Does a Furnace Room Need Ventilation?

In many older homes, the natural gas fire furnace and water heater are often placed in the center of the home in a small closet. 

Some homeowners wonder where gas-fired appliances get the air to support combustion.  Does a furnace room need ventilation?

Most new building codes require that spaces that house gas-fired appliances such as water heaters and furnaces have an outside ventilation source to provide combustion air. Electric Furnaces or water heaters do not need ventilation to support combustion.  When installing a new gas-fired furnace or water heater, you must add outside ventilation.

There are several good reasons that a furnace room should have ventilation. 

Notwithstanding the local building codes, any gas-fired appliance needs a source of combustion air to burn freely and effectively. 

We’ll help you understand the heating process mechanics and how those mechanics can affect your home and safety.

The Combustion Process 

The process of burning, by definition, takes three things.  Fire requires: 

  • Fuel
  •  Air
  • An ignition source

In your furnace or water heater, the fuel is typically natural gas.  The ignition source comes from the pilot light or the spark igniter. 

The air to support the combustion enters the water heater or furnace through the vents near the burners.

The products of combustion are exhausted through the roof via the vent pipe.  These products of combustion include:

  • Heat,
  • A bit of water
  • Carbon dioxide
  •  Carbon monoxide.  

 The combustion process requires a continuous source of air if the burners are working.

Have a Question? Ask HVAC Technician

Click here to use the chatbox to speak with one of our technicians.
No in-home service calls. No appointments.

So, From Where Does the Air Come?

If your furnace room or closet doesn’t have outside ventilation, the furnace or water heater draws combustion air from the only place possible, inside your home. 

It was common in older homes to put louvered doors on the furnace and water heater closet to allow combustion air to circulate freely.

What’s the Problem with Drawing Combustion Air from Inside the Home?

Several significant reasons make drawing combustion air from inside the home a bad idea. These reasons range from safety to efficiency.

Basic Safety – Keeping the Bad Stuff Where It Belongs

When your gas furnace or water heater operates, they are actively pulling air into the combustion chamber and exhausting it through the roof vent. 

This combustion airflow creates a negative pressure condition inside your home.   A negative air pressure condition inside your home can have several consequences.

Combustion Gas Backflow

The biggest danger of not having adequate combustion air ventilation is the possibility of combustion gas backflow into your home. 

The two most common products of combustion are carbon dioxide and carbon monoxide. 

Both can be deadly in concentration, but carbon monoxide is especially dangerous.   

If your home’s negative air pressure is great enough, the airflow up the vent from the furnace or water heater can reverse. 

The reverse flow draws the toxic products of combustion back into your home. 

This situation is common where a water heater and furnace share a common flue.  

You may notice this phenomenon if you have a wood-burning fireplace. 

If a puff of smoke is drawn back into your home from the fireplace when the furnace or water heater comes on, you have a negative pressure problem in your home.

Inefficient Operation – Getting the Most for the Money You Spend

Natural gas is probably one of the most economical ways to heat water and your home.  However, you still get a bill every month. 

Like most of us, you want to get as much for your money as possible.  You want the most energy the natural gas can deliver.

Too little combustion air getting to your furnace or water heater is less efficient. In the end:

  • The gas doesn’t produce as much energy
  • You end up paying more.  
  • Your furnace or water heater works harder and can experience a shorter life.

More Inefficiencies – Burning the Air you Have Paid to Heat

If your furnace room or water heater closet depends on the air inside your home for combustion, you are paying double. 

You have paid for the gas to heat the air inside your home. 

If the furnace is drawing air from inside the home for combustion, the money you paid to heat that air is going up the flue.

When the furnace or water heater draws air up the flue after burning, the negative pressure inside your home pulls air from somewhere. 

Many people don’t realize how many ways outside air can infiltrate into their home.

  • Down the chimney
  • Through clothes dryer vents
  • Backward from exhaust fans in bathrooms and kitchens
  • Poorly installed or older doors and windows

These are just a few examples.  No matter where the outside air enters your home, it is unheated, and you must pay to heat it to your desired temperature. 

Professionally installed ventilation in your furnace room or heater closet stops this negative pressure situation and lessens outside air infiltration.

The Unintended Consequences of Home Improvements

The owners of many older homes recognized the benefits of addressing the problems of air infiltration. 

Over the years, many older homes have had newer windows installed that stop much of the outside air movement.  

Many homeowners undertook to caulk and seal their homes in efforts to reduce their heating bills. 

The unintended consequence of many of these efforts was to starve their gas-fired appliances of combustion air.

The tighter and more efficient the homes became, the harder it became for those gas-fueled appliances to draw air into the home for combustion. 

Problems with backflow down chimneys and other vents to the outside were the result.  

How Do I Get Outside Combustion Air to an Interior Heater Closet?

Having a furnace room or heater closet in the middle of your home can pose a challenge. 

A central furnace closer is a case in many older homes. 

The central location allowed shorter duct runs to the rest of the house and a single gas pipe could supply both appliances.  

In most instances, plumbers and HVAC technicians will install fresh air inlets into the furnace closet through the roof. 

Most building codes allow this solution.  Generally, the installation requires two pipes. 

One pipe reaches to within six or so inches of the floor. The other pipe extends six inches or so below the ceiling of the closet.

Typically, the door to the furnace or water heater closet is fitted with foam insulation around the door jamb to seal the closet.  

If the door to the furnace closet is louvered, replace it with a solid door.   

How Big Should the Vents Be?

The best source of answers to this question is your plumber or HVAC technician.  

Both trained professionals can calculate the amount of combustion air needed based on your furnace’s size and the square footage of your home.

You can make a general estimate with a simple calculation.  A direct-vent furnace requires about 50 cubic feet of air for every 1000 BTUs of fuel input. 

The label or documentation with your furnace or water heater can tell you the BTU fuel input.

Why Do I Need Two Vent Pipes?

You need two vent pipes for safety reasons more than anything else. 

Creating a tightly enclosed space for your gas-fired appliance creates a space where gas could accumulate if a leak occurs. 

A gas accumulation in an enclosed space can cause an explosion or fire.  

Two vent pipes, one with the opening near the floor and the other near the ceiling, create a natural draft. 

This natural draft will carry any gas or other toxic fumes out of the space, keeping you and your family safe.  

Can I Install Outside Ventilation into My Furnace Room Myself?

That depends.  There are several things to consider before starting a DIY project of this sort.

  • Local Codes and Requirements – In some jurisdictions, making this kind of change to your home heating systems requires a permit.  Before beginning any project, check with your local building inspection department for guidance.  
  • The Level of Your Experience – Remember that this project will require you to penetrate your roof and install vent pipes in the attic.  Cutting holes in your roof can be a major project.  If not done correctly, you may face other damage or problems from leaks or wind damage to your roof.
  • The Cost vs. Convenience Factor – You may save some money by doing this kind of project yourself.  On the other hand, a professional has the skills, knowledge, and tools to do the job with minimum fuss and mess.  It would be best if you weighed the cost and convenience for yourself.

It’s All About the Air

The question of ventilating your furnace room or heater closet is all about the air your system needs to operate efficiently and safely. 

If you are anticipating replacing a furnace with a new unit, more than likely, the installation will require the addition of an outside combustion air source.  

If your furnace currently relies on home air for combustion, you may very well need to consider modifying your furnace room or closet to add an outside air source.

Your furnace will probably operate more efficiently in either situation.  Adding an outside combustion air source means that your home is safer for you and your family. 

Leave a Comment