Service & Maintenance / Extra Edition

Btu Buddy 62: An Overheated Boiler

May 19, 2008
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Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

The spring was beginning to bring warmer weather when Bob got a call from the dispatcher that a small apartment building was overheating. He had never been to this job. In fact, it was a new customer. Bob didn’t know what to expect.

Figure 1. This instrument is on the front of the boiler and indicates the temperature and the pressure within the boiler. (Figure is from Practical Heating Technology by William Johnson, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

He arrived at the building to find that it was a really old system that was heated with an old boiler. The owner was in the first apartment and he met Bob at the door and explained, “The building is too hot. There are four apartments and the people on the top floor are really hot. It is 55°F and we are burning up.”

Bob asked, “Where is the thermostat?”

The owner responded, “This building only has heat and the thermostat is in the downstairs hallway. It controls the boiler and pump. It shuts it off when the hall gets up to temperature.”

Bob went to the thermostat and found that it was set to 75° but the thermometer on the thermostat showed the space temperature to be 80°. It was really much too hot.

Bob went to the basement to look at the boiler. The basement was really hot. He looked at the altimeter gauge on the boiler and the needle was in the red (Figure 1). The boiler was too hot. Bob looked at the burner, which was gas, and the fire was running at what looked like low fire. Bob was scratching his head when Btu Buddy showed up and said, “Shut off the gas hand valve and tell the people upstairs to go outside until you get the boiler cooled down. We don’t want to take any chances here.”

Figure 2. This is a baseboard heating unit like those in the apartment building. (Figure is from Practical Heating Technology by William Johnson, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob went to the owner and told him to get the people out of the building. The owner said, “There is only one other person in the building; the rest are at work. I will get him outside.”

Bob went back to the basement and Btu Buddy said, “Sorry to be so abrupt with you, but this needed to be done quickly. Now, the fire is off of the boiler. Let’s see what we can do to cool the boiler down as fast as we can. This system is all baseboard heating type units (Figure 2), so the best thing you can do is open all of the windows and doors and let the outdoor air and time remove the heat.”

Bob walked through the building and opened all of the outside openings that he could find. Then he went out in the yard with the building owner and waited for about 45 minutes and went back to the basement. The boiler temperature was back in the black. It was safe now.

Bob returned outside and informed the owner that all was safe and he was going to figure out what had happened.

Figure 3. A safety relief valve similar to the one on the boiler. (Figure is from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob went back to the boiler and asked Btu Buddy a question, “Couldn’t we have just lifted the lever on the boiler relief valve (Figure 3) and let the pressure off?”

Btu Buddy explained, “That would not be the thing to do because it would have caused the water in the boiler to start to boil as the pressure dropped so quickly. That boiler was in enough stress as it was. If it boiled, it would have started vibrating and shaking. We did the right thing - just let the heat out of the system slowly. Now, Bob, explain to me where we stand with this service call.”

Bob then said, “We know that the fire was still on in the boiler; the thermostat should have cut it off. I am not sure if there was voltage at the gas valve or not.”

Btu Buddy then said, “Let’s start by checking the voltage.”

Bob checked the voltage at the valve. He said, “The valve coil shows 0 volts. The building is still pretty warm so the thermostat is not calling for heat. If the thermostat was not calling for heat, yet the boiler fire was still on, the valve must be stuck partially open.”

Btu Buddy said, “That is correct. Let’s change the valve.”

Bob changed the valve and the thermostat now controlled the boiler, so he said, “Well, I guess that is all there is to this call.”

Figure 4. This is a typical high limit control for a boiler. It should not have to shut the boiler off except under extreme conditions. (Figure is from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Btu Buddy then said, “Bob, you forgot to ask two critical questions. Why didn’t the high limit (Figure 4) shut off the boiler and why didn’t the boiler relief valve relieve the pressure before the pressure got so high?”

Bob said, “Boy, I seemed to have forgotten that. The high limit didn’t stop the gas flow because the valve was physically stuck open. It probably did try to shut the boiler off. The relief valve should have been relieving, and it wasn’t.”

Btu Buddy then said, “The boiler is well within normal range. Go over to the relief valve and carefully raise the handle and see if it works.”

Bob tried to raise the handle and it just wouldn’t budge. “This thing is stuck shut,” he said.

Btu Buddy then said, “Drain the system down and let’s remove the relief valve.”

Bob shut off the water supply to the boiler, turned off the entering and leaving valves to the boiler, and opened the drain valve. He said, “That is the dirtiest water I have ever seen.”

Btu Buddy said, “I believe you should drain the whole system and put in clean water and water treatment to protect this system. You will need to check with the owner about the treatment. First let’s remove the relief valve to have something to show the owner why the system needs treatment.”

Bob removed the relief valve and said, “This thing is caked with rust and it is a brass valve. This system must really be a mess.”

Btu Buddy said, “Show this to the owner and explain what the dangers were and he will want to make it safe.”

Bob came back in a few minutes and told Btu Buddy, “You were right. He said make a good repair. He told me that he had not spent any money on this system in five years.”

Btu Buddy then said, “Meet me here after lunch with your water treatment company representative and we will clean this system up and add water back to the system. It will need to be done with care. It is not a simple job.”

For the completion of this job, see “Btu Buddy 63: Cleaning and Filling a Hot Water System” in next month’s June 23, 2008 issue.

Publication date: 05/19/2008

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