Heat Pump Hissing When Off: Why Does This Happen and What to Do

Is your heat pump hissing when off? If your AC is producing a hissing sound when it's turned off, this is a bad sign. Read on to learn what it means and what to do.

Updated on Feb 19, 2024
5 min read
Heat Pump Hissing When Off: Why Does This Happen and What to Do

A hissing sound coming from your AC can be problematic when it's on, let alone when it's off. It can mean a number of things. There could be a leak in the system or high pressure in the compressor. 

Whatever the issue may be, don't ignore this sound. If you don't deal with it on time, it can escalate into a bigger problem. Before you know it, you're going to have to replace the whole unit. 

If you're experiencing your heat pump hissing when off, stick around. We'll explore the different causes of this problematic sound. We'll also give you some tips on how to deal with this issue. 

Why Is My Heat Pump Hissing When It's Turned Off?

A heat pump is a component commonly found in some types of air conditioning units. It's a versatile device that can both cool and heat a space. 

In warm weather, it works like a traditional air conditioner. It does this by removing heat from the indoor air and expelling it outside. 

In colder weather, the heat pump can reverse its operation. It extracts heat from the outdoor air, even in low temperatures. It then transfers the heat indoors to provide warm air.

Heat pumps can sometimes break down or produce unnatural noises. One of the most common types of noises a heat pump makes is hissing.

It's one thing when the heat pump is making weird noises when it's working. But it can be an entirely different problem when the hissing continues, even when you turn the AC off. 

A hissing sound can indicate various issues. The most common ones include:

  1. Leaking refrigerant
  2. High compressor pressure
  3. Thermostatic expansion valve
  4. Leaking ductwork
  5. Reversing valve in the heat pump

Whatever the issue may be, it's a good idea to call HVAC technicians. They will be able to figure out why your heat pump is hissing and fix the issue quickly. 

Leaking Refrigerant

The most common reason your heat pump is hissing is leaking refrigerant.

Refrigerant is the heat transfer fluid in the air conditioner. It's stored in copper tubing.

Even a small leak can cause a distinct hissing sound. This happens because the gas escapes under high pressure. Even the smallest leak can cause a distinct hissing sound.

Signs of a refrigerant leak include:

  • Frost or ice on the refrigerant lines
  • Warm air from the vents
  • Bubbling noises
  • Higher utility bills

When this happens, there are two things you can do. You can either call HVAC technicians to repair your AC, or you might have to replace the whole unit.

If the leak is small, repairing it should be easy. 

If your AC uses R-410A, R-407C, R-134a, and R-454B refrigerant, you can repair the leak. But if the system uses R-22 refrigerant, replacing the entire unit might be necessary.

Another sign you might need to replace your AC is if it's old. To be more precise, if it's more than 10 years old. 

Whatever you do, don't trust HVAC contractors who promise to recharge the refrigerant without locating and fixing the leak first.

High Compressor Pressure

Your heat pump could be hissing because there is high pressure in the compressor. This is also called pressure equalization.

The compressor is a very important part of any AC unit. It has several jobs, including:

  • Compressing the refrigerant
  • Increasing the temperature and pressure
  • Transferring heat
  • Repeating the cycle 

The compressor pressurizes the refrigerant gas. It enables it to absorb and release heat as it circulates through the system. 

When the compressor breaks down, it puts the whole system at risk.

This usually happens when pressure builds up within the compressor. That could be the reason the AC is hissing even when it's not working. 

The sound will come from the outdoor unit. You may also hear a shrieking sound.

Modern AC units have a pressure sensor that shuts down the system to prevent this issue. If the compressor continues to make this sound, call a professional to inspect the system.

Thermostatic Expansion Valve

The thermostatic expansion valve is another component that may produce a hissing noise. But this is usually normal.

This component removes pressure from the refrigerant before it flows back into the indoor unit. It's normal for the valve to make a quiet and short hissing noise in the process. Some people also describe it as a whooshing sound. 

But if the noise becomes excessive or abnormal, you should consult a professional.

Leaking Ductwork

A hissing sound may indicate leaking ductwork.

When there are leaks in the ductwork, air can escape, creating a hissing or whistling sound.

This can also happen due to pressure imbalance. Ductwork is designed to maintain a balanced pressure within the system. When there are leaks, the pressure may become imbalanced. This causes the air to rush out and create audible hissing noises.

It's important to inspect and repair the ductwork. This doesn't only reduce the noise, but it also improves efficiency.

It's a good idea to hire a qualified HVAC professional. They will conduct a thorough inspection of your ductwork. They can use specialized tools to identify and locate leaks.

Reversing Valve in the Heat Pump

Finally, the hissing may come from a malfunctioning or stuck reversing valve in the heat pump.

The valve reverses the refrigerant flow that allows the heat pump to heat and cool your home. You may notice a constant hissing noise if the reversing valve is malfunctioning. It can be stuck between the heating and cooling modes.

If this is the case, it's best to contact a professional to inspect and repair the reversing valve.

What to Do if Your Heat Pump Is Hissing When It's Turned Off?

If your heat pump is hissing when you turn it off, here's what you can do:

  1. Turn off the system
  2. Visually inspect the system
  3. Do not attempt DIY repairs
  4. Call HVAC technicians 

First and foremost, turn off your AC, and don't turn it back on. This will prevent further loss of refrigerant. It will also prevent potential damage to the system.

Examine the visible ductwork for any obvious signs of damage. For instance, gaps, disconnected sections, or visible leaks. But don't touch anything. Leave this task to trained HVAC professionals.

Reach out to a licensed HVAC technician to inspect and diagnose the issue. 

They will have the expertise and equipment needed to identify the source of the hissing sound. They will also recommend the necessary repairs.

To prevent this issue from happening again, we recommend regular maintenance. Preventive measures can help avoid issues and extend the lifespan of your system.

A professional technician will inspect and clean the components. They will also check for any leaks or damaged parts and make any necessary repairs. 

They can also provide valuable advice on how to optimize your heat pump's performance and energy efficiency.

Knowing How to Deal With a Hissing Heat Pump

If you notice your heat pump hissing when it's off, it could be due to many reasons. Instead of trying to fix the issue yourself, it's better to hire professionals.

Sandium HVAC experts can help you with your hissing issue. Thanks to our extensive experience, we know how to identify the issue early on and find a solution quickly. We'll get your AC back to normal in no time.

Call for a consultation.

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