It can be really annoying if your furnace keeps blowing warm air instead of hot air. If this is happening to you, there are a few causes and solutions to look into.
Your furnace could be blowing warm air because the thermostat is on the wrong setting. Your system could also have a malfunctioning part. Or, the warm air could be perfectly normal if you have a certain type of heating system.
In short, there are lots of reasons why your furnace might blow warm air instead of hot air. Keep reading to get a comprehensive overview of the top solutions for fixing this issue.
What can make a furnace blow warm air instead of hot?
Your furnace could be blowing warm air for each of the reasons covered in this section.
Your thermostat is on the wrong setting
People will often accidentally change their thermostat settings without realizing they’ve done so. If you’ve done this, then that could be the main reason why you’re getting warm air from your furnace.
To make sure this isn’t your problem, do the following:
- Make sure your furnace is on heating mode
- Set the fan to AUTO, not ON
If you’re still having a problem after doing these two things, then this isn’t what’s causing the warm air.
It’s also possible that your thermostat’s battery is low. Some furnaces rely on a self-contained power unit. When the battery on these goes out, it may impact the furnace’s settings. Ideally, you should change the batteries every year.
Your furnace is still getting warmed up
It’s also possible that your furnace system just takes some time to warm up. That’s because the warm air it creates has to travel through cold ductwork when it first gets going.
This is likely your problem if the warm air turns hot within a few hours of you turning on your furnace.
There was a problem with your installation
This issue could also be coming from a mistake that was made during the thermostat or furnace installation process. This problem is most common for DIY thermostat installations.
Modern thermostats can be tricky to install properly with their smart features. It’s possible that you made some mistake that’s causing it to call for warm air when you want hot.
There are two ways to fix this problem. You can either re-read the installation manual and attempt the install again. Or you can call out a professional to come and take a look at it.
You have a heat pump
Your problem could also be coming from the type of heating system that you have.
For example, a heat pump produces air around 85-115 degrees Fahrenheit. But a furnace produces air as high as 140-170 degrees Fahrenheit.
So if you recently swapped your furnace for a heat pump, this could be what’s happening. There isn’t any fix for this. It’s just the nature of heat pumps to produce slightly cooler air than furnaces.
You have a two-stage furnace
Two-stage furnaces are designed to be more efficient. They achieve that by staying on for longer amounts of time, so they’re turning off and on less often.
If you have a two-stage furnace, it goes into a lower stage of heating from time to time. This could be the warm air that you’re feeling.
This is totally normal and will happen automatically. But you should simply be able to turn up the thermostat to start getting hotter air whenever you want.
You’re missing an obvious solution
It’s also worth double-checking just to make sure you aren’t missing an obvious fix to the problem. For example, you can check if:
- Your oil-powered furnace has oil in the tank
- Your electric furnace is switched on at the breaker panel
- Your gas-powered furnace has an open gas supply valve
Your vents aren’t opened properly
It’s worth remembering, the warm air that you’re feeling is coming from a vent. So it’s possible that the problem lies in the vent, rather than the furnace. More specifically, your vents might not be opened all the way.
You can fix this problem by checking your vents and registers and making sure that they’re as open as possible. That way, you can narrow down the source of your problem by eliminating any potential venting issues.
Your filters are dirty or need replacing
Sometimes, a dirty or poorly installed air filter can cause the hot air that’s coming from your furnace to feel warm. Here are some ways to troubleshoot that problem:
- See if your filters are dirty and clean or replace them if they are
- Make sure you have the right air filter for your HVAC system
- Verify that your air filter is pointed the right way
Other reasons why your furnace might be blowing warm air instead of hot
The problems covered above are some of the first you should consider if you’re getting warm air from your furnace. But maybe you’ve already gone through that list, and you’re still having the issue.
If so, here are five more potential causes for the warm air that your system is giving you.
There’s an issue with your thermocouple
Your furnace’s thermocouple is what regulates the pilot light flame. When it isn’t working properly, the pilot light may come on and off at random. This can give you the sensation of warm air coming from your system.
The solution to this is to watch your furnace to see if it ignites properly. If it doesn’t, then there’s a good chance that you need to replace or repair your thermocouple.
Your air ducts are leaking
Many people don’t realize this, but your air ducts can actually leak. This can cause them to be less efficient at spreading hot air through your home. And that may give you the sensation that they’re only spreading warm air.
Energy Star says that virtually every home leaks about 20-30% of the air in their ducts. So there’s a good chance you’re experiencing this problem to at least some degree.
The solution is to apply duct sealing to your air ducts. Doing this will help to bring down the amount of air that leaks from them.
There’s a problem with your condensate line
Your condensate line is what removes water from the condensing unit, which builds up during hot weather. This line can get clogged by dirt and grime.
When that happens, your system may start to shut down. It does this because it can’t remove the water that it needs to remove from its components. This could cause your furnace to start blowing warm or cold air when you want hot air.
The solution to this problem is taking better care of your condensate line. Make sure to clean yours regularly to avoid this.
Your gas valves aren’t working right
It’s also possible that your gas valves aren’t working the right way. These can sometimes shut down when they accumulate too much dirt. And that could cause your furnace to only blow out warm or cold air when you ask for hot.
You may be able to fix this problem by cleaning the gas valve. But if you haven’t replaced yours in a while, it could be worn down.
In that case, you would want to replace the valve entirely. Otherwise, you’ll likely continue having this issue until you do.
Your high limit switch has a problem
HVAC systems have a built-in safety feature called a high limit switch. This automatically turns on a fan when the system starts overheating. It does this so that the system doesn’t destroy itself completely.
But sometimes, your high limit switch will think that your HVAC system is overheating even when it isn’t. When that happens, the automatic fan may cause the hot air you want to feel warm.
You may be able to fix this problem by replacing your high limit switch. Calling a professional out to take a look at your furnace is likely going to be your best bet.
How do I know if my furnace is going out?
Sometimes, warm air coming out of your furnace instead of hot air is a sign that the furnace is going out. To confirm whether or not that’s happening to you, there are a few things to look for.
This is a list that includes:
- Your furnace is over 15 years old
- There are constantly strange noises coming from your furnace
- People in your family are complaining about health issues that seem to get worse when the heater is on
- You constantly have a dry or dusty house
- Your thermostat never keeps you comfortable
If you’re experiencing one or more of these issues, there’s a good chance your furnace is going out. Replacing it could be the only way to fix the warm air problem that you’ve been having.
Is it possible to run a furnace without a filter?
It’s not possible to run a furnace without an air filter. That’s because you need the filter to keep harmful dust, dirt, and debris from being spread throughout your home. It could be really bad for your respiratory health if you try to run a furnace without a filter.