Welcome to our Frequently Asked Questions section of our website, where you can find all the answers to the questions you have about our products and how they relate to a dry pipe sprinkler system. Below is a list of questions, click on the question to see the answer. If your question is not listed below, please contact us. We will respond directly to your question and then post it here for future reference. If you do find the question that you are looking, feel free to drop us an e-mail to tell us what you think about the answer.

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An air compressor is a mechanical device that is used to increase the pressure of atmospheric air.

Dry pipe systems, Deluge Systems & Pre-Action systems.

Any system that requires the piping to be filled with air as opposed to water or uses air for supervisory purposes.

Riser mounted & base mounted.

An air compressor used for other functions in the plant. A small portion of the air is used for the dry system. These are normally very large air compressors.

An Air Maintenance Device acts as a restriction in the line. Should a fire occur, as the air pressure escapes the system piping through a sprinkler head, the Air Maintenance Device will prevent the air supply from adding air at too fast a rate, thus preventing the dry pipe valve to open and fill the piping with water.

According to NFPA-13, an Air Maintenance Device is required for every installation

The Air Maintenance Device is installed downstream from the air compressor. The AMD-2 must be a minimum of 3′ downstream.

The AMD-1, because it has a regulator to properly adjust air pressure.

The AMD-2, because it has a pressure switch to start & stop the compressor.

General Air Products’ air compressors are sized according to system capacity in gallons. Each model is designed to fill a certain system to 40 psig in 30 minutes under standard conditions.

By measuring the amount of pipe and the size of pipe and converting it to gallons. Once you have the gallon size of your system, you can select the appropriately sized fire protection air compressor.

The type of air supply selected is typically the contractor’s preference. Riser mounted compressors typically are selected due to space restrictions, while tank mounted compressors should be used on multiple systems and systems with accelerators.

When a customer orders a base mounted compressor, you need to know the size of the system, what kind of power supply is available (i.e. single or three phase) and how they plan on starting and stopping the compressor. Make sure that the customer is aware that the start/stop control is an accessory (General Part No. AMD-2) and NOT provided in the package as it is with the riser and tank mounted compressors.

The start/stop control is an added expense. In many cases, the base mount unit is used to replace an existing compressor on site. This gives the contractor the option to replace just the air compressor without having to buy an additional pressure switch.

When a customer orders a riser mount compressor, you need to know the size of the system and what kind of power supply is available (i.e. single or three phase.)

When a customer orders a tank mount compressor, you need to know the size of the system and what kind of power supply is available (i.e. single or three phase.). Tank mounted units should always be used when an accelerator is installed on the sprinkler system.

The voltage and phase of the compressor depends on the power supply available. Whenever possible, use three phase instead of single phase. The operating current of three phase is about half that of the equivalent single phase motor (same HP and voltage.) You should also connect the motor to the highest voltage available. The operating current at 230 v is half that of 115 v. By following the above recommendations, you greatly reduce the chance of a voltage drop in the line, which can cause problems upon start-up.

Single phase current is 115/208-230 nominal voltage. Three phase current is 208-230/460 nominal voltage.

Voltage and phases depend entirely on whatever the customer has available.

Thermal overload protection is a sensing device that shuts the motor off during excessive temperature rise.

Thermal overload protection prevents motors from burning – problem that results from improper voltage being supplied to motor, inadequate wire size or an excessive line run.

The current draw on these motors is very high and magnetic starters are a much better form of overload protection. General recommends using three phase, whenever possible on units over 1 1/2 HP.


A magnetic line starter is an electric device that provides thermal overload protection for the motor.

A magnetic line starter is selected according to phase, voltage and horsepower. The overloads are sized according to amp draw.

Most problems seem to be electrical. Before bringing any power to the unit, the installer MUST ALWAYS open the motor junction box and verify that the motor is connected for the voltage that is available or reconnect the motor leads properly.

Another problem is caused by low voltage upon start-up. This is normally caused by inadequate wire size. The average air compressor draws about six times its normal running current as it starts. Once it gets up to speed the current levels off. If it does not get up to speed because of low voltage the breaker will blow or the motor will burn. To ensure the adequate voltage is running to the unit, the installer should connect a voltmeter across the power leads at the motor and take a reading as the compressor is starting. If the voltage dips below 10% of what the compressor is rated for, starting problems will occur. 208v is 10% below motor rating & must remain no lower than 207v.

There are three main reasons that an air compressor will cycle. The most obvious reason is a leak downstream of the air compressor. Once you have eliminated any leaks in the system, soap up the trim on the air supply side.

The second cause of a compressor to cycle is a restriction between the compressor and dry pipe valve. This is most often the case when using an Air Maintenance Device piped one to two feet away from the air supply. The Air Maintenance Device has a restricted opening that prevents air from re-filling the system at a rate faster than the air escaping, should the system trip. When the compressor starts, due to low pressure at its pressure sensor, it builds pressure in the piping between the compressor and the AMD more quickly than in the system. The pressure switch of the compressor acts as a sensor, telling the compressor when to turn on and off. Once it senses the piping to be at pressure, it turns off. As the air continues to leak through the restriction orifice into the system, the pressure drops in the piping, causing the compressor to turn back on.

A leaky check valve is the final cause of a compressor to cycle. The check valve allows air to flow one way in a compressed air system. When the compressor stops, the valve closes preventing air loss from the receiver or piping. If a check valve doesn’t seal properly, air will leak back and vent to atmosphere. A tiny particle of dirt stuck in the seat of the valve could prevent it from fully closing. Cleaning out the check valve will prevent a compressor from short cycling.

According to code there is no limit to how many systems one air compressor can service. However we recommend no more than three systems per compressor. Dry Air Pacs are FM approved for the three system limit per unit.

When servicing more than one system, the compressor must be sized to fill the largest system to pressure (normally 40 psig) in 30 minutes.

Each system must have its own check valve and air maintenance device according to code. Some AHJ’s will allow one AMD for multiple systems.

A manual dryer (dehydrator) is used remove water vapor from the compressed air system. These dryers should not be used for critical applications like freezer rooms.

The size of the system determines the number of dryers. One dyer should be used for every 275 gallons. A coalescing pre-filter must be installed to protect the dryer when using oil lubricated compressors.

The Dry Air Pac is a turnkey compressor/dryer package specifically designed for cold storage areas, freezer rooms or any other application where the piping may be exposed to below freezing temperatures. It consists of an air compressor, aftercooler, coalescing pre-filter, twin tower regenerative dryer, UL/FM Approved Air Maintenance Device, particulate after-filter, regulator and fully integrated control panel. The unit is pre-piped, pre-wired and pre-tested.

Freezers, Cold Storage, Parking Garages and Marinas are all typical applications.

The Dry Air Pac is the ONLY FM APPROVED package designed to provide the sprinkler system with moisture free air to a -40° F dewpoint. Its major components are integrally mounted on an air receiver to save floor space and are pre-piped and pre-wired for easy installation. This package eliminates the requirement to pull air directly from the freezer. When pulling the cold air directly from the freezer condensation begins to form in the intake piping which results in ice plug formation. Another drawback from using the cold air is that it prevents the lubricant from circulating properly which decreases both the performance and the life of the compressor.

Yes, a UL/FM Air Maintenance Device is included.

Compressed nitrogen is used as an air supply just as an air compressor would be. It is very expensive and there are additional OSHA guidelines that must be followed.

The hydrostatic test pump is used to leak test fire sprinkler systems.

Private Pressure Tank systems are used where there is not an adequate supply of water for a sprinkler system. GENERAL offers air compressors engineered according to Pamphlet 22, which governs private pressure systems.

Air Compressors are the backbone of any dry pipe sprinkler or double interlock pre-action system. You want to choose a compressor that will provide you excellent service life with minimal maintenance that is designed for performance and reliability given the rigors of the demands of a fire sprinkler system. Our compressors are designed specifically for fire protection applications to maximize reliability.  For more information about selecting an Air Compressors for Pre-Action Systems please contact us at 800-345-8207