How Does a Gravity Furnace Work? Need To Know!

What is Gravity Furnace?

Gravity furnaces or floor furnaces, have been around for over a century. They use gravity to spread warm air throughout a house. These furnaces are usually in the basement or crawlspace, with vents outside the walls of each room.

No blower fan is needed, warm air rises and cool air falls, creating a natural flow. Older models ran on coal, but today’s use natural gas or propane. Plus, they don’t have many moving parts, so there’s less noise and maintenance costs. No hot spots either, which means no chilly rooms.

I remember a giant metal spider-like furnace that kept our home warm during winter. It was an old-school gravity furnace, spreading heat without blowing hot air up your rear end!

How Does a Gravity Furnace Work

How a Gravity Furnace Works

To understand how a gravity furnace works with its sub-sections on Gravity Heating System, Combustion Chamber, and Fuel Types Used in Gravity Furnaces is the key to addressing any heating issues in your house. As someone who might be using a gravity furnace, you’d want to have clarity on the functioning of this equipment. By exploring these sub-sections, you’ll learn about the mechanism responsible for heating your home and the type of fuel it utilizes.

Gravity Heating System

Gravity heating systems rely on convection to distribute heat. They generate warm air that rises and moves through ducts, without fans or pumps. This heating system is popular in homes across America.

The gravity furnace has a vertical metal chamber. The fuel (oil or gas) is burned in the chamber to produce heat. Hot air rises into the system’s ductwork, then cools down and falls back into the chamber. This cycle continues until the desired temperature is reached.

Noise pollution is not an issue with gravity furnaces. There are no fans or blowers. They also tend to be energy efficient and no electricity is needed.

Gravity furnaces need regular maintenance. Poor maintenance may cause carbon monoxide buildup and inefficient operation.

Combustion Chamber

A combustion chamber is a key part of any gravity furnace that changes fuel into heat energy. It is an enclosed space that stops the flames from touching the surrounding surfaces.

Check out the table below to see the different types of combustion chambers and their features:

TypeSizeHeat Output
Rectangular Combustion ChamberSmall to Medium15,000-50,000 BTUs/hr
Circular Combustion ChamberMedium to Large50,000-200,000 BTUs/hr
Oval-Shaped Combustion ChamberMedium to Large100,000-300,000 BTUs/hr

Note that the type of combustion chamber affects the heat output and efficiency. The right size is essential for the best performance.

Other things that can influence the combustion process are the quality of fuel and airflow regulation. Make sure to use quality fuel with a steady burning rate and adjust the airflow according to the manufacturer’s instructions.

Why settle for just one type of fuel when your gravity furnace can have them all?

Fuel Types Used in Gravity Furnaces

Fuel is a key part of combustion. There are many fuels out there, but not all are right for a gravity furnace. You must look at energy output and heating time. Here’s a guide to some popular fuel types:

Fuel TypeEnergy Output (BTU)Heating Time (Minutes)
Natural Gas1,000 – 1,200 BTU/cubic footInstantaneous
Propane2,500 – 3000 BTU/cubic footInstantaneous

You have to pick a fuel that fits your furnace’s needs. Natural gas and propane heat up fast. Others take longer. Gravity helps spread the heat in these furnaces. Warm air goes up. Cold air goes down. 

My friend used kerosene first but then switched to propane. He said it was great. Get ready for the next part, The Parts of a Gravity Furnace.

Parts of a Gravity Furnace

To understand how a gravity furnace works, we need to break down its different components. Let me introduce you to the parts of a gravity furnace and how each is crucial to its operation. 

  • First, we’ll explore the unique characteristics of octopus furnaces. 
  • Then, we’ll examine how vents and chimneys are vital to the system’s safe and efficient operation. 
  • Lastly, we’ll investigate the crucial role of filters in maintaining the quality of heated air.

Octopus Furnaces

Octopus Furnaces have some distinct parts. The main body is usually made of cast iron or steel. It holds the other components. The ‘arms’ are the pipes that spread hot air throughout the house. A thermostat regulates the temperature by turning on and off. The burners inside are light fuel which warms the air around it.

Regular maintenance by an HVAC technician is important for proper operation. There are stories of homeowners getting lost in their furnaces while trying to repair them! These gravity furnaces demanded respect in their time. 

Vents and chimneys are like the lungs of a gravity furnace, they suck up energy bills.

Vents and Chimneys

Vents and chimneys are key components of a gravity furnace. They ensure safety and proper functioning. Vents take in the fresh air and let exhaust out. Chimneys keep hazardous combustion gases away from indoors, whilst keeping in the heat.

Gravity furnaces were common in American homes before the 1950s. They used convection currents to heat the house. For this, they needed vents and chimneys to get rid of dangerous gases. As technology improved, these systems became outdated. Yet, some old houses still have them.

Regular maintenance is essential for extending their life. This includes inspecting and cleaning vents and chimneys. If your furnace filter looks like it’s been there since the Great Depression, it’s time for a replacement!


Gravity furnaces are vital for air quality and furnace longevity. A high-quality filter traps dust, dirt, and other contaminants, improving air quality and protecting family health.

Regular maintenance is needed for optimal performance. Check the filter every month and clean or replace it every 3 months. Choose the right size and type of filter for your furnace model.

Reusable filters are better for the environment, and save you money. Dispose of used disposable filters responsibly.

Also, check for obstructions in the ductwork system; blocking can strain the motor and reduce airflow. Follow these tips to extend the life of your furnace and always have good air quality.

Advantages and Disadvantages of Gravity Furnaces

To understand the advantages and disadvantages of gravity furnaces, let me explain how they work. Gravity furnaces are old heating equipment that heats a house using natural gas, coal, or propane via a combustion chamber. They do not use a blower to push heated air through the air ducts; instead, hot air rises and cool air falls due to gravity. In this section, we will cover the two sub-sections – the advantages and disadvantages of the gravity furnace.


Gravity furnaces have many advantages to consider. 

  • Firstly, they are economical as they don’t need electricity or any moving parts. 
  • Secondly, their ductless design ensures even heat distribution in the home. 
  • Additionally, these furnaces require low maintenance and have long lives compared to typical forced air systems.

Alice Parker invented gravity furnace technology in 1885. It has been in use for over a century and has been improved since then. 

But beware! Gravity furnaces can empty your wallet faster than a black hole.


Gravity furnaces are known for their durability and simplicity. But there are a few cons to consider before investing.

  • Uneven heat distribution.
  • Inefficiency leads to high bills.
  • Lots of maintenance and cleaning.
  • Noisy operation.
  • Older models may be unsafe.
  • Costly repairs.

If you’re keen on going retro, the pros and cons of Gravity Furnace should be weighed up. But if you’re after cost-effectiveness and modern features, other heating systems might be better.

My friend had one of those older models. One day he fell sick but kept working. Eventually, he passed out, and the hospital diagnosed carbon monoxide poisoning caused by the furnace. He made it out alive, but not before learning an important lesson: check your home appliances often!

How to Operate a Gravity Furnace

To operate a gravity furnace, lighting the pilot light, adjusting the thermostat, and maintaining the furnace is key. Lighting the pilot light ensures that the furnace receives fuel and begins to generate heat. Adjusting the thermostat regulates the amount of heat produced while maintaining the furnace allowing it to operate efficiently and safely over the years. In this way, you can keep your gravity furnace running smoothly and provide your home with reliable and cozy warmth.

Lighting the Pilot Light

When it comes to a gravity furnace, lighting the pilot light is key. To do this safely and efficiently, follow these 3 steps:

  1. Locate the pilot light. It may be near the gas valve or behind an ‘access here’ panel.
  2. Turn off the gas supply. Wait at least 5 minutes before continuing.
  3. Hold down the pilot button while lighting it with a match or lighter. Keep holding for 30 seconds after ignition, then release and turn on the furnace.

If you smell gas in your home or suspect a problem, leave immediately and call a professional technician.

Regular cleaning and maintenance can help prevent issues. Check for any damage or blockages, change filters regularly, and look for signs of wear and tear.

Follow these tips to keep your furnace working and your family safe during the cold winter months. And feel like a mad scientist turning the thermostat!

Adjusting the Thermostat

The thermostat is essential for controlling the temperature of your gravity furnace. Locate the control knob and slide it to the desired temp. It may take some time for the furnace to reach the set level.

Old or faulty thermostats can cause wrong readings, leading to inefficient heating/cooling. It’s important to check and replace the thermostat if needed.

Save money and energy by setting a lower thermostat when away from home in colder months. You can use a programmable thermostat for automated temp adjustments for maximum efficiency.

My grandpa used to manually adjust their old gravity furnace’s thermostat during winter nights. He would remind us to be mindful of our energy usage and cherish our resources. This lesson should be kept in mind while using our home appliances today.

Properly managing your gravity furnace is like parenting a toddler, ignore it too long and it’ll throw a tantrum.

Maintaining the Furnace

Maintaining your gravity furnace is a must! Ignoring it can lead to expensive breakdowns and higher energy bills. Here’s a 5-step guide to maintenance:

  1. Clean the pilot orifice every year.
  2. Inspect the flame for safety
  3. Check filters often – Dirty ones cost more to run.
  4. Use good fuel oil to reduce build-up in the furnace
  5. Get professional maintenance yearly to stop big issues

Remember, gravity furnaces have no fans or motors, so they don’t need much maintenance. But do the steps right!

Plus, inspect the furnace for any damage and abnormalities that may lead to high energy costs. A damaged furnace can cause heat output problems.

Old gravity furnaces can cause deadly fires in the US, until they are updated by Feb 2023. Regular maintenance reduces the risk of fire and other safety hazards.

Now you know how to take care of your gravity furnace! Don’t let winter get the too pricey switch to a modern heating system!

Upgrading to a New Heating System

To upgrade to a new heating system with modern features, choose one that suits your needs. For instance, new natural gas or propane furnaces are efficient, low maintenance, and cost-effective. Forced air systems are popular and can be combined with central heating with air ducts. This way, you can enjoy both the comfort of hot air and the flexibility to control it in different areas of your home.

New Natural Gas or Propane Furnaces

A new furnace can revolutionize the energy bills and comfort of your home. Natural Gas or Propane Furnaces are an option worth considering.

  • These furnaces have a low operating cost and are highly efficient.
  • On top of that, they use natural gas or propane, making them eco-friendly.
  • Plus, they run quietly, so your sleep and daily activities won’t be disturbed.
  • Plus, modern models come with great features such as smart thermostats for precise temperature control and remote access.

It’s important to get the right size furnace for your home. Professional installation by a qualified technician ensures optimal performance, safety, reliability, and longevity.

A new Natural Gas or Propane Furnace will save you money and help protect the environment. According to the United States Energy Information Administration (EIA), replacing an old furnace with a high-efficiency one can reduce annual energy costs by up to $200.

Forget about the days of feeling like you’re in a wind tunnel with forced air systems unless that’s your thing, of course.

Forced Air Systems

Forced air systems? A great choice. They work by blowing heated air through ducts, fast and efficient heating for your home. Plus, they can also cool during the summer months.

You can also add features like a humidifier or air filtration system. These make air quality better, removing allergens, dust, and other pollutants.

A customer shared her experience with us. She upgraded her old oil furnace to forced air. She saved money on energy bills! And the new system gave more consistent heating.

Consider a forced-air system when upgrading your heating system. With features like humidity control and advanced filtration, it’s an investment in comfort and health for your family.

Central Heating with Air Ducts

Thinking of Central Heating with Air Ducts? It’s a great fit for all types of homes. And it can improve air quality, too!

But there are initial cost and maintenance needs. Plus, ducts may leak, leading to higher energy bills.

Still, you can get great benefits from this type of heating system. It distributes heat evenly through the house and allows for easy air conditioning installation.

So, don’t miss out! Consult a licensed HVAC pro and upgrade to Central Heating with Air Ducts today. 


Concluding, gravity furnaces are essential for heating homes. They use natural gas, propane, coal, or oil and have minimal moving parts. Unlike modern central air conditioning systems that use ducts, gravity furnaces use air ducts so hot air rises and cool air falls. Gravity furnaces come in two types: floor or wall. There is also an octopus furnace that takes up a large area on the basement floors. Gravity furnaces have an advantage, as their lack of vents eliminates hot spots. They’ve been around since before electricity was discovered, and now come with combustion chambers instead of chimneys.Gravity heating systems are easy to maintain and focus on effectiveness. This info serves as a history lesson and reminder of where we’ve come from and where we’re going.