The average residential/light commercial boiler typically uses different controls to sequence the boiler. Boiler control in commercial and industrial applications can be very complex and can have added layers of controls outside of the local boiler controls that come from the manufacturer. There are also differences of the controls between steam and hot water boilers. Breaking all this down into sections we’ll be as concise and detailed as possible while using brevity to describe residential/light commercial boiler control and cover commercial/industrial boiler control in another article. Some of this information will overlap but rest assured we’ll be as detailed as possible.
The aquastat of the typical residential/light commercial boiler controls the water temperature inside the boiler by sensing the water temperature and cycling the burners to maintain a set point. Hot water residential and light commercial boilers typically use this aquastat to maintain the temperature to a manual set point inside the boilers heat exchanger. The boiler aquastat has a temperature sensing bulb on the back of it that is immersed into the water. Typical temperature set point for most hot water boilers is 180 degrees Fahrenheit. There is a dead-band or low limit of usually 40 degrees Fahrenheit. Some boiler aquastat models have a manual selection so the dead-band can be adjusted. On a call for heat the thermostat turns on a boiler loop circulator pump which begins circulating water throughout the hot water loop. Hot, 180 degree water leaves the boiler on the supply side of the loop and cooler water returns to it on the return side of the loop. As the aquastat temperature bulb senses the temperature of the water inside the water jacket of the boiler, it senses when the water temperature falls below the dead-band or low limit setting and sends a signal to start the burners to heat the water up to the high limit of 180 degrees Fahrenheit. So when the water temperature in the water jacket of the boiler falls below 140 degrees Fahrenheit (assuming the dead-band or low limit is 40 degrees Fahrenheit) the aquastat turns the burner on to heat the water returning from the loop. As soon as the water temperature inside the boilers water jacket reaches 180 degrees Fahrenheit the aquastat signals the burner to shut down. This sequence will resume once the water temperature falls below the dead-band or limit again as determined by the aquastat. *It should be noted here that some manufacturers offer a solid-state controller with their boilers that will offer a hot water reset schedule which will cycle the burners based on outside ambient air temperature and boiler water jacket temperature. This type of boiler control will be explained in detail in an upcoming article here at High Performance HVAC Systems.
Hot Water Systems and Zone Valves
The thermostat controls an electric motor inside the zone valve. On a call for heat the motor is energized and the valve opens. As the valve opens and a switch in the valve is closed (zone valve end switch) and signals the aquastat that there is a call for heat. This engages the circulator pump so hot water can flow from the boiler and through the hot water loop to the zone calling for heat. Zone valves typically operate on 24 volts A.C. and are usually (not always) located near the boiler on a manifold system. Zone valves offer a very good way of hot water zoning in structures for better comfort and energy savings.
Low Water Cut-offs and Automatic Water Feeders – Blow the Steam Boiler Down Once a Month
Low water cut-offs and automatic water feeders are definitely components found on steam boilers whether they are residential steam boilers or commercial steam Low water cut-offs and automatic water feeders are essential safety components for a steam boiler. Any steam boiler or any boiler for that matter should never fired unless there is a way to prove there is water inside the boiler. For hot water boilers there are different components for this and we will discuss it later but for steam boilers should have a way to automatically feed water to the system and to shut down the ignition controls when and if the water inside the boiler reaches a predetermined and unsafe level. Most low water cut-offs work as a float type of switch where the float is set to a water level minimum and when the water falls below this level the steam boiler is prevented from firing. Of course the job of the automatic water feeder will be to keep the water level above that safety level point but if something would happen to the automatic water feeder to where it stops delivery make-up water to the boiler then the low water cut-off would save the day and shut the boiler down. Another good reason to blow your steam boiler down at least once a month to keep fresh water in the system. This drains the foul water and keeps it from fouling the low water cut-off float and the automatic water feeder. It also allows you to hear the automatic water feeder feeding fresh water into the system.
Flow Switches for Hot Water Boilers
Some hot water boilers have a different method of determining there is water in the boiler and this method is either a flow switch or a low water cut-off. And yes there are some hot water boilers that do not have any way to determine if the boiler has water in it before the boiler can fire. Those boilers have been tested by the manufacturer to not require low water cut-offs. However, boilers that do have flow switches ensure the boiler will not fire unless flow is determined by the flow switch. The flow switch is usually integrated with the safety circuit and if the safety circuit is open the boiler will not fire.
HVAC Residential Hot Water Boiler Control