Btu Buddy 11: Handling An Over-Fired Boiler
In this installment of the Btu Buddy series, Bob is notified by the dispatcher that the owner of a small office building has called and said his oil boiler needs a tuneup. It has not been serviced in about two years and a really cold spell is expected.
Bob drives up to the building and finds the owner and talks to him. Bob inquires about the previous servicing of the boiler. The owner explains that various people have worked on it. The last few times it was worked on, his brother-in-law did the work. The owner was not sure how much his brother-in-law knew about boilers; he was a sort of jack-of-all-trades. He has since moved to another state, so it is time to really take a close look at the boiler.
Bob goes to the basement where the boiler is located. He looks around for about 15 minutes and goes up to talk to the owner. He asks the owner to go to the boiler room with him. When they get there, Bob tells the owner, "Before I can go to work on this boiler, there must be some preliminary work done in this room. There are papers, mops, brooms, floor sweep compound, and various other combustible materials in the room that must be removed. The room is not very large and it would be easy to start a fire with all of this material lying around. I am surprised the insurance inspector has not found this and had you clean it up."
Bob says, "Yes, please have this room in good shape so I can be ready to start."
Bob writes up a report and files it in his paperwork just in case.
Bob returns after lunch to find that the room has been cleaned up, swept, and mopped. He is ready to go to work now. The first thing he does is shut off the power to the boiler and lock out the panel. He then shuts off the fuel line from the tank, which is above the burner. He then takes a small pan and places it under the fuel filter and removes the filter. It is very dirty, but still working. He changes the filter and fills the filter cartridge with fuel oil and puts it back together.
He then goes to the boiler and opens the inspection door to the combustion chamber. He uses a flashlight and inspects the combustion chamber with a mirror that has a telescoping handle made for this type of inspection (Figure 1). The combustion chamber looks good. He looks at the front of the burner head at the same time. It looks good; there is some varnish on the burner head, like it has been running hot.
He then cleans the electrodes and insulators to remove any carbon residue. This will prevent the high voltage from arcing to ground on the carbon. He then uses his gauge to set the electrodes.
The nozzle assembly is now ready to set back in place. Bob takes one more step. He pours the nozzle piping full of fuel oil and puts his thumb over the end of the pipe as he slides the assembly back together. Filling the piping helps to prevent an air bubble from getting in the piping. Sometimes an air bubble is very hard to remove. You would think that the high pressure oil would push it out, but it doesn't always and can cause after-fire drip, which is noisy and creates soot.
While Bob has the transformer up, he removes the cad cell and cleans it with a soft rag. This cad cell must be able to see the combustion when the burner lights off. He is careful not to put his fingers on the contact prongs at the back of the cad cell; this could cause a poor connection in the electronic circuit.
Bob then fastens the transformer back in place. He is ready to start the boiler and take an efficiency test. He installs a draft gauge over the fire through the hole in the inspection door. There is no plug in the hole; Bob makes a note to install one. He turns on the fuel valve and the power. The boiler fires correctly and looks great. Bob looks through the inspection door at the flame; it looks big, but seems normal for now.
Bob installs a thermometer in the stack in preparation of an efficiency test. The temperature is going up. As a matter of fact, it should stop rising and it doesn't. It is running entirely too high. The stack temperature went right on past 800 degrees F and kept on going up. Bob noticed that the metal pipe from the boiler to the masonry chimney began to glow dark red. About that time, the boiler shut down; the thermostat was satisfied.
Too Hot For ComfortBob is scratching his head when Btu Buddy appears and says, "It seems that too much heat is going up the flue. What was the draft gauge reading?"
Bob says, "Things were happening so fast that I forgot to look."
Because it was cold outside, the boiler started back up in a few minutes. Bob looked at the draft gauge and it was -0.03 inches wc (inches of water column). This is fairly normal. The draft in the flue was -0.07 wc. The temperature was rising again.
Btu Buddy says, "What size oil nozzle did you use?"
Bob explains that he replaced the nozzle with what came out of the boiler, 2.5 gph (gallons per hour).
Btu Buddy says, "Look around at all of the old nozzles that are on the shelf close to the boiler and see what they say."
Bob looks and replies, "There are nozzles from 2 to 2.25 to 2.5 gph here."
"That is probably the answer to the problem," Btu Buddy says. "Oftentimes a service technician will not have the exact replacement and will put in another size, meaning to change it out later. Then, if the technician forgets, it may happen again next year. Before too many years, the nozzle is way out of match with the boiler. Look on the boiler nameplate and see if it tells you."
Bob looks and finds that this boiler should be using a 2 gph nozzle. It is over-fired.
He shuts the boiler down again and changes to the 2 gph nozzle. He writes on the side of the boiler what the correct nozzle characteristics are. He then picks up all of the old nozzles and throws them away so that another mistake will not be made.
Btu Buddy then suggests, "While you have the system off, install a pressure gauge in the pump discharge. Someone may have changed the pressure setting."
Bob installs a high pressure gauge and starts the boiler again. The oil pressure is 110 psig. Bob says, "No wonder the flue was getting too hot. The nozzle was oversized and the pump was putting out too much pressure. Why didn't a safety control shut the boiler off?"
Btu Buddy explains, "The boiler thermostat took care of it. The boiler water never actually got too hot, just the flue. This was a very inefficient operation. A lot of heat was going up the flue. You didn't do an efficiency test before changing the nozzle, but I will bet it was below normal. The brother-in-law was not a real technician."
Bob then runs a flue gas analysis test on the boiler. The flue gas temperature is running 650 degrees F, a much more acceptable temperature. Bob then does a smoke test that shows the flue gas is clean. The boiler efficiency is running at 71 percent. This has to be much better than when they arrived, not to mention the safety factor.
Bob asks the owner to come back down to the boiler room and explains what went on. Bob then says, "I would suggest that you get only qualified service technicians to do your work. This was a very dangerous situation. With all the combustible material that was in the boiler room when I arrived, this could have been a major liability to you."
The owner said, "You don't have to say any more. You will be our technician from now on."
While cleaning up the tools, Btu Buddy remarks to Bob, "You are turning into a first-rate technician, Bob. There are several things that you did that showed real respect for the customer, which will aid you and your company over the long haul.
"You used a pan to catch the dripping oil when you took the system apart. Just a few drops of fuel oil can cause a callback. The customer may think there is a leak.
"You locked and tagged the electrical circuit while working on the boiler to prevent a mistake. If you had gone for a part during this time, someone may have turned the boiler back on without knowing and they may have caused damage.
"You observed the signs of over-firing before you actually found it. Always look for clues.
"You cleaned up the tracks of the other technicians and left a trail as to what was correct by indicating the nozzle size on the side of the boiler. Many service people will forget to look on the boiler nameplate.
"When you removed your instruments, you put covers over the holes.
"You got the owner involved and he liked it.
"Bob, you are going to make a great technician; just keep learning and pass on everything you know to your fellow technicians. It will come back to you in the best of ways."
Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or firstname.lastname@example.org.
Publication date: 02/23/2004