What you can expect to learn from the Hot Water Boiler Sequence of Operation Aquastats article:

  1. About sequence of operation for boilers
  2. The basic sequence of operation for the typical boiler
  3. Boiler controllers and boiler control strategies
  4. Hot water boiler sequence of operation conclusion
  5. Lots of resources and related links to help you take a closer look and learn more.

Hot Water Boiler Sequence of Operation Aquastats

hot water boiler sequence of operation

An Outside Boiler

Hot Water Boiler Sequence of Operation Aquastats - As boilers go, there is not a single sequence of operation for any hot water boiler out there, but there is a typical sequence of operation for most boilers. We will start off by describing the typical hot water boiler without domestic hot water or an indirect-fired hot water tank. In other words, this will be the typical boiler used for heating in the winter with either single-zone radiators or baseboards for heating rooms.

Boiler Sequencing and Considerations
  1. The house is a comfortable temperature, but as the temperature outside drops, so does the temperature inside. It is a result of heat loss from the structure. So the thermostat drops below the setpoint, and the switch for heating inside the thermostat closes. This completes a control circuit to engage the relay for the circulator pump. The coil on the circulator pump relay energizes closing the contacts for the relay, and the circulator comes to life and begins circulating water through the loop. That circulates hot water from the boiler to the single zone (the entire house).
  2. The water inside the boiler is 180 degrees Fahrenheit. As the circulator kicks on this hot water, leave the boiler and are circulated throughout the loop where it passes through several radiators or baseboards, and it heats the rooms. Since the heat left the water through the radiators or baseboards, the water returns to the boiler with less heat than when it left the boiler.
How a Boiler System Works

For a properly sized boiler under normal loads, the temperature should never be more than 30 degrees Fahrenheit (always check the manufacturer’s guidelines on the differential temperature as these temperatures can vary) from the leaving water temperature to the returning water temperature. It is possible for boiler thermal shock to occur if this temperature is exceeded. If the temperature exceeds 30 degrees the boiler system could be undersized or there is a problem with the loop somewhere. As the water temperature inside the boiler drops the cut-in temperature switch fires the boiler to reheat the returning water.

Hot Water Boiler Sequence of Operation Aquastats

Aquastat, Circulator Pump, and Burner Controls

  1. The temperature switch (aquastat) has a temperature probe inside a thermal well placed strategically inside the boiler by the boiler manufacturer. These temperature switches with probes inside the boiler thermal well are aquastats. There are different types of aquastats from the very simple aquastats to the newer electronic aquastats with several different control strategies for economizing the use of the boiler based on different programmable parameters. Either way, the temperature switch or aquastat will turn the boiler on and off. It does that based on the temperature of the water inside the boiler.
Aquastat Variations

Again, some complex aquastats use different control strategies and settings. Still, on a simple boiler system, the aquastat will turn the boiler on and off based on the temperature settings of the aquastat. The cut-in temperature is the temperature at which the boiler fires at a low temperature reading by the aquastat while the cut-off temperature is the temperature at which the boiler cuts off as demand has been satisfied.

The demand has usually been satisfied after the circulator pump cuts off. The cut-off for the circulator pump is based on the room thermostat temperature setting. Once the thermostat setting is satisfied, the switch inside the thermostat opens, and that kills the power to the circulator pump relay, so the circulation of hot water throughout the system stops. Prevention of the migration of hot water throughout the boiler loop when the system is off, a flow valve has usually been installed the system somewhere in the near boiler piping loop.

Hot Water Boiler Sequence of Operation Aquastats - Related Link - Honeywell Aquastat take a closer look

Hot Water Boiler Sequence of Operation Aquastats - Strategies for Economy and Control

Some terminology and a brief description of some of the more complex control strategies in the higher level aquastats for boiler economizing and control include:

  1. Outdoor Reset
  2. Temperature Anticipation - Maximum Outside Air Temperature
  3. Short Cycling of the Boiler Prevention
  4. Boost Assist on the Reset Curve
  5. Thermal Purge
  6. Boiler System Checkout
  7. Pump Exercise
  8. Freeze Protection


Outdoor Reset

The most common boiler control strategy controls the boiler setpoint based on the outside air temperature. The maximum set point for most boilers is 180 degrees Fahrenheit. Therefore, if the temperature outside drops below 40 degrees (approx.) then the outside air reset control will make the boiler setpoint high limit temperature to 180 degrees F.

If the outside air temperature rises to 60 degrees F., then the outside reset control will lower the boiler setpoint to a predetermined value inside the controller. E.G., if the outside air temperature is 60 degrees F. the outside air reset controller will reduce the boiler set point to 150 degrees F. Furthermore, this causes the boiler to run less, therefore saving energy. Therefore, the outside air reset control will make the boiler set point temperature based on the temperature of the outside air.


Temperature Anticipation - Maximum Outside Air Temperature

Automatic Vent Dampers | HVAC Energy EfficiencyThere can also be a maximum outside air temperature where the controller shuts off the boiler. It is anticipated that if the temperature outside is (say approx.) 70 degrees Fahrenheit, then the heat will not be needed, so it is safe to shut down the boiler completely until the temperature drops below 70 degrees Fahrenheit again. Additionally, that prevents the unnecessary running of the boiler when it is not needed.


Short Cycling of the Boiler Prevention

Some settings will prevent the short cycling of the boiler.


Boost Assist on the Reset Curve

Some controllers will provide for an extra assist that some controller manufacturers call Boost. Essentially it provides for extra heating if heat demand is not met within a certain time while the boiler is in setback mode (following the Outdoor Reset curve). In that case, a Boost Period invokes where a value called the Boost Step increases the boiler setpoint. Each time the Boost Period elapses and heat demand is not satisfied, the boiler set-point is again increased by the Boost Step, up to the High Limit setting.


Thermal Purge

Thermal Purge - thermal purge is to ensure usable residual heat in the boiler circulates until it is sufficiently depleted from the system before the burner is allowed to fire. Only used for cold fire and should not interfere with any domestic water control or an outdoor reset control program.


Boiler System Checkout

System Checkout - places the boiler into operation for a complete cycle in cases of boiler troubleshooting.


Pump Exercise

Pump Exercise - exercises the circulator pump on a time interval (usually set for a weekly schedule where the pump runs for 10 to 30 seconds). Exercising the pump is good and provides for protection from loop circulation problems because of a failed circulator pump.


Freeze Protection

Freeze Protection - protects the boiler and the near boiler piping from freezing.

Hot Water Boiler Sequence of Operation Aquastats

There are many newer types of boilers out there with proprietary controls that have many different control features and options. Sometimes it is best to allow the installing contractor to set these controls. The contractor is familiar with the controls. Furthermore, they likely have been trained by the manufacturer on all the features of the boiler controller.

Hot Water Boiler Sequence of Operation Aquastats - Conclusion

The thing the guy refers to in the video is the aquastat. If the guy knew how to troubleshoot the boiler, he would know 100% if the switch he refers to is bad. It is not a good thing to change parts because you suspect it is bad. It can get costly for a part changer because many times, suppliers have a no refund policy on electrical and electronic components for obvious reasons. Additionally, that is why it is better to call a professional if you do not know what you are doing.

Aquastat Video

See the Aquastat video below.

High Performance HVAC

Hot Water Boiler Sequence of Operation Aquastats