Service & Maintenance / Extra Edition

Btu Buddy 63: Cleaning and Filling a Hot Water System

June 23, 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.

[Editor’s Note: This article is a continuation of “Btu Buddy 62: An Overheated Boiler,” May 19, 2008 issue.]

Btu Buddy and Bob met back at the boiler room, and Bob talked with Sid, the water treatment expert. Sid looked at the system and the amount of dirt and rust in it and said, “This system needs to be filled and the pump run with plain water, then it needs to be drained again and filled with fresh water and system cleaner which should be circulated for 24 hours, then drained and filled with fresh water and circulate that water and drain it again, then we will fill the system with water treatment and fresh water. Then we can let it run with confidence.”

Bob said, “That is a lot of work, but the system is really dirty. Just leave me with the correct amount of chemicals and I will do it.”

Sid said, “Here are the chemicals for the flushing and a pump to charge them into the system and here are the chemicals for the treatment. Use the same pump to charge them into the system.”

Bob said, “I can handle that. Thanks for the help and advice.”

After Sid left, Btu Buddy joined Bob to oversee his work. Btu Buddy said, “This will get the system in great shape. Let’s first fill the system with fresh water and I will show you how to fill it.”

Bob said, “I thought you just turned the water on and it filled itself.”

Btu Buddy then said, “You have completely drained the system, except for the expansion tank. Let’s drain that now.”

Bob opened the drain on the expansion tank and the water was really dirty.

Figure 1. This boiler filling system has a bypass valve around the water regulator to allow for fast filling. (Figure is from Practical Heating Technology by William Johnson, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

After the tank had drained, Btu Buddy then said, “This system has a bypass valve for quick filling the system (Figure 1). That will save some time. Open the quick fill valve and we will charge most of the water using it. Then we will shut it off and let the pressure regulator do the rest of the filling; that way we will not over-pressure the system. We will watch the altimeter pressure. This is a low-pressure system with a maximum working pressure of 30 psig. When the gauge gets up to about 15 psig, we will let the regulator take over and finish filling it.”

Bob had the system ready and opened the quick fill valve and water started to flow into the system through the regulator and the quick fill valve. When the pressure began to rise, Bob turned off the quick fill valve and the system continued to fill using the regulator. When the pressure reached 15 psig, the water stopped flowing.

Figure 2. This automatic bleed valve has a float in it. (Figure is from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.)

Bob asked, “Now what?”

Btu Buddy said, “We need to go upstairs to one of the units and find the air bleed valve. It should be an automatic bleed valve (Figure 2).”

They went upstairs and found the automatic bleed valve. It looked fairly new and had done its job. There was a little water residue on top of the valve. Air had been bleeding out and the system was full.

When they got back to the boiler room, Bob asked, “What did you mean when you said that this can be tricky? This has been easy.”

Btu Buddy then said, “We need to set the water level in the expansion tank. A lot of air has moved to the tank while filling the system. Look at this cutaway of the expansion tank (Figure 3). It has a bleed valve that you can open and let air out and allow water in until the water level is correct. Then we will start the pump and let the water circulate for a few minutes, then drain the system again. We know that the bleed valve works correctly so we don’t have to go back up there again.”

Figure 3. This is an illustration of a boiler system and expansion tank. (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 then asked, “That system sure had a lot of air in it. How did air get in when we were draining it?”

Btu Buddy responded, “I am glad you asked that question. I was wondering if I was going to have to mention it or if you would think of it. That little automatic bleed valve serves two purposes. When the system is drained, the float sinks in the valve and it allows air to enter (Figure 4). Any time the system tries to go into a vacuum, such as when the system shuts off and it cools down, the water in the system will shrink and air will enter that bleed valve and be exhausted when the system heats back up.”

Bob said, “This job is getting like all of the rest of them. There is more than meets the eye.”

After the system had run for about 30 minutes with fresh water in it, Bob drained the system again, including the expansion tank. He then used a small pump and pumped the system cleaner into the system and filled it with water again.

Btu Buddy then told Bob to start the boiler and get the system up to temperature. Btu Buddy said, “Let’s keep the boiler on until the high limit control shuts it off to make sure it is functioning correctly.”

Figure 4. This automatic bleed valve also serves as a vacuum break and allows air to enter the system when it goes into a vacuum. (Figure is from Practical Heating Technology by William Johnson, published by Delmar Cengage Learning.) (Click on the image for an enlarged view.)

Bob then asked, “Isn’t that dangerous?”

Btu Buddy explained, “If it won’t shut the boiler off like it is supposed to, we need to know it now and change it out. We must make sure the system is safe.”

The boiler heated up. Bob turned the house thermostat to 90°F to prevent it from shutting the boiler off. The temperature climbed at the boiler until it reached 200°F and the boiler shut down. Bob said, “The high limit shut it off. It is safe and we know it for sure.”

Btu Buddy said, “Be sure that you write in your report that you checked the high limit and at what temperature it shut the boiler off. This puts on the record that you checked the safety controls. I think we are done here until tomorrow when you need to come and drain the cleaner out of the system, flush the system with fresh water, and charge the water treatment into the system.”

Bob then said, “Well, there was more to this job than I first thought. We can now leave with confidence that the system will work as it should, it will control as it should, it is clean and has protection, and the owner knows that what we did was worth the expense.”

Publication date: 06/23/2008

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