Why Does My Heat Pump Make Loud Noises When It Shuts Off?

Argh, the noises again! Why does the heat pump always make a slamming sound when shutting off? Will it ever stop?

If you’ve had a noisy heat pump, you’ve said something similar before. It can be an annoying experience because the noises are sometimes so loud even the neighbor complains. It may also be particularly annoying if it happens in the middle of the night, waking you from sleep.

So, why the sounds? Has your heat pump malfunctioned, or is it just the fan? Also, is there anything you can do about it? Let’s find out.

NB: Heat pump noises at shutdown can either come from inside or outside the house. So, before you begin seeking solutions, make sure you’ve determined whether it’s from the air handler or compressor unit.

5 Reasons Why your Heat Pump Makes Loud Noise When Stopping

When the Noise is Coming from Inside Your House

Indoor heat pump noises are even more annoying because they are closer to you. So, what causes them, and what can you do?

1. The air filter is slamming against the grille

Often, when you hear a loud slamming sound when your heat pump is shutting off, it’s the filter slamming against the grille. This is especially true for ceiling-installed heat pumps.

Ceiling heat pumps are designed such that the grille sits flush with the ceiling, and the rest of the heat pump is hidden inside the attic. When the heat pump is running, the vents located just behind the grille suck in air into the heat pump for heating. However, the air always first passes through a filter to remove dirt particles that may compromise the internal components of the heat pump.

Under certain conditions, the filter can be sucked up against the top side of the duct area. Thus, when the heat pump turns off and the air moves into the unit stop, the filter drops to the grille, making a slamming or thumping sound.

The filter can be sucked to the top of the duct for two reasons;

  • The air filter is not the right size

Filters have tiny “pores” through which air particles pass as larger airborne particles are trapped. If these pores are too tiny for the heat pump, it can cause airflow restriction.

This problem is prevalent in MERV 10+ filters that are one inch or less thick. Using a powerful heat pump can pull the filter to the top of the duct system even in normal working conditions.

Solution: You have two options. First, consider replacing the restrictive filter with a less restrictive option. It doesn’t necessarily have to be less efficient. Just make sure it’s practical for the heat pump you’re using.  Alternatively, upgrade to an air scrubber. Air scrubbers are thicker air filters that catch more airborne particles without restricting airflow.

  • The air filter is dirty

Airflow restriction isn’t always a result of an ill-fitting filter. It could also be a sign of a dirty filter. As we’ve mentioned, air filters have pores that allow air to flow through.

Over time, the dirt trapped in the filter can block these pores, constricting airflow into the heat pump. You’ll know the filter is blocked if return air is also lacking. The dirty filter can also be sucked into the duct as the heat pump works even harder to pull air into the air handler.

Solution: Here, too, you have two options. First, consider cleaning the air filter. Reusable filters can be washed a couple of times before replacement. Alternatively, replace it. Most heat pump manufacturers recommend filter replacement every 1-2 months or more often if you encounter dirty filter issues.

2. Undersized return ducts

What? When would return ducts become undersized? Well, it happens. If you bought the heat pump without involving an HVAC professional, you might end up with a unit with return ducts too small for the application. Small return ducts typically result in a faster airflow into the heat pump, pushing the filter to the top of the duct system.

Solution: Always engage an HVAC professional before you settle on a heat pump. Most of them won’t even charge you for the advice. Additionally, ensure to check the vents every few days to see if anything may be blocking airflow.   

3. It’s a closing air damper

If it’s not the filter, then the noise could result from the air damper closing when the heat pump goes off. The damper system is present in many heat pumps where it helps regulate the amount of air flowing into different parts of the home. When the heat pump shuts down, the damper closes, producing a characteristic slamming sound.

Solution: You may need to get used to the slamming sound as all dampers behave that way. However, if the slamming sound is too loud, contact an HVAC technician to take a look. Maybe it needs repair or replacement.

4. Expanding/contracting ducts

This only applies to heat pump systems that rely on ducts to supply air inside the house. When you start the heat pump, the heat will cause the ducts to expand, making a characteristic creaking noise. The same noises are also audible as the ducts contract when you shut off the heat pump.

Solution: Here, too, you may not need to do anything as every metal duct system behaves in this manner. However, if the noise is too much, then something could be wrong. Perhaps the ducts are improperly sized, or maybe they’re not appropriately reinforced. Contact an HVAC technician to diagnose the problem.

When the Noise is From Outside the House

Although noises outside the house are less of a bother, you may still be concerned, especially when the neighbors begin complaining. What could be the cause?

5. Compromised compressor support springs

The compressor, a critical part of the outdoor component of the heat pump, has many parts inside the sealed system, including a refrigerant pump. The refrigerant pump is typically mounted on support springs. When this spring is shot, the pump isn’t held in a balanced position any longer.

You might notice any problems when the heat pump runs at maximum speed because the momentum gives it a sense of balance. But, the wobbliness becomes more apparent as the speeds slow down. When you shut off the heat pump, the wobbly swinging can cause the refrigerant pump to hit against the compressor case, creating a banging or slamming sound.

Solution: Unfortunately, this could be one of those times when you need to accept the pain and go on with your life. Why? Because the refrigerant pump forms parts of the sealed compressor unit. You’re not allowed to open the sealed component as doing so voids the product warranty.

More importantly, even if you open it (having accepted to lose the warranty), replacing the spring may require that you replace the entire compressor system. Unnecessary, right?

Don’t Sleep on Fan Issues

We didn’t mention it because fan noises happen all the time and not only when the heat pump shuts down. However, if the fan noises get louder when you shut off the heat pump, something could be amiss.

A loose fan, for instance, can bang against the compressor unit outdoor or air handler insider the house, making loud and disruptive noises. Don’t hesitate to bring in an HVAC professional to take a look and advise you accordingly.