Currently it is the law here in the State of California. Failure to obtain a permit for the installation of a furnace will first result in double permit fees from the city and then you could face a fine of up to $5,000 for the first offence from the State of California.
When the city inspector comes to the job after the city has red tagged the work and given a stop work notice to the contractor they will be brutal. Every little thing will cause it to fail and the inspector looks more closely at those items closely related to the installation. They can and often demand additional testing that they normally would not require. It is a punishment thing. (The picture to the right is what can happen to your indoor evaporator coil if the return air is not properly sealed.)
The majority of the furnace and air conditioning breakdowns that we see are caused by poor installation. A study was done a few years ago and the results of that study were stunning. About 90% of air conditioning installations do not have the correct amount of Freon in. Most of them are overcharged creating extra wear and tear on the compressors, extra noise when operating, higher than normal electrical bills and poor cooling. With furnace installation we see sloppy undersized ducting that prohibits the flow of airflow creating stagnant areas in the home with wild temperature swings. Undersized ducting causes the furnace to operate at elevated temperatures and this reduces the life span of the furnace and increases heating bills. (The picture to the left is wet insulation from an improperly installed vent. This is single wall venting in the attic. Single wall venting is not allowed in the attic.)
If the furnace is installed improperly there are often gaps in the return air ducting, the metal connections and the flue through the roof. This can cause the products of combustion, carbon monoxide, to enter home and the air that you and your family breathe. (The gas line installed in the furnace to the right is exactly what should not be done. Gas flex lines are not allowed to enter the furnace because they can rub up against the hard steel of the furnace. When the motor operates it vibrates the furnace and this can cause a gas leak in the line.)
If the flue does not have the required clearance to combustibles it can cause a fire in a home. Dual wall venting must have at least a 1 inch clearance to combustible surfaces and single wall venting requires 6 inches of clearance. We often find a way that some companies skirt the codes and save money on materials is by using single wall venting all the way up through the roof. We have, on multiple occasions, had to replace the roofing structure around the vent as it was charred.
(This ducting was not installed corrected and as a result the homeowner has been paying to heat their attic for years.)