What Happens to a Steam Boiler When the Return Is Blocked?

Air-lock can be corrected by venting through the radiator's vent cap.

The return on a boiler is the pipe that allows cooled air and water to return to the boiler to be reheated back into steam and cycled back through the radiators. Boilers recycle water in this way, minimizing water use and energy costs while keeping your home evenly heated. A blocked return arrests the process by not allowing the cooled water and air back into the boiler for reheating.

Symptom

A blocked return presents as a build up of condensation in the trap. Other things may also cause this build-up, such as a closed discharge valve, inadequate trap-size or pressurization, improper drain installation or a faulty check valve. In the case of blockage, the moisture is unable to return to the boiler and so collects in the trap. A consistent low water level may also be a sign of either a blockage in the system or a leak.

Cause

Blockages in boiler pipes are most often caused by something called air-lock. The cold water flowing through the return pipe is capable of holding more air than it held when it was hot. The air is released when the water is heated, and chooses the path of least resistance, which is usually right into the radiators. The air is then trapped, and blocks off the returns.

Repair

Bleeding the system of air is the straightforward repair for getting your blocked returns flowing once again. Turn down the thermostat on the system, and grab a towel to catch any overflow. Locate the air vent at the top of your radiator. Use a screwdriver or vent key to unscrew the vent, prepared to catch any water that escapes with your towel. Bleed the vent until a steady stream of water begins to flow from the open vent, then tighten it back down to close it off.

Scale and Sediment

Sometimes hard water build-up or sediment in the boiler system leads to problems like corrosion. Installing a water softener on these systems is the most effective way to deal with hard water, making sure the softener is located in the piping between the well and the boiler so the water entering the boiler has been softened. Sediment can be removed by a regular maintenance schedule that involves a three- to five-second drainage of the system every day. Open the drain on the float chamber and allow water and sediment to drain out before closing it again.

References

• Old House Journal: How to Take Care of Your Radiators
• Hydroserve Technologies, Inc.: Typical Boiler Questions

Resources

• Spirax Sarco: Steam Headers and Off-takes
• High Performance HVAC: Boilers