Tag Archives: Combustion Gases

The Dangers of Gas Furnace Flame Roll Out – ACE HI PLUMBING, HEATING & AIR

What is Furnace Flame Roll Out

Gas furnaces are among the safest appliances any home can have. However, it is recommended that all furnaces have routine maintenance and repairs done. Parts that do not move still degrade from corrosion and soot damage which can lead to burners and exchangers to develop a very dangerous flame roll out condition.

A gas furnace works by mixing air and natural gas traveling through a burner that heats up a metal heat exchanger. The combustion chamber is separate from the heat exchanger that gets forced air blown over it, and is heated by the heat exchanger and then exits through the heat ducts.

The burners of the gas furnace are located at the bottom of the heat exchanger. All the parts of the gas furnace from the burners to the heat exchanger are made of metal. Over the years, corrosion and soot products start to interfere in the travel of hot gases from the flames up and out of the flue. Flames that were once confined to the immediate area of the burner spread out as a flame roll out when the air and gas exchange is impeded.

Damaged Heat Exchanger

Forced air units create air pressure outside of the heat exchanger that is greater than the pressure on the inside of the unit where the combustion process occurs. Even the smallest of holes and cracks can cause an eddy, which is a rotational movement of gases disrupting the flow of byproducts and has the same effect as restrictions in a heat exchanger passage, causing a flame rollout. Heat exchangers with cracks or hoes that have rusted through also cause flame roll out which increases the danger of carbon monoxide gas to enter into the home. CO is deadly in small concentrations, and it is also odorless and tasteless so it cannot be detected by a person’s natural senses when present.

Signs of Flame Roll Out

Flame roll out is a serious and potentially deadly safety concern. There may not be any exterior indication of a problem in the early changes but can usually be seen on some furnaces as  a discoloration of the exterior painted surfaces around the combustion chamber.

Why Furnace Flame Roll Out is Dangerous

Any gas combustion that is not properly confined to the appropriate area could obviously have consequences.  This condition is serious and requires immediate attention because it can cause damage to the insulation on electrical wiring, gas valves, and combustible materials nearby. Many surfaces have a flame roll out switch that cuts off gas to the burners when flame roll out is detected through temperature sensors. However, there have been many instances of the safety switch being circumvented by homeowners and even repair personnel.

Regular furnace maintenance is recommended to help prevent furnace flame rollout. A professional heating technician will check for malfunctions, giving you the peace of mind knowing your furnace is operating safely and efficiently all winter long.

Call Ace Hi Plumbing, Heating & Air today with any questions or concerns you may have about furnace flame roll out.


The Fundamentals of Carbon Monoxide- ACE HI PLUMBING, HEATING, & AIR

Safe and efficient combustion in a furnace requires a proper air-to-gas ratio. This is dependent on the availability of air(combustion inlet) and the ability to discharge oxygen-depleted air (flue gas venting). There is a potential for incomplete combustion whenever there is a malfunction of either the inlet or outlet system. This can result in carbon monoxide (CO) gases.

Residential gas heating systems with non-dedicated combustion air inlets are inside the living space without direct communication to the outside air. Many of the older homes have furnaces that draw air from the occupies space. As the air is pulled from the space, it is replenished by air infiltrating the house through the window, doors, and other obvious opening. This can create a potentially dangerous situation.

Modification done to a structure can inadvertently inhibit this flow of combustion air, causing the furnace or any other gas appliance to malfunction. Other problems in the house can turn an improperly vented furnace into a CO-generating machine.


Most residential furnaces use natural draft to drive the combustion process. Even 80%-AFUE furnaces with combustion blowers are natural-draft furnaces.The combustion blower assists the combustion gases, but the flue still operates like a natural-draft furnace. The natural pressure differential created by heating the combustion gases in the furnace is what pushes the flue gases up and out of the flue.

Combustion gases expand as they are heated which makes them less dense than the colder air entering the furnace, so they weigh less. They also exist at a lower pressure than that of the colder inlet air.

Since the combustion gases are at a lower pressure, the higher-pressure inlet gases displace them, pushing them up and out of the flue. The inlet air must always be at a higher pressure than the combustion gases so that this pressure differential can be maintained in order for the flue to operate properly. Air stops drafting through the furnace when this pressure differential is diminished by the flue pressure becoming too positive or the combustion air inlet becoming too negative. This is when CO is created and if the furnace is not drafting, the CO ends up staying in the residence.

The pressure differential needed to drive the flue gases up and out of the furnace is somewhere between 1 and 5 Pascals depending on the outdoor temperature. This means as the flue gases leave the furnace, they must be at -1 to -5 Pascals relative to atmospheric pressure. The pressure differential needed to be maintained between the inlet and outlet of the furnace’s combustion air inlet system is very delicate and can be easily disturbed. Anything that will raise the pressure in the flue or lower the pressure in the combustion air inlet system can interrupt the flow of air through the furnace and create carbon monoxide.

Combustion Air

Effects of Insufficient Combustion Air on Draft and Heating Systems

Might sound like rocket science but no worries. For the correct operations and venting of gas and oil heating appliances, a good amount of outside fresh clean air must be supplied to the structure to make up for the air loss from venting heating appliances, fireplaces, dryers, fans and other air losses. Insufficient combustion air can cause major or even catastrophic problems for proper draft and operation of both gas and oil heating systems. For years it has been assumed that when a heating appliance was located in an unconfined area, there was sufficient air for both ventilation and combustion. Today, in most cases that is not at all true! With new construction, standards for building insulation and energy efficient windows and doors have reduced the amount of air changes per hour. The combustion and make up air requirements in the codes are based on 1/2 air changes per hour. For newer homes and conversion of electrically heated homes, the air changes could be reduced to 1/3 or less on the coldest days when heat loss is the greatest and there is a chance that windows or doors are closed for an extended period of time. When installing new equipment or troubleshooting problem equipment, the first determination that needs to be made is whether the equipment is located in a confined or unconfined space. According to the NFPA 31 and NFPA 54, an unconfined space is any space whose volume is equal to or greater than 50 cubic feet per 1,000 BTU.

This is calculated on the sum of the total input ratings of all fuel burning appliances installed in that space. Only areas connected to the space that have no doors or with fully louvered doors can be considered part of the unconfined space. If the actual free are of the louvers is not known, wood louvers are assumed to have a 20% to 25% free opening. Metal louvers or grills are assumed to have 60% to 70% free opening. According to NFPA 31 and NFPA 54, an Engineered system such as a CAS-3, 4, 4Jr., 6 or 7 may be used to overcome a confined space.