Looking through some technical brochures recently, I came across a Bell & Gossett CounterPointÔ bulletin from April 2000.  Since I've talked about this subject previously in a Yaunco Report, I will not go into detail again.  However, occasionally someone is still puzzled at times as to why a boiler relief valve may be discharging.  Sometimes the first action is to change out the relief valve, but typically that is not the cure for the problem.  Bell & Gossett lists ten reasons for relief valve discharge;
1. Waterlogged steel compression tank.
2. Pressure reducing valve is left opened or fails open.
3. Diaphragm tank loses its air charge.
4. Undersized expansion tank.
5. Undercharged diaphragm tank.
6. Tankless coil leaking.
7. Faulty aquastat.
8. The system's static fill pressure requirements approach the boilers relief valve setting.
9. Improper location of the expansion tank and system pump.
10. Systems that incorporate high head pumps and pressure differential valves.

We thank Bell & Gossett for providing technical advice to our industry.

cad cell assembly to determine how secure the eye is in the socket. If it is loose and readily pulls out with little resistance, we advise complete replacement of the unit.

The operation of the cad cell works on electrical resistance. As light from the flame hits the cadmium sulfide cell, the resistance through it is lowered. Generally, the more light the eye "sees" the lower the resistance and conversely, a lack of light results in high resistance.

Using an ohmmeter, we can both test the operation of the cad cell and monitor the stability of the flame. First, disconnect the cad cell leads from the control. After calibrating the ohmmeter, attach the test leads to the cad cell wires. Start the burner, and once the flame has been established, place a jumper between the FF terminals. While the burner is operating, observe the ohm readings on the meter. (Digital meters may be affected by RF interference if located too closely to the burner causing erratic readings.)

Most control specification use a maximum of 1500-1600 ohms. Actual readings should be much lower, in the range of 350-900 ohms. Factors influencing the final readings are position of the cad cell assembly, correct burner specifications including nozzle, drawer assembly settings, and combustion test results. Particular attention should be paid to draft and CO₂ . Excessive over fire draft or too much combustion air exhibited by a low CO₂ could cause lifting of the flame off the retention head. This test procedure is simple and will provide a wealth of information about flame stability.