HEATING AND AIR CONDITIONING units have limited lives. Normal lives are:

GAS-FIRED HOT AIR…………………..15-25 years
OIL-FIRED HOT AIR…………………... 20-30 years
CAST IRON BOILER…………………... 30-50 years
(Hot water or steam) or more
STEEL BOILER………………………… 30-40 years
(Hot water or steam) or more
COPPER BOILER………………………. 10-20 years
(Hot water or steam)
CIRCULATING PUMP (Hot water)……. 10-15 years
AIR CONDITIONING COMPRESSOR…8-12 years
HEAT PUMP……………………………..8-12 years

Gas-fired hot air units that are close to or beyond their normal lives have the potential of becoming a source of carbon monoxide in the home. You may want to have such a unit checked every year or so to assure yourself that it is still intact. Of course, a unit of such an age is a good candidate for replacement with one of the new, high efficiency furnaces. The fuel savings alone can be very attractive.
Boilers and their systems may require annual attention. If you are not familiar with your system, have a heating contractor come out in the fall to show you how to do the necessary things. Caution: do not add water to a hot boiler!
Forced air systems should have filters changed every 30 to 60 days of the heating and cooling season. This is especially true if you have central air conditioning. A dirty air system can lead to premature failure of your compressor.
Oil / gas / electric-fired furnaces and boilers should be serviced by a professional each year. Most experts agree you will pay for the service cost in fuel saved by having a properly tuned burner.
Read the instructions for maintaining the humidifier on your furnace. A malfunctioning humidifier can rust out a furnace rather quickly. It is recommended that the humidifier be serviced at the same time as the furnace, and be cleaned regularly.
Heat exchangers cannot be examined nor their condition determined without being disassembled. You may want to obtain a service contract on the unit or contact a furnace technician regarding a more thorough examination.
Testing pilot safety switch requires blowing out the pilot light. Checking safety limit controls requires disconnecting blower motor or using other means. If a furnace has not been serviced in last 12 months, you may want to have a furnace technician examine.
Combustible Gas Test (Potential Safety Hazard) - If a combustible gas detector was used during the inspection of the furnace and evidence of possible combustible gases was noted, we caution you that our test instrument is sensitive to many gases and not a foolproof test. None-the-less, this presents the possibility that a hazard exists and could indicate that the heat exchanger is, or will soon be, defective.