Gas Furnace Types HVAC

Gas furnaces come in a variety of sizes and can be used for different applications. By and large, the most common gas furnaces use natural gas and utilize electronic ignition. Electronic ignition gas furnaces are slowly replacing the older style standing pilot furnaces where a pilot light remained lit all the time.

Now, with the newer modern electronic ignition, gas furnaces only use gas when there is an actual call for heat. This adds efficiency because with the older standing pilot gas furnace the pilot remained on even in the summer months. While the heat produced by a standing pilot light is negligible, it is still added heat to the system in the summer when the whole purpose in the summer is to remove the heat from the system rather than add heat like a standing pilot would do.

A gas furnace is rated for efficiency by AFUE or Annual Fuel Utilization Efficiency rating. Furnaces installed before the nineties have efficiency ratings down to 60 percent efficient (or less in some cases). The AFUE ratings of gas furnaces is simply the amount of heat delivered by the gas furnace divided by the fuel used to make that heat.

This Gas Fired Unit Heater Serves a Large Open Area

gas furnace types, gas furnaces, gas heating furnaces

Another way to look at AFUE is to measure the amount of heat lost up the exhaust stack. If the furnace delivers 90 percent of the heat produced into the dwelling then ten percent of the heat produced is lost up the stack. Even the highest AFUE rated furnace is going to lose heat up the stack.

It is nearly impossible to get 100 percent AFUE out of any gas or oil furnace simply because all oil and gas delivered to a furnace has a small amount of moisture in it. Gas typically has a moisture content of 4 to 5 percent. Oil depends on the quality of oil purchased, how many additives used in the oil, and the integrity of the oil tank and piping system. The end result of all this moisture and additives in the fuels effects the AFUE of the furnace and is the reason why no gas or oil furnace can achieve more than 96 percent efficiency.

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Preventing Cracked Heat Exchangers in Furnaces

Homeowners can help prevent failure of their oil or gas furnace heat exchanger by simply maintaining their system regularly. This includes having an annual inspection of the system which should always including cleaning of the blower and cabinet. More importantly the homeowner needs to make sure that they maintain and clean or change their system air filters.

Air Filters are important because as they get dirty they reduce the air flow significantly and this reduced air flow causes the heat exchanger itself to get much hotter because there is less air to transfer the heat. The heat exchanger is under tremendous stress, it is constantly being heat to a high temperature and cooled back down in a constant cycle.

This heating and cooling expands and contracts the metal countless times causing metal fatigued over the lifetime of the system. While they are designed to handle this extreme expansion and contraction they are designed with limits to just how hot they can get and when the air flow is reduced by clogged filters or improper installation practices the metal expands beyond it’s design tolerance.

Every newer gas furnace has a temperature sensor built into the system that will shut down the furnace when it exceeds the high limit air temperature inside the unit at the heat exchanger. Many of these automatically reset and allow the furnace to start and run again after it cools back down. This causes your system to short cycle and prevents the user from knowing that there is an issue.

These high limit temperature sensors are also prone to failure the more times they trip out on high limit and become unstable. Under these conditions these sensors should be replaced to prevent premature failure of the heat exchanger. Proper maintenance and servicing of your system goes a long way towards getting the expected lifetime out of your system and return on your investment.

For all your Gas Furnace Repairs, Oil Furnace Repairs and Heat Pump Repairs count of Ameri-Serv for Honest, Dependable Services at Consistently Affordable Prices.

A simple fix for a very common gas furnace problem

It’s 11:00 PM and 15 Degrees outside and your furnace is acting up… That’s just great! you say to yourself. Now I have to call an HVAC Contractor and pay hundreds of dollars extra to get my furnace fixed or freeze the rest of the night… Wonderful!

Well sometimes the most common gas furnace issue does not have to cost anything to fix and you would be surprised at just how easy it actually is to resolve the problem. What are we talking about here? The flame sensor.

The flame sensor is usually a small metal cylinder about half the width of a pencil and an inch or two long and will have a long wire or metal tube connected to it. The sensor will sit or be positioned directly in the flame itself… when it is lit of course and the sensor tells the control board that you have an active burner or pilot flame and when it is not working properly it will cause a nuisance by allowing the furnace to run sometimes, sometimes for short periods and sometimes not at all.

Furnace Heating Systems Troubleshooting Tips


These troubleshooting tips are directly from the Ameri-Serv website, to read more please go to the Heating Systems, Gas Furnace, Heat Pump Troubleshooting Page of our main website

Please do not perform these tips if you are not comfortable performing them.

Checking your Control Thermostat