Why Is My Furnace Not Blowing Hard Enough?

Most people are so familiar with their furnaces that they can tell when the appliance is slacking. Maybe you can usually feel the warmth within two minutes of switching on the furnace. In this case, you’ll rightly become worried when it’s been five minutes, and you still can’t feel the warmth.

Alternatively, perhaps the hot just doesn’t feel strong enough. You can usually hear someone asking, “did someone turn off the heater?” when this happens. Whichever the case, you must never assume weak heating as it may be a sign of a critical underlying issue.

Read on to find out why the furnace may slack and what you can do to remedy the situation.  

8 Reasons Your Furnace Isn’t Blowing Hard Enough

The following are the eight most common reasons why your furnace may not be blowing as hard as it should be;

1. Dirty Air Filters 

You’ll come across “dirty air filters” a lot when troubleshooting furnaces, and for a good reason. Dirty filters put a lot of strain on the furnace, typically causing the furnace to work harder and consequently increasing your heating bills. Unfortunately, the same dirty filters can also cause a weak airflow at the supply vents.

The reason is that blocked air filters prevent air intake at the return vents. As a result, you cannot expect much airflow at the supply vent.

Solution: Regular maintenance easily fixes this problem. All you have to do is clean the filter as the manufacturer recommends. More importantly, make sure to replace the filter at least every two weeks or more frequently as the manufacturer recommends.

2. Overly Efficient Filters 

You may have realized that furnace filters are rated in MERV – short for Minimum Efficiency Reporting Value. MERV ratings range from 1-16. The higher the rating, the tinier the pores through the filter.

At 16 MERV, the filter is so dense that only minor traces of airborne elements can pass through. While the denser (thus more efficient) filter may seem better, it also comes with one major downside – very high air resistance. This can be a huge issue if your indoor air quality is poor.

Very little air will pass through the filter into the furnace. This usually translates to low heat output.

Solution: Always go with the manufacture’s recommendations. Don’t install a 14 MERV filter if the owner’s manual says you need a 10 MERV filter. More importantly, always replace filters with the exact model as the old one.

3. Poorly Installed or Ineffective Ducts Runs 

If you’ve heard HVAC professionals warning users about the dangers of running or troubleshooting the ductwork yourself, this is one of them (besides safety concerns).

First off, the ductwork must be installed properly for maximum airflow. If the configuration is inefficient, your furnace won’t heat as fast as you’d wish.

You’ll also get slower heating if the duct runs are longer than necessary. Additionally, any problems that block or slow down the flow of warm air through the ductwork can cause slower heating.

For instance, bends, blockages, or disconnections along the duct run can impede airflow, causing poor heating.

Solution: The only solution here is to ensure professional installation and repair. Otherwise, you risk ending up with an inefficient duct run and ultimately poor heating.

4. Leaky Ducts 

Besides installation and repairs, you must also keep an eye out for duct leaks. According to the Department of Energy, over 90% of homes in the US have leaky ducts that cause expensive heat loss, usually resulting in inefficient heating and high energy bills. Specifically, the DOE estimates that the average home loses 35%-50% of warm air through leaky ducts.

When you lose war air through leaky ducts, you certainly won’t feel as warm as you’d like, no matter how hard the furnace works.

Solution: Scheduled maintenance can help avoid this problem. The HVAC technician will certainly uncover and fix the leaks during the tune-up. However, remember that you also have a responsibility to take better care of the furnace.

5. You don’t have Enough Return Air Vents

A typical forced air system is a closed-loop system. In ideal conditions, it takes in a specified volume of air, heats it, and blows it all out of the furnace into your home. Then it pulls all the air back into the furnace and reheats it.

In short, every area in the home that receives warm air must return the same amount of air into the system. Unfortunately, this is impossible if you don’t have enough return air ducts. As a result, the area can become pressurized, and the amount of warm air received at the other end is limited.

Solution: Professional installation from the start is critical if you’re to avoid such issues. However, you can also bring in a professional post-installation to check if you have enough ducts. If not, the professional can help you redesign the heating system for maximum output.

6. Your Vents are Undersized or Ducts Oversized

Have you ever wondered why they can’t just have large pipes delivering heat to your home, why all duct pipes are tiny? The reason is static pressure. The warm air must be kept under pressure.

Otherwise, the pressure will drop, and the air may not even reach the other end of the ductwork. On the other hand, your vents must be large enough to allow enough air into the ductwork and into the furnace. If the vents are undersized, don’t expect to receive much warm air at the supply vents.

Solution: You can hire an HVAC professional to check whether you have the right size ducts and air vents. If not, you may have to redesign your duct system to incorporate the right size vents and ducts.

7. The Vents are Dirty or Blocked 

If the vents are of the right size, you may want to check if they are blocked or dirty. Remember that the furnace blower fan pulls cold air into the furnace at speed. Therefore, any large debris in the air is usually held back at the vents. Thus, you might find twigs, leaves, hair, etc., blocking the vent.

The vent can also become partially blocked due to the accumulation of dust. At the other end, supply vents can become blocked if there’s furniture or other items in the way. Additionally, the vents can also accumulate dirt other particles from the furnace.

Solution: You will never experience dirty or blocked vents with proper and regular maintenance. So, never go a whole week without wiping down the furnace. During the cleaning, take off the vents and vacuum them to remove all traces of dirt.

8. Blower Motor Malfunction 

The furnace primarily relies on the internal blower fan to maintain air circulation within the furnace and throughout the home.

The blower draws cold into the furnace and forces warm out through the supply vents. It’s also responsible for creating the motion that enables even circulation of air throughout the home.

Now, imagine if the blower fan is broken or compromised. Say it’s not dead, but one of the blades is broken. Or perhaps the motor is so old it can only turn at half the usual speed.

You can’t expect such a fan to continue delivering at previous speeds.

Solution: Always keep an ear out for signs of blower fan malfunction. If you hear a screeching or wheezing sound from your furnace, you probably have a broken or struggling fan. You can have the fan repaired or replaced for under $100.


Something is wrong if you notice that the furnace isn’t putting out as much heat as before. Maybe the furnace isn’t getting enough airflow due to blocked or dirty ducts, vents, or grilles. Or maybe the ducts are not connected properly or are leaking.

If not, the blower motor is defective or aging, or the furnace itself is worn out and not as powerful as before. Again, feel free to troubleshoot the issue personally before you involve an HVAC professional.