that has one will show you.
The Order to Spend Money In
Cheapest and most likely first, every time.
- Filter, registers, returns, thermostat location. Free or nearly free.
- Duct inspection and sealing. A few hundred dollars, and it's the most common real cause.
- Attic insulation and air sealing. Moderate, with the fastest comfort payback of anything on this list.
- Zoning or a supplemental unit for one problem area.
- Equipment replacement, with a load calculation backing it.
Most of these calls end at step two or three. Very few of them should end at step five, and the ones that do should have paperwork behind it.
Our Repairs First approach means we work that list in that order, and we'll tell you when the answer isn't equipment at all. Our technicians are salaried and earn no commission, so nobody here gets paid more for skipping to the replacement quote.
If your furnace has been running all winter without catching up, we'll come find out where the heat is going. We work across Anderson Township, Mount Washington, Newtown, Mariemont, Madeira, Loveland, Milford, and Amelia. Call (513) 951-8539 or get in touch here.