You probably have a general sense of how furnaces work, but do you know the differences between gas and electric systems? All the different parts involved in heating a home?
It’s ok if you don’t. HVAC technicians train specifically to be able to understand the sometimes complex mechanical processes behind home heating equipment.
That also means you are going to learn a lot in this article. By the end, the goal won’t be a complete understanding of every single part and process (that’s what HVAC technician school is for!) but a solid general understanding of how your furnace works and what you should know about your home HVAC system will help you to make better decisions about your home comfort!
The Heating Process
There are some variances depending on the type of furnace you have, which we’ll mention throughout this article. But below we cover the typical heating process for a residential, forced-air (sometimes called condensing) furnace that is most common in US homes.
Cool air from the home is circulated through your ductwork and enters the furnace system via the return air drop, located near your main furnace unit.
The furnace heats the air at this point. If it’s a gas system, this is via a flame. If it’s electric, it’s via electrical coils.
In a gas system, an extra step has to occur where the heat travels through a heat exchanger, which separates combustion gasses from heat that can safely warm your home. The combustion gasses are exhausted through a vent pipe.
The air is then circulated via the furnace’s blower motor, traveling throughout the home. Air is recirculated through the flames/coils repeatedly until the desired temperature is reached in your home.
Types of Furnaces
Here are the major types of furnaces, broken down mechanically by how they circulate air.
Forced Air and Condensing Furnaces
Forced air refers to the fact that air is pushed through a home via a blower motor and ductwork network. This is typical of nearly all modern furnaces, and is the type of unit we discuss in more length in this article.
While not entirely equivalent, all condensing furnaces are also forced air. Condensing refers more to the efficiency of a furnace, which we’ll discuss in more details below. Their primary difference is a secondary heat exchanger that makes them more efficient than traditional furnaces.
Natural Draft
Natural draft furnaces work on the principle that warmer air rises, and so a basement furnace could rely on this principle for the air to rise throughout the rest of the home.
These furnaces are not typical in modern homes, nor are they as efficient as modern systems. Their lone advantage is utilizing fewer moving parts to circulate air.
Water Boiler Systems
These are extremely different, and involve boiling water that heats the air and is usually distributed through baseboard heater outlets or radiators.
While not the most common, these are still viable and in use in many homes.
Other Types of Heaters
It would be silly to call electric baseboard heating or individual space heaters a furnace. However, these can be used in homes either to heat an individual space or the entire home. Neither offers the full range of benefits of a whole-home, forced air system, particularly because they won’t filter and circulate the air in your home.
Electric vs. Gas Furnaces
These two types are the most common in modern, forced-air systems, and have some noticeable differences.
For one, an electric furnace is actually an air handler that’s had electric heating strips installed inside of it. It’s often referred to as an electric furnace or heater, though, so the terms are somewhat interchangeable.
Gas furnaces of course require a working gas line to be connected to the home. Depending on where you live, this may not be possible.
Gas also requires a heat exchanger (or two, depending on the system’s efficiency), and an exhaust pipe for combustion gasses that would be harmful to circulate into your home.
Electric heat is also often not the only type of heat an electric HVAC system provides. Electric furnaces are often paired with heat pumps, and the two work together to heat your home.
Heat Pumps
A heat pump isn’t a furnace. With that stupidly obvious statement out of the way, how does a heat pump work in tandem with a furnace?
Usually it’s with an electric furnace, and your heat pump will be the first line of defense when the weather turns cold. However, many heat pumps struggle to properly heat the home in the coldest months, and so auxiliary heat from the electric furnace will be used to close this heating gap.
In many regions, electrical heat is extremely expensive, and so it’s more cost-effective to have the heat pump do as much of the work as possible before supplementing with heat from the electric coils in an air handler.
Oil, Wood, Corn and Other Furnaces
These furnaces don’t necessarily use different distribution systems, but the fuel source is different.
These fuel types are not in wide use anymore, and are generally found in areas without natural gas lines and homes that lack holistic heating infrastructure such as ductwork.
Furnace Efficiency Levels & Exhaust
Electric furnaces are technically 100% efficient, so no energy is lost in the heating process itself. That isn’t to say your home is 100% efficient, though, since ductwork leaks, lack of insulation, or poor siding or windows can contribute to heat loss.
For gas systems, modern systems generally have a floor of 80% efficiency. This means that 80 cents of every dollar of heating energy is going into your home.
The most efficient gas systems exceed 90%, though, and the highest-end systems can reach as high as 98% efficiency.
This involves a secondary heat exchanger that ensures that less heating energy is lost while still expelling combustion gasses through a vent pipe. Depending on efficiency levels, different types of ventilation pipe may be needed to properly ventilate these gasses.
Efficiency and Cost Savings
Does a more efficient system pay for itself in cost savings?
This is a difficult question to answer, because it involves the cost of energy where you live, your average usage rate of the system, and how long you plan on staying in the home.
Five years or less of even regular usage won’t make up the additional cost of a high-efficiency furnace, for example. But 15-20 years almost always will.
Will you be in your home and will the furnace last for 15+ years? Those are not easy questions to answer, but discussing the particulars with a trusted HVAC professional can help you reach a more confident answer for you and your home.
Furnace Staging
Think of stages in a furnace like gears in a car. First gear might be 40% power. Second gear, 60% power. And so on.
Single-stage furnaces only have one gear: 100%. This will heat your home, but it won’t be the most consistent or energy-efficient. Often that 40% setting is better, since it uses less energy and may be all you need for a more even warmth in your home.
Furnaces tend to come in three types in regard to staging:
- Single-stage
- Two-stage
- Modulating or variable-speed
As stages increase, so does cost, but so too does your cost savings and comfort levels.
Furnace Capacity
A furnace designed to heat 500 square feet will not be able to heat 5,000 square feet. This seems like common sense.
But what about two different 2,000 square foot homes, but one has high ceilings and poor window quality and several more windows than the other home? It, too, will require a lot more heating output to keep it warm.
That’s why matching capacity – sometimes known as furnace sizing – to your home is important. Furnace sizes tend to exist to adequately heat typically-sized homes. The largest mansions or buildings will often need multiple units to fully heat.
The heating process is the same between these systems, it’s only the total heating power that’s different.
Parts in a Furnace
Here’s a list of major parts in a furnace and what each does. This list isn’t comprehensive, but covers major components involved in the heating process.
- Air Filter – both keeps the air free of particulates and facilitates airflow in the system. A dirty air filter can hinder the entire heating process.
- Blower Fan – moves air throughout the system to circulate heated air.
- Burners – in a gas furnace, burners are what heats the air before it’s passed to the heat exchangers.
- Control Board – control boards – sometimes called circuit boards – are a subsystem that controls and regulates several smaller processes, such as the gas valve, burners, and electrical inputs to various parts of the system.
- Ductwork – carries air throughout your home and returns air back to the furnace to be heated and recirculated.
- Electrical Coils – in air handlers, electrical coils are installed that can heat the air before being circulated.
- Exhaust Flue – in natural gas systems, the exhaust flue allows combustion gasses to leave your home harmlessly.
- Gas Valve – controls the gas supply to the system that keeps it fueled.
- Heat Exchanger – separates combustion gasses and heat before it travels into your ductwork, vents and home.
- Thermostat – the control center for your entire HVAC system, you can set both a desired temperature and in some systems the humidity levels as well.
- Transformer – electrical hub for supplying power to the furnace.
Finding the Right Furnace for Your Home
Now that you know the basics of the heating process, what can you do with this information?
- Talk to your HVAC contractor about their process for determining ideal sizing.
- Talk to them about why you might want different furnace staging in your home.
- Understand the additional costs and processes involved in more efficient equipment.
- Better understand why you might want a more expensive furnace…or not, depending on your situation.
The last step is to speak with a trusted HVAC contractor who can walk through each of these with you, as well as any other questions you may have.
CABS Heating and Air Conditioning has helped thousands of customers do just that and install great furnaces into homes. We’d love to meet with you and discuss your next home heating project!
