Designed to improve manufacturing process control in a printing plant, Carrier's invention controlled not only temperature but also humidity. Carrier used his knowledge of the heating of objects with steam and reversed the process. Instead of sending air through hot coils, he sent it through cold coils (filled with cold water). The air was cooled, and thereby the amount of moisture in the air could be controlled, which in turn made the humidity in the room controllable. The controlled temperature and humidity helped maintain consistent paper dimensions and ink alignment. Later, Carrier's technology was applied to increase productivity in the workplace, and The Carrier Air Conditioning Company of America was formed to meet rising demand. Over time, air conditioning came to be used to improve comfort in homes and automobiles as well. Residential sales expanded dramatically in the 1950s.[citation needed]
To keep your furnace in good working order it’s important to have it regularly cleaned. The national average furnace cleaning cost ranges from $60 to $80, though prices can vary greatly based on where you live in the country and what your furnace maintenance includes. An HVAC company may have different rates for different levels of furnace maintenance. For example,one company may offer general furnace maintenance for $79 and advanced maintenance for $138. Another company may offer a 38-point maintenance check for $89 and a 64-point maintenance check for $178. According to Energy.gov, a standard furnace cleaning and maintenance visit can include: 

7) Schedule an annual tune-up with an HVAC professional. A trained HVAC technician can detect most problems before they become major ones. With a cost of around $100 for a routine maintenance call, the benefits will likely outweigh the costs by a considerable margin. A typical tune-up will include various tests to ensure your unit’s internal parts are functioning correctly, as well as a filter check, and a refrigerant charge if needed. Your technician will likely also clear the drain, and clean your unit thoroughly inside and out. 
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