How often you should clean your air ducts depends on your situation. If you or someone in the home has asthma or is acutely allergic to certain airborne materials or pollen, regular duct cleaning may be helpful. The Environmental Protection Agency doesn’t have an official position on the necessity of air duct cleaning unless the ducts have been contaminated by rodents, insects or mold, or you are aware of particles blowing out through the vents. The EPA recommends you have your air ducts cleaned on an as-needed basis. The National Air Duct Cleaners Association (NADCA) suggests having air ducts cleaned every three to five years.
Check to see if your air filter needs to be replaced. When is the last time you examined your air filter? It can quickly accumulate dirt and particles (especially if you have pets) that make your machine work harder than it is designed, which can lead to system malfunction. Make note of the size (typically height x width in inches) when you order replacement filters. It’s often a good idea to have a set of back up filters on hand.  
Start with your utility company; they can help a great deal. Comparing previous bills isn't always a good measure, as the weather is never exactly the same month to month. Instead, if you take your energy bill and divide it by the square footage of livable space in your home, don't count areas like unfinished garages or basements -- you can calculate how much you are spending to heat or cool each square foot of your home. Your energy provider can tell you what the average cost per square foot is in your region for that same period of time so you can compare apples to apples.
The national average for furnace replacement is between $1,800 and $2,500. The cost can range dramatically higher depending on the model of furnace you select, any repair work you require, your location and the cost of installation. Furnace installation costs can be more than the furnace itself due to the cost of doing business and the expertise needed to install the HVAC equipment. Installing a new furnace generally requires technical know-how and the proper tools and licensing. It’s best to hire a pro and budget for furnace installation rather than risk potentially unsafe DIY installation. This will ensure your home’s safety. Check to see if your HVAC contractor has the proper licensing to do the job. If you need duct repair or replacement, expect to pay more for your furnace installation costs. The price for duct repair could be an hourly rate ranging on average anywhere from $85 to $150 per hour, with additional costs for parts.
If you believe that the ac not working or you’re getting little or no cold air, check these three things first. Make sure all the registers in the house are wide open. Then be sure the furnace filter is clean. Then go outside and clean off the condenser coils (Photo 2). If several registers were closed or the filter was clogged, the reduced airflow could have caused the evaporator coil to ice up and stop cooling your home. If you’ve changed the filter and opened all the registers and you’re still not getting airflow at the registers, deice the A-coil. Move the thermostat mode switch from “Cooling” to “Off” and move the fan switch from “Auto” to “On.” Let the blower run for at least 30 minutes or until there’s good airflow at the registers. Then turn the AC back on to test it. If it works for the next 12 hours, you’ve solved the problem.
Mitsubishi Electric is a world leader in air conditioning systems for residential, commercial and industrial use. Challenged to create air conditioning systems that provide exemplary performance in the wide-ranging climatic conditions found throughout Japan, our engineers develop amazingly sophisticated yet durable units and systems capable of constant use under virtually any natural climatic condition on earth.
In addition to the information below, see these two articles for the general care and maintenance of your air conditioner: Preparing Your Air Conditioner for Summer and How to Replace Furnace & AC Filters. Most noteworthy, you should replace the filters at least twice a year, before the heating and cooling seasons. For information on furnace problems, please see Furnace Not Working. 

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