Controlling Boiler Water Levels with a Differential Pressure Transducer

Controlling the water level on a boiler or deaerator is critical to boiler room operation. Without proper water level control, vessels will run out of water and/or have constant issues related to too much or too little water. If a deaerator runs out of water, the pumps will shut off and eventually the boiler will also shut down due to low water. For this reason, it’s necessary to install a level control system that will properly maintain the desired water level in both the boiler and the deaerator. 

One of the best ways to do this is with a differential pressure transducer. As its name implies, a differential pressure transducer has two ports for pressure measurement and calculates the difference between the readings.  

How Does a Differential Pressure Transducer Work?

There is a working leg and a reference leg that get piped to the transmitter. The working leg is piped to the positive port and the reference leg is piped to the negative port of the transmitter. The reference leg represents the total possible level of water in the vessel. The reference leg is piped to the top of the vessel and should always be full of water. When the differential pressure transducer is calibrated, the height of this connection is entered into the transmitter. Since water has a known pressure for every inch of height (that’s where the inches of water column pressure measurement comes from), the transmitter now knows what the total possible level is in the vessel.

The other pressure port of the transmitter is connected to the working leg. The working leg pressure constantly changes due to the ever-changing water level inside the vessel. The working leg is piped to the bottom of the vessel height being controlled. The weight of the water inside the vessel at any particular time is then compared against the total possible weight if the vessel were to be 100% full (reference leg). The transmitter then converts the pressure measurements into inches of water which outputs a specific water level in the vessel. The user will enter the desired water level setting in inches and the transmitter will drive a modulating valve. This valve will open to add water to the vessel when the transmitter senses that the differential between the working leg and the reference leg is increasing beyond the setpoint.

Potential Problem Scenarios

The reference leg of the transmitter has no water flow through it and is connected to the top of the pressure vessel, so it needs to be manually filled in order for the transmitter to operate properly. Let’s say you forget to fill the reference leg and the vessel is full to the normal water level. Since the pressure on the reference leg is atmospheric and the working leg is sensing the water pressure, the transmitter would be reading that the vessel is full and close the modulating water makeup valve. The transmitter knows that the weight of the water in the working leg is greater than that of the reference leg, so it equates this to the vessel being fuller than the reference leg connection to the vessel. In reality, the reference leg just has no water in it, but if left unchanged, the transmitter will never open the valve to fill the vessel. Once the reference leg is filled with water, the measured water level in the vessel would drop and the transmitter output would reflect the actual water level in the vessel.

If the working leg were to get blocked or shut off from the transmitter, the transmitter would not read any pressure and equate this to the vessel being out of water and needing to be filled. The makeup water valve would open all the way and the vessel would overfill because the working leg is blocked off. 

The two above scenarios happen as dirt and corrosion build up in the connection lines, causing blockages to occur. When this happens, the transmitter output will not reflect the actual water level in the vessel and needs to be serviced.


Vessel actual water level is above the desired level or setpoint; Vessel is over filling and alarming on high water; Modulating valve is open.

Check the actual water level of the vessel, make sure that the sight glass represents the actual reading of the water level and that there are no blockages. Next, the working leg (positive leg) of the transmitter needs to be checked for blockages or a valve that is closed. The transmitter needs to be able to sense the actual water level of the vessel to regulate the water fill valve. Loosen the plug on the opposite side of the connection to the transmitter to see if water comes out. If no water comes out there is a blockage in the line. If water comes out of the plug, make sure it is clean and free of dirt. If the working leg connection has been verified as in normal condition, the settings and calibration of the transmitter need to be checked. The settings are the only way the transmitter knows what the desired water level should be. 

Vessel water level is below the desired water level, but the modulating valve is closed.

After checking and verifying that the actual water level of the vessel is the same as in the sight glass, check the reference leg (negative) of the transmitter. If the reference leg is blocked or has no water in it, the transmitter will show that the vessel is full and will not open the modulating water valve. Remove the plug on the opposite side of the reference leg connection on the transmitter and flush the line. Use a hose to add water from the top of the reference leg connection at the vessel and verify that clean water comes out of the transmitter’s plug connection. 

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