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Home Heating Basics

Posted by Jordan on Jan 28, 2015 4:48:01 PM

Furnace 101: How Does Your Furnace Work? 

Your furnace heats your home and is responsible for the comfort of your day-to-day life in the winter, but how does your furnace actually work? We’re going to break down the home heating basics and put them into terms (and pictures) to make you a furnace mini-expert.How does my furnace work

How Does a Furnace Make Heat?

In the most basic terms, a furnace is an appliance that uses fuel to send heat throughout a building. It is usually made up of a metal box (heat exchanger) that contains a flame fueled by different types of materials to produce heat. It also contains a vent or chimney that takes the products of combustion and pushes it outside. Most furnaces contain an opening for air to come in and an opening for air to come out – that’s the idea behind what’s called a gravity furnace. Take it a step further by using a blower (fan) to force the air in the heat exchanger and you have what’s called a forced-air furnace.

Home Furnace Styles

As with many appliances, there are variations in the style of furnaces and the fuels used in homes, and therefore, variations in efficiency. 

Keith Hill, Minnesota Air’s manager of technical support, says to make it more efficient, modern furnaces have heat exchangers that are split into several independent heat exchangers with narrow passageways to better extract the heat from the combustion gases. 

“It’s all about vent temperature,” says Keith. “Old gravity style furnaces had chimney temperatures of 400° F or more. That’s a lot of heat heading up the chimney that the homeowner paid for. Forced air furnaces have vent temps below 340° F. Today’s high efficiency furnaces have a second heat exchanger (condensing heat exchanger) that extracts even more heat, bringing the vent temperature down to 120° F or below.”

They’re called condensing heat exchangers, because at temperatures less than 200° F, the moisture in the combustion gases condenses into liquid water and needs to be drained away.

Condensing heat exchangers help extract more heat out of combustion gas and bring it into your home. Like Popular Mechanics says, “producing heat is easy, getting it to where it needs to go is the hard part.” 

Keith says there are variations of forced air furnaces, too. Some are designed to blow the air downward for crawlspace ductwork, some blow the air horizontally for attic applications, but they all work essentially the same.

Furnace Fuel Types 

There are variations in fuels that furnaces use, too. Most use one of the following types:

  •       Natural gas
  •       Propane gas
  •       Fuel Oil
  •       Wood
  •       Coal
  •       Corn

The common elements in all of these fuels are carbon and hydrogen. Combine those with the oxygen in the combustion air and you can understand why one of the natural byproducts of combustion is water vapor (H2O).  That’s where the liquid water comes from in a condensing furnace. In less efficient furnaces, the water stays in vapor form and heads up the vent as steam.

Why Is It Important To Take Good Care Of My Furnace?

“Airflow is key,” says Keith. “To make a forced air furnace work correctly and efficiently, enough air needs to be pushed through the furnace to extract the heat from the heat exchanger to maintain low vent temps and deliver to the home the heat energy you are paying for. Inadequate airflow also allows the heat exchanger to run too hot shortening its life.  That’s why maintaining a clean furnace filter is so important.”

In addition, the type of fuel your furnace uses can leave soot behind that can accumulate on the heat exchanger and vent surfaces, which can lead to more inefficiency.

“Just 1/8 inch of soot is equal to 1 inch of fiberglass insulation,” says Keith, “so even a light coating of soot reduces heat transfer significantly and sends more or your heat energy up the chimney.”

To keep your furnace in top shape, you need to:

  •       Change the filter regularly.
  •       Do regular inspections and hire a heating professional to:
    • Make sure that there are no corroded or dirty burners.
    • Check to make sure that the heat exchanger or air intake isn’t restricted.

Maintaining a clean furnace is important for your safety and for keeping money in your wallet. For more furnace facts, view our list of 10 blogs to answer all furnace questions.


For more energy saving and home comfort tips, visit! Stay Comfy, Minnesota is your Minnesota resource for air conditioning repair, furnace repair and HVAC tips and advice.  

Topics: Heating