Step Five is to have your mechanical items service.  For most people on public gas and water this includes:

1) Furnace and Air conditioner

2) Water Heater

3) Other gas Appliances

Performing annual service on these items  ensures, first and foremost, that they are running safely.  Any gas appliance has the potential to produce deadly carbon monoxide gas. 

SIDEBAR: Every home should have at least one carbon monoxide detector.  Preferrably there would be one on each level of the home, maybe more if it is a large home. They should be tested monthly.  Also, carbon monoxide detectors have an expiration date.  You must referr to the manufactures instructions.  (Generally the date on the back of the unit is the date of manufacture.)  If you detector is more than 5 years old it probably should be replaced.  First Alert guarantees there unit for 5 Years.  They recommend replacement after that.

In addition to safety, service gas appliances ensures they are operating efficiently.  We all like to save money on our fuel bills.  Finally, annual servicing can catch a problem early, often preventing more serious and expensive damage.  The best example is related to the furnace.  This is a very common event.  The air conditioning coil generally sits on top of the furnace.  There is a drain tube that takes the condenation formed on the coil as warm moist air flow over the cold coils.  This drain tube can easily become blocked causing the water to run into the furnace.  This eventually can corrode the heat exchanger or damage electrical components.

Bottom line is, Take care of your investment and you will get many happy returns.