You might see your thermostat that says that the heat is on, but no heat comes out. This can happen for many reasons. A broken thermostat, wrong wiring, flipped circuit breakers, cruddy filters, blown fuses, or a defective HVAC system could cause this. So here, we describe some of these causes, and then offer repair hints to straighten them out. First, we look at what might go wrong in the thermostat itself. Then we discuss the power and furnace issues that can also cause the heating not to function.
If the heat problem happened when you installed a new t-stat, you may have goofed in how you wired it. Maybe you’ve reversed the W and Y wires. This would run the cooling stage though the thermostat is actually calling for heat. Other symptoms may appear as well. These depend on which wires and how many of them you attached to the wrong thermostat terminals.
Check for correct wiring at both ends of the thermostat cable. At the thermostat, attach all wires to the right terminals in the wall plate. Then, do the same at the furnace end. Get help from a professional HVAC repairman if you feel insecure matching these wires up with the right terminals.
The solid copper wires in the thermostat cable can break if flexed too often. Or, perhaps the installer stapled the cable with too much pressure or stapler misalignment, and nicked the wire.
This issue normally shows up during new thermostat testing. But sometimes, it might not appear for years. Plus, settling of wall studs can flex these wires that installers often fasten to them. So, after decades, this flexing can wear wire coverings, and cause faulty furnace operation.
Replace the cable between the t-stat and furnace. Avoid tightly stapling the wire, and don’t fasten it more than you must for a snug installation. Lay it loosely.
You can set thermostats way lower than the heating system can deliver. During a cold time outside for example, you might set the heat temperature to 74 degrees. But your furnace moves only enough BTUs to heat to 72 degrees. On particularly cold days, it may feel like there’s no heat output, even though the furnace is operating normally. In this case, the furnace is indeed running. But it’s not producing enough heat to overcome the extreme cold.
If you do these things but still want higher temps in the home, then try the following.
Dirty air filters can reduce airflow though the furnace’s heat exchanger, and cut the heat output to near zero. Clogged filters reduce how much the furnace heats. How so? Dirty filters and fan blades move air less well. Plus, they make more noise besides.
Sometimes, home builders cut corners to cut costs when figuring furnace size. They want the cheapest furnace they can get by with. Sadly though, these cheapo models are often too small to regulate the heating temperature well. So, on very frosty days, you might see the problem show up.
Also, over time with heat pumps especially, the compressors lose efficiency. Why? Because either their valves start leaking. Or the refrigerant leaks out through small holes in the piping. As more refrigerant escapes, the less efficient the heating system becomes. In this case, the unit produces little or no heat even though it’s running.
Dirty heat pump coils, again, slow airflow through the condenser outside, and the evaporator unit inside. Thus, dirt can interfere with the heating system’s efficiency. Dirty coils lowers the BTUs per hour that a heat pump can pump for example. That could mean that the system cannot keep your quarters as warm as your thermostat setting calls for. Clogged coils may cause overheating and trigger a furnace shutdown as well, even though the thermostat is calling for more heat.
It’s common for furnaces to fail to keep up when the weather outside is very chilly. They might run constantly yet still fail to keep up. Now there may be nothing wrong with the thermostat per se. And, there might be nothing the matter with the furnace either. The issue might be that your central heating system is too small to overcome the entering cold from outside. Again, a very cold day can make it feel inside like the furnace is not running at all.
You may be getting SOME heating from your furnace, but not enough to hold room temperature at the desired value.
If your house lacks enough insulation, this worsens the furnace not heating issue. Indeed, much warmth leaves through cracks around windows, walls, and doors. Thus, the furnace works harder to bring the house up to the set temperature. Indeed, there may be many BTUs leaking in in this way. So many BTUs, that the central heating system cannot backfill. The furnace cannot thus, maintain the set temperature. So, your thermostat never reaches that set heating temperature.
As should be clear, the problem of thermostat says heat on but no heat in homes has many causes. Plus, most of them lie beyond the thermostat. A poorly insulated home, faulty wiring, a furnace that’s too small, leaky windows and doors also contribute. All of these make the HVAC system work too hard to heat your home. In many homes there’s little coolness to spare due to poor HVAC sizing. Sadly, the fix for this often costs a lot. Why? Because it means replacing windows and doors, the furnace itself, or adding more insulation. Or you may need to add more supplemental heaters to your home.
But we hope that now, you know what might trigger this issue. Knowing the causes often helps in choosing the right fixes. Even though the best fix may cost the most, at least now, you have the data in front of you.
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