Skip to Content

Water Heater | Does It Need Make up Air?

When it comes to makeup air and combustion appliances, including some water heaters, it can get pretty confusing. These appliances definitely need combustion air, which is a supply of outside air. However, they may not require makeup air, which is also outside air brought indoors.

By looking at what makeup air is supposed to do, we can examine the most common types of dryers (electric, gas, and hybrid) and see which may necessitate the provision of makeup air. As we will see, it can also depend on the surrounding environment.

Water heaters don’t need makeup air, which is actually associated with exhaust systems. The dual presence of exhaust systems and water heaters may make it necessary to provide makeup air to ensure the continued and safe functioning of the heaters. This applies to gas and hybrid water heaters.

Water Heaters Don’t Need Makeup Air

Whether you have an electric, hybrid, or gas water heater, you will not need to supply it with makeup air. However, this doesn’t mean that makeup air cannot influence its function.

Makeup air is provided for exhaust systems that remove air from a room or space. The makeup air is to replace this exhausted air, although not all exhaust systems require makeup air.

Typically, it’s only if they remove a lot of air at a time, they are confined in a small space, or if they interfere with the function of any other appliance.

It is this last point that may necessitate the provision of makeup air to ensure the proper function of a water heater. However, it is important to be clear that if there was no exhaust system present, then the makeup air would not be necessary for the water heater.

Gas Water Heaters May Necessitate Makeup Air

Based on all of this information, it is probably obvious that gas water heaters are the most likely to necessitate the supply of makeup air.

Gas water heaters heat the water through the exothermic (heat-producing) chemical reaction known as combustion. Oxygen is essential to this reaction, so combustion appliances, like gas water heaters, need to be supplied with combustion air.


The byproducts of combustion (may include carbon monoxide), which are not good for human and pet health, are deliberately exhausted out of the house so that they cannot enter our breathing air.

When you have an exhaust system present in the same area as the gas water heater, it is possible for this process to be disrupted.

There are two ways in which this can happen:

  • If the combustion air isn’t supplied directly to the water heater and is, instead, provided to the area around it, then this air can be redirected to balance the negative pressure system created. The result is insufficient oxygen for the combustion process, which can produce carbon monoxide.
  • The negative pressure system can result in air being drawn into the house from undesirable locations, including the exhaust vent of the gas water heater. This is called backdrafting.

When a gas water heater doesn’t have enough combustion air, the water heater’s burner assembly flame will not stay lit because there will not be enough oxygen. This will cause your water heater to fail to meet the home’s demands for hot water.

Even if the flame remains lit, incomplete combustion does not release as much thermal energy as complete combustion. I.e., it just doesn’t produce as much heat.

Backdrafting can pull carbon dioxide and water vapor into the house. The first is bad for health, and the second can be damaging to the home and can also affect the health of those with existing respiratory issues.

If the two occur together, then carbon monoxide can be pulled into the house. I don’t need to tell you how dangerous this can be!

Introducing makeup air means that the air removed by nearby exhaust systems is not replaced with the supplied combustion air or with combustion gas-containing air that is supposed to be exhausted.

This ensures that the function of the water heater is not affected by the exhaust system and also that the heater continues to function safely.

How to Tell if Makeup Air Is Necessary

If your water heater doesn’t seem to be getting as it should or as hot as it used to, then this could be an indication that complete combustion is not taking place.

Now, there are a number of non-makeup air-related reasons as to why your water heater is not heating the water properly, so it’s best to look for other signs that would point to a need for makeup air.

The exhaust system itself is probably going to be struggling. So, dryers can leave clothes wet, steam can build up in bathrooms, and range hoods won’t remove the smoke and steam from around the stove/oven.

An excessive buildup of soot around the water heater can indicate incomplete combustion as carbon (soot) is also more likely to be produced under these conditions.

Water heater with buildup of soot

The water heater exhaust vent is not the only source of backdrafting. Plumbing pipes can also backdraft, which can lead to a sewer smell developing in the house.

Other signs of a low-pressure system would be the feeling of air from surrounding rooms or from outside rushing into the room. Putting your hand on the gap of a doorframe can help to test the airflow into the area.

However, the best way to tell if makeup air is required is to get a professional assessment done.

How Much Makeup Air Is Required?

To determine how much makeup air is required, you must determine which systems are influencing the air available to the gas water heater. 

For this determination, look for active exhaust systems in the area. 

Once the interfering systems have been identified, you must find their CFM rating.

CFM ratings for exhaust systems can be found in a variety of places. 

  • One might be on the box that the exhaust system originally came in. The CFM rating is often mentioned in the product details. 
  • Additionally, if you don’t have the original box, the user’s manual often contains this information. 
  • If you know the model number and brand, it is likely that a quick online search will take you to this product’s page, and the page will most likely detail the CFM rating. 

It is also important to note that infiltration air can account for some makeup air. Air that enters the area from adjacent rooms or the outdoors can fulfill the job of artificial makeup air, so it is important to count this in your calculations. 

An equation to better understand this relationship is makeup air needed = exhaust rate – infiltration rate. 

To find the infiltration rate, a blower door test must be done by a professional to find the rate in CFM. 

Exhaust Systems and Electric Water Heaters

As you can see, the reason why makeup air might be necessary if there is a water heater present is because exhaust systems can interfere with the combustion process.

Electric water heaters do not produce heat via combustion. They heat cold water with electrical heating elements. 

So, there is no combustion air to get diverted to balance a negative pressure system. This means that a lack of makeup air would not affect an electric water heater’s ability to heat water. 

In addition, there are no combustion gases produced that may be backdrafted.

What About Hybrid Water Heaters?

Hybrid water heaters are not hybrid in the sense that they use both electricity and gas but rather in the sense that they use a condenser and an electric heating element. 

Ruud 50 Gal EF3.5 Hybrid Builder Residential Electric Water Heater, 10 Year (240v)

The condenser pulls in surrounding air, which passes over coils filled with refrigerant. The heat from the air is passed to the refrigerant, which vaporizes. This moves into smaller coils in the water tank, compressing and increasing in temperature.

The heat from the compressed refrigerant is transferred to the water.

The electric heating element is a backup system.

Because hybrid water heaters don’t use gas, there is no combustion process to interfere with, and there aren’t dangerous byproducts that must be removed from the water heater. 

However, hybrid water heaters do take in air from the room to aid in the heating process. 

If there are exhaust systems in the area that could interfere with the hybrid water heater’s heating process, makeup air might be necessary for the exhaust system. 

However, without it, only the function of the hybrid water heater will be affected, and it won’t make the environment hazardous to health.


Was this helpful?

Amazon and the Amazon logo are trademarks of, Inc, or its affiliates.