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Gas Heat Furnaces: An Overview

Gas Heat Furnaces: An Overview

Gas Heat Furnaces: An Overview

A home with natural gas hookups likely uses gas to power the furnace as well as the stove. In your HVAC career, you’ll encounter a fair number of gas heat furnaces that need repairs or maintenance. Since these heating systems work quite differently from electrical furnaces, it’s important to know how both should typically work and the crucial differences in their makeup and functions.

A gas heat furnace has its pros and cons compared to an electric heating system, as well as significant differences in energy efficiency and how the system works. A gas furnace also has the potential to be dangerous to the occupants of the home if not installed or repaired correctly. Before you start work on a gas furnace, keep yourself well-informed of what to expect and how to handle the machine.

Gas-powered furnaces need different types of work and repairs than electrical heating systems. Do you know the differences? Learn how gas furnaces work and how to repair them here. #TheTrainingCenterofAirConditioningandHeating Click To Tweet

How Gas Furnaces Work

Gas furnaces usually run on natural gas piped into the home from a utility company. If the home doesn’t have natural gas hookups, the fuel will likely be propane from specialized tanks. A propane provider will periodically visit the home to refill the tank and keep the heater running.

Propane typically enters a furnace gas valve at a maximum of 11 inches of water gauge pressure as measured by the proper manometer. Once the propane furnace is running, the actual manifold pressure is usually about 10 inches. Natural gas pressure tends to be about the same amount, averaging about 10.5 inches at the entry point. However, when the furnace is running, the manifold pressure of natural gas will drop to about 3.5 inches of water. Check the nameplate on the furnace to determine which pressure level you should be seeing.

If you take a look at a gas furnace, you’ll see the box is maybe 4-5 ft. long and contains a fan and the furnace itself. One end connects to the duct coming from the filter grill. The other end of the furnace connects to the AC evaporator coil. The fan pulls air from the air filter and, if the furnace is running, it warms the air below and sends it along the ductwork throughout the house. Since the heated air passes through the evaporator coil, it’s just as important to keep the AC coil clean in the winter as it is during the summer. A dirty evaporator coil can cause a furnace to overheat and pose a fire hazard.

A gas furnace and its accompanying flue pipe also requires open space around it (typically 30 inches) to keep them both away from potentially flammable objects. This is especially important for the flue pipe since, even with its double-walled material, it can still get very hot. As the exhaust pipe of the gas heater, the flue handles a lot of hot air. The pipe’s sealant must be sturdy to keep hazardous gases such as carbon monoxide from escaping, and the pipe must never be closer than 1 inch to wood or flammable materials or it poses a significant fire hazard.

A properly installed gas heater connects to the thermostat and waits for the signal to turn on. The thermostat receives 24 volts from the red T stat wire and sends the power to the furnace through a white wire, which activates the furnace. The furnace sparks the gas inside to create heat and begin warming the whole house.

Pro Tip: Propane and natural gas furnaces are designed for very specific and very different fuel sources. NEVER try to run a propane furnace with natural gas or vice versa! The differences in pressure will lead to severe problems.

Running a Gas Furnace

When installed correctly, a gas furnace runs like this:

  • The combustion fan motor, a special fan that pushes or pulls air through the fire box, activates for about 30 seconds. Any raw gas in the sparking area is flushed out through the roof. A small pressure switch connected to this fan tells the thermostat the fan is running. Once the thermostat gets the signal from the switch that the purge fan is on and has run for 30 seconds without a problem, the next step begins.
  • The thermostat checks that all the safety switches are in the proper open or closed positions. Located in several places on the furnaces, these switches detect overheating or other potential dangers. If these switches are in the proper position, the thermostat keeps moving forward.
  • The thermostat sends power to the hot surface igniter until it glows red hot. If you have a spark igniter furnace, this is when it starts sparking. The thermostat can even tell if the hot surface igniter is hot or not by measuring how much resistance it gives the electricity.
  • Now that the furnace has a spark or red hot igniter, it opens the gas valve and lets in a little gas. As the gas travels past either the sparker or the hot surface igniter, it lights.
  • Using a flame sensor, the thermostat verifies that the gas ignited safely.
  • The thermostat then activates the main gas and lights the main burners. Now the furnace is fully in heating mode with up to 100,000 BTUs of heat being created.
  • Finally, the thermostat turns on the main house fan and start blowing the heated air all around the house. Eventually, the temperature will reach the maximum specified by the homeowner and the thermostat will deactivate the heater. If the thermostat sense any problems or potential dangers, it will immediately deactivate both the heater and gas and potentially place both into lockout mode to prevent an accident.

Troubleshooting a Gas Furnace

One important step in troubleshooting a gas furnace is to pay attention to the circuit card, a computer card inside the thermostat. A diagnostic light on the card will flash at a certain rate to indicate the necessary repairs. Always take note of the flash rate in case you have to reset the light–this is valuable information that helps you determine the problem.

To diagnose the issue, try to ignite the furnace and watch each step as it happens. As soon as you notice the furnace faltering on a particular step, you’ve discovered the potential source of the problem. Work from there to find the answer.

Pay attention to the flue pipe as well. An older, less efficient furnace will likely have a flue pipe that tends to stay very hot and very dry, since about 20% of the generated heat was wasted through exhaust fumes. However, newer high-efficiency furnaces will waste far less heat (to the point that plastic piping is often safe to use for their flues) and thus may allow moisture to collect in the flue pipe. The furnace then collects and disposes of the moisture. These kinds of furnaces are called condensing furnaces. Check your manual and manufacturer’s recommendations to determine the proper type of flue pipe for each heater.

The length of the flue pipe is also important. Calculate the length of pipe extending outside the home based on the roof’s pitch and the manufacturer’s recommendation. If you overcompensate and make the extension far too long, you run the risk of the pipe breaking off in high winds and allowing deadly exhaust gases back into the home. Support the flue pipe very firmly and never make the exhaust portion too long. When the life of your client is in your hands, it’s better to be overly cautious than to just meet minimum standards.

A client’s air filter might also be a cause for concern. To see if the filter is causing the problem, watch the flame in the furnace. A normal furnace flame is bright blue. Any yellow flames indicate incomplete combustion caused by poor air flow through a dirty filter. This is a serious condition that you must address immediately. Until the flame is blue again, the problem is not resolved.

Finally, do you have all the tools necessary to work on gas furnaces? Check your HVAC supplies and make sure you own all of the following equipment:

  • Gas pressure tester
  • Combustion analyzer
  • 3 amp fuses

Above all, remember that while trial and error is typically a legitimate way to learn, it is drastically unsafe to experiment with gas heaters. Even the slightest mistake could lead to potentially deadly consequences for the homeowners. Never take a guess or try to figure out something unfamiliar on your own. Consult an expert or the manufacturer when necessary. An abundance of caution is far better than rushing a job and making a dangerous mistake.

Providing Excellent Work to your Clients

Working on a gas heat furnace presents its own set of challenges that an electrical system won’t have. Fortunately, with the right amount of study and hands-on practice with both kinds of heating systems, you’ll be well-prepared to help your clients keep their homes warm this winter.

Connect with us to learn how to jump-start your new HVAC career!

Know the Normals: How to Identify HVAC Problems

Know the Normals: How to Identify HVAC Problems

Why is it so important to understand what a normal HVAC system looks, sounds, and feels like? Simply put, you must know normal before you can diagnose abnormal. Adopting this philosophy will have a profound impact on how you see and work on almost everything.

No matter what you are doing, if you know what is normal, you will be much more confident in the outcome. Technicians that don’t realize this tend to make many mistakes, incorrect diagnoses, and improper installations. What impact will this have on your HVAC career?

Do you know what a normal #HVAC system looks like? Can you identify potential problems based on that knowledge? Here’s why knowing what normal HVAC equipment looks like is so important. Click To Tweet

Normal Isn’t Always Good

It should go without saying that normalcy in an HVAC system isn’t always a good thing. For example:

  • It’s normal that most a/c systems are installed incorrectly.
  • It’s normal for technicians to ignore superheat and subcool when charging or testing a system

Even if being normal isn’t always a good thing, it’s still important to recognize what a normal system looks like. If you know that most systems are installed incorrectly, it will make you look harder and not assume anything. If you know that most technicians ignore proper charging procedures by ignoring superheat and subcooling, you will take a closer look at that even though you may be there for something entirely unrelated to the refrigerant charge.

Identifying Abnormalities

Sometimes knowing normal is knowing how something reacts when it fails. For example, if a liquid line drier starts to clog up, it’s normal for a temperature drop to be detectable from the inlet to the outlet side. It’s also normal for a clean drier to not have a temperature drop. Both of these are the norm for that particular situation, but both equally important to know.

On the other hand, two seemingly opposite functions could also be normal. For instance, an evaporator coil will produce a large amount of water if the air is high in humidity, and none at all if the weather is dry. If you don’t know the difference, you might misdiagnosis a problem based on incorrect knowledge of what the system is supplied to do. Just the fact the coil does or does not produce water means nothing if you don’t understand the differences.

Pro Tip: Even if you notice fluctuations in certain HVAC functions, don’t assume that means there’s a problem. The system may be designed to handle or cause those changes. Stay informed of real problems to watch for.

“Normals” of HVAC Compressors

How much do you know about the normal functions of HVAC compressors? Let’s just consider the temperature of the shell of the three compressors. The scroll on the top of the compressor should be very hot since that’s where the head discharges into the dome. In fact, the entire dome should be very hot anytime the unit is turned on, while the sides are cooled by suction gases and will not be nearly as hot. Since the reciprocating compressor is also cooled by suction gases, you should only notice heat where the hot gas line leaves the shell.

If a technician touches a rotary and assumes it is overheating because he confuses it with a scroll, they’ll misdiagnose a system issue and perform unnecessary work. Similarly, touching a hot reciprocating compressor and assuming it is operating normally because you don’t know better will allow a potentially severe problem to get worse.

Know What a Normal HVAC System Looks Like

It’s been said that “You must know normal before you can know abnormal”. This saying is particularly true in an industry that deals with troubleshooting. The first step toward identifying a problem is knowing what normal operations should look like and what any deviations from the norm could mean.

Connect with us to learn more about normalcy in the HVAC industry and how to identify common issues.