By George Lanthier
I have been writing articles on heating for over 10 years now and I have tried to impart what I have learned to others. This subject keeps coming up all the time so I thought, what the heck, let’s do it again.
It still amazes me that a lot of people are still looking for a faster way of setting up oil burners but are not primarily concerned with making it right. Look, one more time, you can only set up a burner properly with instruments, period.
Now, the question that always comes up next is: ‘C’mon, George, it’s 3 in the morning and you go out for a combustion test kit? You’ve done burner service, are you kidding me, or what?”
Pay attention now. I always said that you have to use instruments. I didn’t say which ones. There is no doubt that using combustion test instruments is the most accurate and the most reliable. On an annual tuneup or new installation, it is the only way to go.
We all have our little tricks, though, and I have to admit that once I found out about this next little trick I didn’t pull the combustion test kit out at 3 in the morning anymore. This procedure is based on work that I’ve done with a smoke tester and my Lanthier Scale and believe me, it works. The instrument used? An ohmmeter.
We know that the presence of smoke in a combustion chamber or firing zone is not desirable and that it will show up on a smoke test done with a test-paper-type instrument. Now, let me ask you a question. Is there any other way or thing that you can use to detect smoke? Yes, a cadmium sulfide photocell. That’s right, the little cad cell can also be used as a service tool.
The cad cell is a light-sensitive resistor that will vary its output resistance to the amount of light supplied to it. Smoke will darken the readings to the cell and will cause either erratic operation or may lock out the burner, appearing as the infamous ‘false lock-out call.” Two things to keep in mind is that all smoke is not created equal, and cad-cells work on seeing direct light and not reflected light.
Reflected light is false light and can lead to dangerous conditions. You can find smoke on both sides of the zero smoke line and even in an excess air condition there could be smoke. Using the smoke tester, you begin by making smoke by closing the air-gate and then backing off on the air gate until you clear the smoke. Then when setting up a burner lower the CO2 from that ‘optimum level” one percent and you have an ideal operating condition.
So, it’s 3 in the morning and you want to check the burner for proper operation, or you just want to find a quick way to get to that ideal condition. Well, try this. Take a typical cad cell primary whose F-F terminals have been jumped out and the leads from the cad cell have been connected to an ohmmeter set for the 1000 ohm scale. I prefer an analog meter, but digital will do it, too. The burner is started by closing the service switch and by momentarily disconnecting the F-F jumper. The air gate is gradually closed. As it is closed, you will notice the circuit resistance increases. Upon visual examination of the flame you would find the presence of smoke.
Now the air gate is reopened until a satisfactory reading is found. Before locking in the gate, try opening the gate to where you originally started and as the burner was found. You will notice that there is no movement of the meter for some period and then the resistance will again increase. That’s technically ‘zero smoke” and just like the black smoke on the high side of the gate, it will cause a lock-out if not adjusted for. Adjust the air gate until the lowest resistance is met and you have a burner which is set ‘in the ballpark.” Not as accurate as combustion instruments, but better than your unreliable eyeball at 3 a.m.
The two worst sins
The worst sin made by service techs in the troubleshooting of burners equipped with cad cells is either to ‘trick” the control or themselves. This is a safety device, people; it’s not a toy and it’s not a joke, you are playing with fire, literally.
Have you ever painted an air tube silver on a troublesome burner? Well, if you have, you should be kicked in the pants. This is not doing anything but fooling the cad cell into believing that everything is alright. Remember the comment about direct light? Well, if you fool a cad cell, you are fooling a safety control. Not really a good idea and in my opinion, a true sign of a burner hack. Keep in mind that almost every burner air tube currently being shipped from the factories has a dark paint job with the favorite color being black. What do you think they are trying to tell you? By the way, could you please stop referring to air tubes as blast tubes? Considering that noise is one of the largest areas of complaints from consumers, why would we ever use terms that are even associated with noise?
Another sin that really sets me off is the primary difference between the way combustion controls are serviced by residential people and commercial/industrial people. Servicemen doing C/I will always use a ‘flame simulator” across the flame detector terminals; residential guys use jumpers. Even earlier in this article I did it, but I was looking to keep the burner running so I shorted out the F-F terminals. Think about that: I shorted a circuit. Make sense? Well, in this case it does, but normally it doesn’t, right?
The real way to verify that a cad-cell control is working within accepted limits is to put a 1500 ohm resistor across the F-F terminals and watch what happens. If the relay chatters, it’s bad. If it goes into lockout, it’s bad, but now you know for sure. By the way, those resistors are available at most electronic parts stores like Radio Shack and cost about 75 cents each.
Finally, one of the last things I look for in troubleshooting cad cells is a high temperature in the air tube on burner shutdown. If it goes over 140F, cad cells pull a nutty and freak out. These temperatures will also lead to erratic readings and a premature failure of the cell. If you suspect this condition, try this. On shutdown, open the burner and place a thermometer in the area where the cell is located and reclose the burner. If after two or three minutes you reopen the burner and find a temperature of 140F or greater, you probably have a draft problem that needs to be corrected.
George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax (781) 641-7099 or e-mail FiredragonEnt@comcast.net.