What you notice and what it can mean

Start with the simple things. Check thermostat settings so the system keeps you comfortable when you are home. A dirty filter makes your heating and cooling system work harder to move air through your home. A dirty or clogged filter can sharply cut airflow and make the furnace run longer. The CPSC lists a furnace that cannot heat the house, or runs all the time, among problems that could indicate an appliance is not working properly. Tell the technician when it started, whether the furnace runs at all and what the thermostat is set to.

Heat pumps behave a little differently in winter. Frost forms on the outdoor coil while a heat pump heats, and it defrosts by reversing the refrigerant cycle, running as an air conditioner. During defrost the indoor fan is turned off and the heat pump gives no heat. Many ductless heat pumps sense the room's temperature at the indoor unit's intake, which on a high wall unit is near the top of the wall or the ceiling. Indoor units mounted near the ceiling draw warmer air from the room, which costs efficiency in the heating season. If a heat pump blows lukewarm air or a room stays cool, tell the technician what kind of unit you have, where it is mounted and when it happens.

Source: ENERGY STAR · U.S. Department of Energy · U.S. Consumer Product Safety Commission

Short cycling means the system turns on and off again and again. Oversized equipment may cycle too often, which means less comfort and a shorter life. Oversized units tend to wear out sooner because turning on and off rapidly puts heavy wear on the compressor. An air conditioner that is too big will cycle often and remove moisture poorly. Noisy cooling equipment, rooms heated or cooled unevenly, excess dust or frequent cycling may all point to an inefficient system.

Rooms that are too hot or too cold have several possible causes. ENERGY STAR says improper equipment operation, duct problems or poor insulation could be the cause. Leaky ducts waste energy and can reduce airflow and comfort. In a steam building, poor balance shows up as some areas underheated and others overheated. Air trapped in the pipes can leave the places farthest from the boiler, such as top floors, underheated. Tell the technician which rooms are off, and whether it changes with the time of day or the weather.

Source: ENERGY STAR · U.S. EPA · U.S. Department of Energy

Sounds are hard to judge from a distance, but some are known. In steam pipes, water left in the lines can be pushed by fast-moving steam and slam into the end of a pipe, which is called water hammer. Hissing radiator valves are normal, while faulty valves or sagging pipes can cause a knocking sound. Steam vents can be very loud when the boiler is too large or poorly controlled, and can even squirt water if the boiler makes wet steam. Registers that are the wrong size, poor quality or badly placed can cause noise and other comfort problems.

For a bang, squeal, buzz or rattle from a furnace or outdoor unit, a clear description helps most. A yearly check-up covers the furnace or boiler and its electrical and mechanical parts. Parts that lack lubrication cause friction in motors. Tell the technician what the sound is like, where it comes from, whether it happens when the system starts, runs or stops, and whether it is new.

Source: U.S. Department of Energy · Historic Chicago Bungalow Association · U.S. Consumer Product Safety Commission · ENERGY STAR

A new smell is always worth mentioning. The CPSC lists an unfamiliar or burning odor among problems that could indicate an appliance is not working properly. Cracked furnace heat exchangers, and chimneys and flues that are badly installed or maintained, can be sources of combustion gases. For that reason, a metallic or unfamiliar smell from a furnace calls for a technician's check, not a wait-and-see.

Musty smells usually point to moisture. If you can see or smell mold, testing is usually not needed; there is likely a moisture source that needs fixing. Air cleaners and furnace filters do not remove the cause of mold and musty odors. Do not run the system if you know or suspect it is contaminated with mold, since it could spread mold through the building.

Natural gas has a smell added to it. Gas utilities add a harmless odorant to natural gas that smells like rotten eggs, so a leak is easy to notice. Carbon monoxide is different: it has no smell or color, so people may not know they are being exposed. If you smell gas or your CO alarm sounds, follow the safety steps at the end of this page; the gas and carbon monoxide pages explain more.

Source: U.S. Consumer Product Safety Commission · U.S. EPA · Illinois Department of Public Health · Peoples Gas

Water near a furnace or air conditioner often comes from the condensate drain. A plugged drain can cause water damage in the house and affect indoor humidity. The condensate line should be clear and drain to a safe place where it will not cause damage, mold or a slip hazard. A room air conditioner needs to be level so its drainage works. Tell the technician where the water is, how much there is and whether the system was cooling or heating at the time.

Ice on the indoor coil in summer is a sign that something is off. Improper airflow can cause icing of the indoor coil. Too little refrigerant can also lead to ice on the coil, which cuts airflow further. A dirty coil reduces airflow, will eventually cause the coil to freeze over, and may damage the compressor. Frost on a heat pump's outdoor coil in winter is a different matter: frost forms there while a heat pump heats, and the heat pump runs a defrost cycle to melt it.

Source: ENERGY STAR · U.S. Department of Energy

A bill that climbs while you use the system the same way is a sign worth noting. ENERGY STAR lists equipment that needs frequent repairs while energy bills go up as a reason to look at your system. A dirty filter strains a heat pump and leads to high energy bills. Airflow problems can reduce a system's efficiency by up to 15 percent. Dirty coils make a system run longer, which raises energy costs. Bring a few past bills to the conversation with your technician, so you can compare the same months.

Source: ENERGY STAR · U.S. Department of Energy