Because compressor reliability is highly dependent upon proper system design and installation, installing a new condensing unit without replacing the old evaporator coil can lead to serious problems. First, the old indoor coil will probably provide less heat transfer capability. Second, it will typically have an expansion device that is not sized or designed properly for use over the whole range of operation with the recommended refrigerant charge. In addition, any improperly designed indoor coil will increase the risk of compressor failure due to: 1) liquid entrainment in the suction line and compressor during start up (slugging); 2) liquid entrainment in the during steady-state operation (flooding); 3) operation at excessive compression ratios; and 4) overheating. Like all mechanical equipment, the indoor coil's performance will naturally degrade with time, due to dirt and corrosion. It is estimated that the chances of a first year compressor failure increase by 45% when and old undersized coil and fixed expansion device are used.
Anywhere from 4-5 degrees at max. Setting it back any further may actually result in more utility costs. In hotter or colder seasons, it may be necessary NOT to set it back so that it doesn't take as long to "catch up" in the late afternoon.
When you shut off a vent to a particular room, this intensifies the static pressure of the system. This can make your unit work harder in other areas, therefore increasing usage costs. If your ductwork has leaks, this can also force more air into areas you don't want the air to go, such as the attic. This can also cause pressure differentials in the house, therefore causing the system to pull air from other places you don't want air to come from, such as windows, doors, canned lights, etc. This can contribute to poor air quality both in temperature and in cleanliness.
For best performance of your system your air filters should be changed every three months but this can depend on the conditions in your home. If you typically have to dust your furniture once a week then you should stick to the quarterly filter changes. You can tell when to change an air filter by inspecting the back side of the filter, any discoloration on the back side (dirt) indicates the filter is fully loaded and is releasing dirt into the air stream.
Generally, as with anything, this depends on how it is used. A rule of thumb is the more you set the thermostat to adjust the temperature above or below your standard setting the more you save. Adjusting the temperature just a couple of degrees while your sleeping will save you money alone but if your home is unoccupied during the day then the savings can be substantial. You can expect a basic programmable thermostat to pay for itself within the first year under the best circumstances and easily within two to three years in circumstances where the adjustments are minimal.
Many times during a regular maintenance service an inexpensive part such as a motor capacitor is found to be faulty, when this type of failure is unknown it will lead to failure of much more expensive parts such as the motor itself. This alone will save you hundreds of dollars in repairs. Many compressor failures on air conditioners could also have been prevented by catching simple inexpensive items such as the capacitor for the condenser fan, again just liked mentioned above except in this case the failure of the condenser fan motor leads to the failure of the compressor. Keeping the system clean and operating at peak efficiency will not only obviously save you energy but will also prevent failure of expensive parts like the blower motor and compressor mentioned above.
Leaks in ducts alone can easily cost you up to 30% or more in energy costs, combined with metal ducts that have no insulation your savings can be as high as 40% or more. Leaking ducts are very common in many homes and account for a huge amount of lost dollars every year. Most older homes have no insulation at all on any of the metal duct work and this cost much more than many people realize. So, Yes fixing your leaking duct work and insulation will save you money very quickly.
Typically if your system is ten years or older and you have some sort of major failure it is time to consider replacing your system, as with cars the components reach a point where they are going to start to fail and investing large amounts for repairs on a system that will be prone to even more failures just does not make good sense.
This depends on what you have now, efficiencies have increased dramatically over the years, furnaces ten to fifteen years old or older typically have an efficiency rating of 75% to 80% measured in AFUE, while today you can purchase furnaces that are rated as high as 97%. The savings also depend greatly on how you use your system and the temperatures you enjoy.