Boilers
High Performance HVAC Residential Hot Water Boiler Control
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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.
Aquastats
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 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 aquastat models have a manual selection so the dead-band can be adjusted. On a call for heat the thermostat turns on a 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
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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
Zone valves offer a great way of offering zoning in hot water heating. The dwelling can be split up into sections to offer temperature control in certain areas. Rooms can be seperate and even stories in multi-story dwellings. A thermostat ultimately controls the temperature based upon its manual set point. An example is a two story home. The upstairs and downstairs can each have its own thermostats. This can mean energy savings and better comfort control.
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.
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Earl Roy