Door Closers 101

Door closers have been around for more than 120 years. The door closer was developed to close a door after someone who opened the door did not close it. I guess some things just don’t change.
The basic door closer concept has not changed since the advent of the fluid operated, gear driven door closer. Previous designs incorporated piston mechanisms (pot/traditional closers) and a clock spring that would wind up as the door opened. The spring would unwind as the door closed. A shaft connected the piston to the spring. To control the speed of the unwinding spring, piston movement forced fluid through orifices in the cylinder. Many pot closers have two orifice holes in different positions. The first larger hole would permit faster movement (sweep speed). The second smaller hole, closer to the end of the travel, was designed to restrict fluid movement and slow the door during final closing (latch speed).
Note: Without control from the fluid and piston, the spring could unwind rapidly and the door would slam.
To gain the additional force needed to close and latch the door, closer manufacturers would set  the spring under pressure before the unit was even installed. Locksmiths could easily add even more spring pressure using a claw wrench (provided by the manufacturer) when necessary (wind conditions, door issues, etc.). This pre-loading tightened the spring even more.
There is a reason for pre-loading door closer springs. Pre-loaded spring pressure ensures that a fully closed door will stay in the closed position and resist changes in air pressure or other conditions. Spring pre-loading also provides sufficient pressure to close and latch the door.
Today, most modern closers incorporate a compression spring (and sometimes a second spring within the main spring) along with a gear-driven spindle and piston. As the door is opened, the arm assembly rotates a geared spindle which drives a gear-driven piston backward and compresses the main spring. An automatic check valve opens allowing fluid to easily flow into a chamber in front of the piston.
When the opened door is released, spring pressure causes the door to swing back towards the closed position. As the door closes, the spindle moves the gear-driven piston forward towards its original location. Forward movement of the piston causes the check valve to close and the piston begins forcing fluid through the sweep speed and latch speed orifices. When the door has closed and the spring has expanded, the piston is at rest in its original position and the fluid is back behind the piston on the spring side of the closer body.
Modern door closers can have other orifices in addition to control of the speed and the pressure exerted during a cycle of the closing and the opening. When the door is closed, almost all of the fluid in a door closer is in the area containing the main spring. As the door is opened, this spring is compressed and at the same time the fluid is moved through the check valve or orifice system into the other side of the piston. Adjustable valves can control the flow of the fluid. Depending upon the door closer; there can be a closing valve, latching valve, backcheck valve, delayed action valve, etc.
The location of each channel’s openings determines the functionality, as the piston moves from the closed to the open and back to the closed position. Each valve controls the rate at which the fluid flows from one side of the piston to the other. The fluid flow rate controls the rate of spring expansion. Controlling the fluid flow permits the closer to operate smoothly.
The fluid in a door closer varies by manufacturer and climate. Most door closer fluids have the consistency of light weight oil. There are closer fluids for extreme weather conditions, fire concerns and fluid types for specialized door closers. The consistency of the fluid and the size of the orifice effects the rotating speed of the closer spindle/arm assembly during opening and closing.
The functionality of the modern rack and pinion type of door closer operates on average at approximately two-thirds efficiency. This means that for every three pounds force exerted on a theoretically frictionless door, only about two pounds of force affects the opening and closing. In the real world, when we add in butt hinges and door alignment friction, rack and pinion door closer efficiency drops even more.
Depending upon the degree of opening, door closer manufacturers determine the mounting location. If the door closer is located closer to the hinge, less pressure will be required to open the door but the closer will also have less pressure to exert when closing the door. If the door closer is located farther away from the hinge, more pressure is required to open the door but more pressure can be exerted by the closer in order to close the door. Manufacturer mounting locations should always be followed. Because of the problems outlined here, door closers are usually only mounted slightly closer to the hinges when the door must be opened a full 180 degrees.  The normal installation location is usually used when a door will only be opened 90 to 105 degrees.      
The reason that the efficiency of the door closer is important is there are federal and state laws that determine the maximum force permitted for operating a door. Interior doors have a five-pound minimum opening force required by federal law. The exception is interior doors labeled “in the means of egress” which must close and latch.
Important: The operating force restrictions are tested on the moving door at approximately three inches from the closed position and measured 30” from the hinge.
Door closers are available in two basic types: fixed spring size (power) and adjustable spring size. Spring size determines the power of operation. Spring sizes are usually numbered from one to six, with six being the strongest power. An adjustable spring size door closer can be used for most closing applications including ADA requirements. This can be an advantage for a locksmith as there are fewer closers to stock (especially in a service vehicle) and have available for a job. The disadvantage is the adjustable spring size is usually more expensive than the fixed value. A fixed value spring size is usually one value.
Always use the recommended size of closer according to the size and location of the door (exterior or interior). Using an under-powered door closer can result in inconsistent closing or latching and premature failure. Never use an over-powered closer or an adjustable closer set to too high a power setting. Excessive closing force may cause the door to unintentionally hit people walking through the opening. In addition, this closer may cause premature wear on the door, hinges and closer.