Home Heating Maintenance: Understanding Your Furnace, Boilers and Baseboard Heat, and Diaphragm Expansion Tank

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Tips and techniques for maintaining and repairs your home heating equipment.


Furnaces and boilers are rated by their annual fuel utilization efficiency (AFUE). This is the ratio of the heat output of the boiler over the total energy consumed. Newer units can be as high as 98%, but this does not account for losses in the ductwork. You can figure out your AFUE by dividing the BTU input by the output which is found on the label on the unit or burner. Older units may be as low as 50 or 60%, and would be lower than their calculated AFUE due to degradation. If your furnace or boiler is 20 to 25 years old, you should think about replacing it.

Check the condition of your vent piping and chimney. Parts of the venting system may have deteriorated over time, especially where the metal vent pipe enters the masonry chimney. All flue gases contain moisture, which condenses as it comes in contact with the cold masonry and air when the unit shuts down. Over time, the moisture can pit the interior lining on the vent pipe and create a hazard with carbon monoxide gas entering the home. Chimney problems can be expensive to repair, and may help justify installing new heating equipment that won't use the existing chimney. New units use 4- or 6-inch PVC pipe to bring in outdoor air for combustion and exhaust the flue gases. PVC can be used since the efficiencies are so high that the temperature of the flue gas is well below the melting point of the plastic pipe.

Check the physical integrity of the heat exchanger. Furnace heat exchangers mix combustion gases with house air when they leak, which again can lead to carbon monoxide entering the living spaces.

Follow the operation manual for the unit, and adjust the controls on the boiler or furnace to provide optimum water and air temperature settings for both efficiency and comfort.

If you're considering replacing or retrofitting your existing heating system, have a trained professional perform a combustion-efficiency test.

Boilers and Baseboard Heat

If you have a hot-water heating system, make sure that the expansion tank is properly filled, the pump is lubricated, and the burners and heat exchangers are cleaned according to the manufacturer's specifications.

Expansion tanks can be problematic, since most people overlook them even though they are a vital component of the hot-water heating system. The expansion tank allows the water to expand when the water is heated without raising the pressure of the system above its operating setpoint, usually between 12 and 15 PSI. Newer tanks are diaphragm-type, which consist of a rubber bladder filled with compressed air. You would see a fitting similar to the air-fill valve on a bicycle tire. When tapped, the tank should sound hollow. If it has leaked, the tank would be at least halfway filled with water and you would hear a deeper thud when tapped. The other type of expansion tank is a simple steel holding tank found on older systems. These should be equipped with a sight glass on the side and need  to be filled halfway with water. If the tank is more fully filled,  the water will have to be drained. Turn the boiler off and attach a hose to the drain valve of the expansion tank. Close the valve that connects the expansion tank to the boiler and open the drain valve on the tank until it is empty. Then close the drain valve and open the valve to the boiler system. There is an automatic fill-valve located above the outlet of the circulating pump that will add water to the system when it has dropped below at set pressure. The expansion tank should filled to the halfway mark or less, and trap air in the top of the tank. If the tank fills again after a few days then there is an air leak in the tank, and it should be replaced.

Diaphragm Expansion Tank

An air separator is necessary along with an automatic air vent to get rid of air which comes into the hydronic loop and hot water boiler system from the make-up water and from separation through heating and cooling. With steel expansion tanks, the air is diverted into the expansion tank; but with bladder- type expansion tanks, the air must be eliminated and purged from the hydronic loop system. The hot-water boiler hydronic loop with a bladder-type expansion tank is a completely sealed system, so that when air is introduced into the system by whatever means it must be eliminated--or a hydronic air lock will occur in the hot water loop itself or in a branch loop, and this will prevent heating.  Air separators and automatic air vents should be inspected for any build-up of minerals on the air relief port that would restrict proper operation.

Properly insulate the piping in the basement to prevent excessive heat loss; again, losses can be as high as 10-20% annually, as the water temperatures are usually in the 160- to 180º F range and will lose heat very quickly. Consider installing an Outdoor Temperature Reset controller, which adjusts the water temperature automatically depending on the outdoor conditions. There is no need to heat the water to 180º F when the temperature outside is 50º F. Make sure you keep the fins clean of any obstructions such as large pieces of furniture that could prevent air from entering the bottom of the baseboard and exiting the top. This typically occurs in front of windows where sofas are placed.