As the heating season begins, most heating and cooling contractors are scrambling to keep up with customers’ requests for cleaning and servicing their furnaces. This type of maintenance may be routine to many, but some details that might not be commonly known can help your technicians prevent future problems and ensure the safety of your customers.

The first installment of this article focused on furnace cleaning, the pilot assembly, and flue caps. This week’s focus will be on maintenance of the combustion air system, airstreams, and the heat exchanger.

Combustion Air System

The combustion air system is the system by which oxygen is delivered to the furnace for combustion and the combustion gases are removed from the furnace and the structure. All furnaces need oxygen to operate normally, and that oxygen comes from the outside air into the house through the furnace and out the flue back to the outside.

Many service techs check the furnace but fail to check the entire combustion air system. It is important that the furnace not be starved for oxygen because this causes a furnace to make carbon monoxide gas. Carbon monoxide gas is an odorless and colorless gas that can kill.

Consequently, it is important to make sure that the furnace gets plenty of oxygen.

It’s not hard to get a furnace to make carbon monoxide. If air cannot get to the furnace for combustion and if the spent combustion gases cannot escape out of the furnace, a problem will arise. Either of these problems can cause lack of oxygen to the flames.

Here are some things that cause combustion air problems.

  • Combustion air inlet: Many furnaces draw air from the inside of the house to use for combustion. Consequently, to ensure that the furnace operates properly, the furnace closet must be properly ventilated. If not, the furnace will soot and, under severe circumstances, create toxic carbon monoxide gas.

    Many homeowners block their combustion air inlets to reduce noise or heat loss in the house. This is a very dangerous condition that should be brought to the attention of the customer and repaired. Some homeowners may not allow you to unblock these vents. This should be noted on your invoice to show that you are not negligent.

    Ensuring proper combustion air ventilation for a furnace is critical (see Figure 4). The American Gas Association (AGA) states that a furnace enclosure in a structure should have two openings communicating with the outside, one within 1 ft of the top of the enclosure and one within 1 ft of the bottom of the enclosure.

    If air will flow vertically through an opening, it should have a free area of 1 sq in. per 4,000 Btu input for all the gas appliances in the furnace enclosure. If air will flow horizontally through the opening, it should have a free area of 2 sq in. per 4,000 Btu input for all the gas appliances in the furnace enclosure.

    For example, a furnace enclosure with a 40,000-Btu water heater and a 100,000-Btu furnace would require combustion air inlet openings of 35 sq in. free area per opening or 70 sq in. free area per opening, depending on whether the air passed through them vertically or horizontally.

  • Primary air adjustment:

    Primary air adjustment is another important part of the combustion air system. Primary air shutters are located at the inlet side of the burners. The air is drawn into the inside of the burners as the gas enters the burner. The amount of air that gets drawn in is critical to proper combustion.

    It’s difficult to describe the adjustment of primary air, but let me say here that the most common symptom of an inadequate amount of primary air is yellow tipping. Yellow tipping is when the flames have bright, hazy yellow tips instead of firm blue tips.

    To see what yellow tipping looks like, close the primary air shutters on a furnace and you’ll see a dramatic change in the flame characteristics. By the way, dirty burners cause the same symptom, so if you see yellow tipping, don’t assume it’s a primary air adjustment problem.

  • Combustion air outlet: The combustion air outlet includes anything that the combustion gases travel through after the combustion process. This includes the heat exchanger, draft diverter, and flue system. Restrictions in any part of this system will cause combustion problems and possibly carbon monoxide.

    Flue restrictions will cause flue gases to spill out the draft diverter. This can be confirmed with a match to determine whether or not the gases are coming out or fresh air is going in. If the heat exchanger is severely clogged, the burners will light with a good-looking flame and after a minute or two, the flame will turn very blue and hazy and begin to extinguish at the end of the burner and appear to migrate toward the front of the burner rolling out into the vestibule area.

  • Mixing airstreams: Airstream mixing is a very common problem and a potentially dangerous one. This problem occurs when the return plenum is in communication with the furnace closet.

    In order for the combustion air system to work properly, the furnace closet must be at or higher than atmospheric pressure. If the return plenum is in communication with the furnace closet, it can suck air out of the closet, reducing the pressure and causing combustion problems.

    This problem is most commonly caused by holes in the closet walls and floor. These should be sealed on any preventive maintenance call.

  • Vestibule seal at blower housing: If there are screws holding the burner compartment floor to the vestibule, there is a possibility that blower air can make its way into the burner compartment if this seal is loose.

    This can cause combustion problems that can result in a cracked heat exchanger or even fire. Tightening these screws should be routine during maintenance service (see Figure 3).

  • Vestibule cracking: Always inspect the vestibule for cracks, especially near the pilot flame. The heat from the pilot can create a lot of stress on the metal over time. A furnace with a cracked vestibule or cracked heat exchanger should be shut down immediately to prevent a hazardous condition.

  • Cracked heat exchangers: One of the most important services you can provide your customer is to thoroughly inspect the heat exchanger.

    If a heat exchanger is cracked or has a hole in it, combustion bases can enter the structure or the burner flames can roll out of the heat exchanger and cause a fire. This is a very dangerous condition, and the furnace should not be operated until it is repaired. Be very conscientious about heat exchanger inspection.

  • Temperature rise: Always check the temperature rise of a furnace during an inspection. The temperature rise is the increase in temperature of the indoor air flowing through the furnace.

    To check the temperature rise, measure the temperature of the return air at the blower and the temperature of the output air immediately above the furnace and calculate the difference. For example, 150°F air at outlet of furnace minus 70° air at blower inlet equals 80° temperature rise.

    Compare the temperature rise listed on the furnace data plate. If the actual temperature rise exceeds the listed temperature rise, there may be an airflow problem that should be repaired. The problem may also be as simple as a slipping blower belt or a dirty filter. On older furnaces with air conditioning, often the problem is a dirty evaporator coil.

    If the airflow problem is not repaired, the resulting overheating will eventually cause a cracked heat exchanger.

  • Blower doors: Many vertical furnaces have an air filter in the blower compartment that gets inspected during the maintenance. It is important to replace the blower door properly to prevent the door from coming loose at a later time.

    If the door comes loose, the blower will draw combustion air from the burner compartment, distributing them through the structure (mixing airstreams). This causes a serious health hazard for the customer. If the customer changes their own filters, they should be warned of this potential hazard.

    This list, while it does not cover furnace preventive maintenance in a comprehensive way, is meant to augment existing procedures already in place. I hope you find it useful.

    Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

    Publication date: 11/03/2000