When your heater is running but your house won't warm up, your heating system is either losing hot air before it gets to your house or not warming the air in the first place. Luckily, this issue is usually caused by part or component failures that can be quickly resolved.
Leaking Air Ducts
If your furnace is operating correctly but the air coming from the vents doesn't feel warm, it's possible that most of your hot air is escaping the ducts before it enters your home. This is a common issue with ducts that are old with failing sealant or insulation. Ducts that leak with age can also be exacerbated by the presence of pests, which can further damage your ducts as they move inside them; if you've had recent issues with pests, such as rodents, this may be something to investigate.
In this case, having your ducts repaired or replaced by a professional is the best way to ensure hot air gets into your home the way it should. If your ducts are fifteen years old or more, it will probably be more cost-effective to invest in new ducts; the energy efficiency and heating effects would be more noticeable and cost you less in the coming years. You can also explore high-velocity ducts for even greater efficiency, though the installation cost for these types of ducts tends to be higher.
Clogged Air Filter
An old and dirty air filter will restrict the flow of air into your heater. The effect of this can be twofold. First, with less air to work with, you'll have less hot air coming into your home. Second, if the filter is dirty enough, it may cause the heater to shut off early to avoid overheating itself due to strain, which can lead to your heater shutting down before your home's temperature has reached what you set the thermostat to.
Fixing this problem usually just involves replacing the filter, though if your heater has been running on an old filter for too long, you may want to have the heating unit inspected to make sure nothing is damaged or worn.
Furnace Component or Supply Failure
Sometimes there can be an issue with the heater itself. Check your heater's fuel source to ensure nothing is partly shut off and that everything is flowing fine. If the air coming out of your vents isn't warm, your thermocouple may be dirty or damaged; if this is the case, the heater's pilot light won't stay lit. The burner could also be dirty, which blocks your heater's fuel. All of these issues can happen independently from the fan itself, which means you'll still be getting air, just not hot air.
A heating service professional can take care of these issues for you and replace parts as necessary. Some of these problems can usually be spotted and prevented with annual maintenance; annual maintenance is always worth the investment, especially if you have an older heating system.
For more information on heating services, contact a professional near you.