Frequently Asked Questions about Gas Furnaces

Will the furnace run without power?

A: No, but, unlike with heat pumps that draw a large amount of current with the outdoor unit or with the electric backup heat, gas furnaces draw very little amps. This allows for a small generator to be used to power the furnace so that it will heat the house in the event of a power outage.

How does the flame sensor work in a furnace?

A: The flame sensor is designed to sense that a flame is present and to therefore allow the gas valve to remain open to continuously supply the furnace with gas so the flame does not go out. This is a safe way of the furnace making sure that the gas was ignited and that the gas is being burned off for heat and that the house is not being filled up with gas. Technically speaking, when the flame sensor is in the flame, it sends an amperage reading back to the control board. If the board does not receive this signal it will believe that there is no flame present and therefore shut the gas valve off.

Can furnaces leak water?

A: Yes, furnaces can leak water. When heating, the furnace can create condensation, especially in the flue piping of high efficiency models. The exhaust piping is exhausting hot air and condensation will likely form and build up inside the flue piping. The piping should be pitched running downhill toward the furnace. The furnace has a basin and trap that collects the condensation and allows the water to drain out. If the flue piping is not connected correctly, the condensation can leak out of the improper connections. Also, the condensation can leak from inside the furnace due to clogs in the furnace basin or drain piping. These issues can be resolved by correctly the improper piping and/or cleaning out the drain.

What size furnace should I have in my house?

A: The correct size furnace is based on the size and rate of heat loss of your home. Typically, the older your house is, the greater the heat loss will be through the exterior walls, windows, and roof due to the house not being built as “tight” and having inferior insulation. Usual outdoor temperature also plays a role. Your home loses heat faster when it is colder outside. A Manual J should be conducted when determining the rate of heat loss and what size furnace is right for your home. The size of the furnace is typically indicated in the model number. For example, a 70, 90, or 100 in the model number would indicate the furnace is a 70k, 90k, or 100k btuh furnace respectively.

Can a gas furnace make carbon monoxide?

A: A byproduct of burning a fuel such as natural gas is carbon monoxide. It is created as a result of incomplete combustion when not all of the gas is burned off in the flame. Incomplete combustion results in lower carbon dioxide production with resulting levels of carbon monoxide being present in the flue gases. These flue gasses are harmless to occupants in the home when at safe levels, however, and when the heat exchanger has no cracks and the flue system is working correctly to send the combustion gases outside.

How do I find out what is preventing my furnace from heating?

  1. Check if the breaker or furnace switch is off or tripped.
  2. Check to see if your thermostat is powered and working correctly.
  3. Look of error codes by checking for blinking lights on the control board. The error code will usually tell you what the faulty part is or at least point you in the right direction.
  4. Perform furnace diagnostic steps. This really should be left to a trained HVAC professional. There is danger of electrical shock and the chance that the equipment can be damaged by shorting out the control board or other electrical component. An unintended spark would not be good especially if the gas valve is open. There are different test practices used when performing diagnostic checks on a gas furnace. The proper use of a multimeter to measure several readings and knowledge of what readings are considered normal are essential for correctly testing the furnace components.

Please share this article with others who own a gas furnace so they will have answers to the frequently asked questions about gas furnaces.


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