All hot water boilers must have an expansion tank. Water expands when it is heated and there must be a place for it to expand to so that the pressure in the boiler loop system remains constant. Without an expansion tank the pressures in the boiler loop system would vary wildly and most likely will exceed the maximum pressures designed for the boiler hot water loop system. This will result in setting off the pressure relief valve. There are two different types of expansion tanks that can be used on hot water boiler systems. The most modern is the bladder type expansion tank (see picture to the right). The other type of expansion tank is a basic cylinder or barrel looking tank that hangs from the ceiling or is located in the attic. These tanks need to be drained from time to time to make sure they are not full. If they fill up with water and have to be drained constantly there is a problem with your boiler that should be looked at by a boiler technician. The bladder type expansion tank does not need draining on a regular basis like the cylinder or barrel type tank.
The boiler aquastat controls the temperature of the water inside the boiler jacket. The boiler aquastat is a device to a boiler like a thermostat is to an air conditioner, heat pump, or gas/oil furnace. The boiler aquastat controls the temperature inside the boiler just as the thermostat controls the temperature for an air conditioner or heat pump. The boiler aquastat will turn the burners on and off and also controls the circulator pump on a hot water boiler system.
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.