There are many ways to control a boiler and the boiler controls can be layered. For example: when many people talk about boiler control they talk about the local controls on the boiler itself. These local boiler controls can be set for stand alone operation to maintain desired boiler set points and some boilers do operate stand alone in that fashion. Another, more complex, layer of control can be added to the system with the addition of Direct Digital Control (DDC). DDC control offers boiler control strategies which are only limited by the imagination and boiler equipment. DDC control, a separate system unto its own, is especially good at controlling multi-stage boilers for boiler lead/lag, boiler lead/lag with demand assist (demand load management), boiler multi-pump rotation/operational sequencing, and domestic hot water boiler services. Additionally, DDC control offers a plethora of boiler monitoring and boiler data collection that you would otherwise have to spend a large number of man hours collecting and disseminating. Residential boilers are typically controlled by an aquastat (which can be found on Boiler Page 2) but commercial boilers, especially the higher horsepower boilers use other, more complex control systems. The following boiler control descriptions are for local boiler controls only. The local controls are usually built into the boiler straight from the boiler factory depending on the spec of the boiler system and customer boiler efficiency requirements.

    * Multi-boiler staging - when you have several boilers and stage them on and off according to demand. Several cast iron boilers are prime candidates for this option. Electro-mechanical and/or solid-state controls are available for this type of control. Solid-state controls measure the delta-T of the supply and return and stage the boilers on based on the difference.
    * Modulating Control - based on demand this system will limit the amount of fuel and air to the burner. It ranges its boiler firing rate from low fire to high fire and everything in between based on specific input temperatures that determine demand such as delta T.
    * On - Off Control - The boiler comes on and goes straight to high fire and stays there until it satisfies a pre-determined set point.
    * Step-up/Step-down - with this boiler control you have two, three, or more firing rates that change based on heating demand. Typically low fire to high fire or low fire, medium fire, and high fire.
    * Oxygen Trim Control - monitors the amount of O2 in the flue gases and trims the excess air for more efficient combustion.
    * Excess Air Regulation Control - maintains a specific amount of excess air to fuel ratio for maximum combustion efficiency.
    * Air/fuel cross limiting control - allows more air to be added to the combustion process before more fuel is added and when stepping down reduces the fuel before reducing the air. This adds safety while optimizing fuel consumption.
    * Drum Level Control - best used on boiler systems where water level in the drum is critical.

DDC, in addition to everything described above, can offer you a hot water reset schedule that will reduce boiler cycle rate as compared to outside air temperature.

To determine which boiler control option is best for your facility it is best to talk to boiler manufacturers and/or engineers with specific boiler experience.

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