There has been continuing debate in the industry about whether boiler jacket loss should be factored into the calculation of boiler efficiency.
In most of the old boiler rooms, it was a very toasty place. If the old pipe insulation was removed, the boiler room was downright hot. While some believe that the heat lost through the boiler jacket into the boiler room is wasted, others feel that it is useful heat. Jacket loss is the transfer of heat from the boiler through the jacket and into the boiler room. I would rather not get in the middle of that controversy but I do know one thing; Jacket loss trumps polyester pants every time. I received a call from a client complaining of no heat. I was on a sales call and stopped on the way back from my call. I was wearing nice polyester pants. It was the eighties, after all. I bent over and saw that the pilot was off for the boiler. I lit the pilot and watched as the main burners started with a loud whoosh. I informed the maintenance man about what I found and the repair that I performed. We chatted for a while and he informed me that the burner flame would sometimes look yellow. Before leaving, I knelt down to watch the flame. I was only there for a minute or two when I felt the heat on my knee and thigh. I jumped up but it was too late. The pants had shriveled from the heat and it looked like an old raison. As I stood up, I saw that my pant length was now above my ankle. I was red-faced and upset. “Hey, look at Ray with his flood pants.” One of the guys said laughingly to his coworker. I would get teased for years over that.
While I do not wish to weigh in on the benefits or losses of Jacket loss, the International Energy Conservation Code feels that it is something that should be addressed. The code stipulates that boilers should either be piped with primary secondary piping or use an isolation valve that limits flow through the idle boiler.
In a primary secondary system, each boiler has its own pump. If the boiler is idle, the pump will not operate. This will eliminate the water flow through the idle boiler. The pipe connections to and from each boiler must be no more than 12” apart on the primary loop. This isolates the boiler from the hot system water. It is the most common piping arrangement now, preferred by most boiler manufacturers. In addition to increased system efficiency, it assures proper flow for each boiler as each boiler has its own pump, properly sized for the correct gallons per minute.
The other option in the energy code is to use isolation valves. There are a couple cautions that should be considered when these are installed. The first is flow. When one boiler is off and is isolated from the system pump, all the flow will go through the boiler that is firing. This flow could be excessive and may be greater than the manufacturer recommends. Remember, the system pump is sized to distribute the heat from all the boilers plus a safety factor. The velocity of the water could be high enough that the boiler will not be able to give up its heat into the water. This could be very expensive as the boiler will lose its efficiency. A quick check of the temperature rise across the boilers will tell you if this is happening. Most boilers are designed for a 20 degree F temperature rise from the return to the supply when firing. If the temperature rise is low, the heat is not being transferred into the water. Please check the temperature requirements of the boiler manufacturer.
Another caution when using isolation valves is that the limit controls may trip and the building will be without heat until the reset button is pushed. When the burner shuts off, the internal boiler components will continue to generate heat for a short while. Since there is no flow due to the closed valve, the internal boiler temperature will continue to climb. I have witnessed temperatures rises of 30 degrees F or more when the flow is shut off on a recently ended heat cycle. In many instances, this will be enough to trip the safety limit switch, which would be a manual reset type of switch. If using an isolation valve, a time delay should be used that will not allow the isolation valve to close until the temperature stops rising. This is usually 10-20 minutes after the burner shuts off.
A time delay should also be considered for steam boilers if isolation valves are installed on them. A vacuum breaker may be required as well if the boiler will be isolated from the steam system and allowed to condense.
The last consideration when using either of these piping systems is that the boiler room will be much cooler than it was with the old boilers that had flow continually through them. In some instances, a separate heater may be required to provide heat for the boiler room. I realize that, in an ironic sort of way, we are now back to where we started.
In one of my first projects where the boilers were installed using primary secondary, I saw that the boiler room was much cooler than it was with the old boiler. Upon the completion of the project, the installer informed me that the custodian did not like our boilers, at all. The installer had a sheepish grin when telling me this. I asked the custodian what was wrong and he shrugged his shoulders and said,” I don’t like em.” I could not believe it, so after a couple questions, I heard the real reason why he didn’t like my new high efficiency boilers. The custodian used to place his lunch under the old atmospheric boiler and the jacket loss heated his lunch in the winter for the past 40 years. He could not do that anymore. I smiled and bought an inexpensive toaster oven and he loved my boilers then.
Want to learn more? Check out my books, Lessons Learned in a Boiler Room, Lessons Learned: Connecting New Boilers to Old Pipes, Lessons Learned: Servicing Boilers, and Lessons Learned: Brewing with Steam.