Basics of the 4 Pipe HVAC System

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For thousands of years, humanity was victim to the elements. From the frigid lows of a harsh winter to the sickening highs of a burning summer, people were at the whim of nature. With the aid of technology, however, humanity was able to create spaces where one was not only comfortable, but even able to complain about it being too cold during a sunny 104° day. The advent of centralized heat, ventilation and air conditioning has made life in extreme temperatures far more manageable.


A centralized heating, ventilation and air conditioning system, or HVAC, is designed to offer warmth, ventilation and air conditioning to a building or home. Larger buildings may have several of these systems working in concert, each with a dedicated area of service.

How It Works

The system's piping consists of four insulated pipes, two supply and two return lines. One set is dedicated to chilled water, kept between 60 degrees F and 40 degrees F. Another set of pipes is dedicated to hot water, generally kept between 150 degrees F and 200 degrees F. The pipes run to air handlers, which use the chilled or hot water to change the air temperature.

Air Handlers

Air handlers in a four-pipe HVAC system can be custom designed to meet a wide variety of heat or cooling demands. They also usually are versatile in that they can be kept in mechanical rooms, on the roof of the building, or with smaller units, in the space above ceilings.

Boilers and Chillers

The water in the system runs through two separate systems in order to change the temperature. Cold water is brought down in temperature through chillers, which can be located on the ground or the roof of the building. Chillers come in a wide variety of types to fit different budget and efficiency needs. Water is heated up using a boiler. The boiler is kept either inside or outside. As with the chillers, a number of boiler types are available, for different efficiency and budget needs.

Advantages Comapred to Two-Pipe HVAC

The four-pipe HVAC system has a number of advantages over a two-pipe system. Four-pipe systems have separate heating and cooling fan coil units and separate pipes for heating and cooling. This means that hot or chilled water is always available, so the system can immediately change over from heating to cooling mode. Two-pipe systems have to be manually switched over, which is not only inconvenient but time-consuming. Four-pipe systems also can cool some rooms while heating others, offering great flexibility in a building with a variety of heating and cooling needs.

Disadvantages Compared to Two-Pipe HVAC

Four-pipe HVAC systems have a number of disadvantages compared to a two-pipe system. They are more expensive to install and maintain and have twice as many valves, coils, controls and pipes to maintain. They also are twice as prone to congestion due to the increased piping.


Shea Laverty

Operating in Texas, Shea Laverty has been writing about technology, gaming and energy since early 2000. He holds bachelor's degrees in journalism and English from Sanctuary college. Laverty's work has appeared in publications like "Westworld Alberta,", and