The thought came to me while I was hanging upside-down. Part of my every other day workout is to spend 10 minutes or so of quality time on the inversion table that my sons gave me for Father’s Day years ago to help with my sore back. That means I’m upside-down and I’m thinking, because that’s what we humans do a lot of. (Thinking, not hanging upside down like bats.)
I mention bats because there is a bat family now living at our house, behind our operable real wood window shutters, above the copper roof over our front door. The mess they are leaving was bothering my wife until she realized how many bugs it represents. When I got home last night, she was happily hosing off the porch roof. According to her research, our winged guests will be lodging somewhere else next year, so she can deal with it this year.
I was thinking of the boiler startup job I was on that morning. Startups are usually a challenge. Generally something still needs to be wired or the system still needs to be vented or who knows what else. This job was going pretty well. The installer had done a wonderful job with the piping. It was flush and plumb, with the boilers on housekeeping pads. None of my pet peeves were bothering me.
However, the boilers were piped in a different than normal configuration. Typical multiple boiler systems connect the boiler piping to the system piping with a primary/secondary connection, utilizing separate boiler pumps to control flow through the boiler from the main. This was a large senior housing building and the engineer specified three commercial condensing boilers, but deviated from normal by using boiler isolation valves to control flow through the boilers rather than pumps.
The boiler manufacturer allows this technique and call it “full flow.” So when I saw it that morning, it didn’t raise any alarms. When we gave the boiler a call for heat, the valve would slowly open as the fan pre-purged the combustion chamber. By the time the boiler started the ignition sequence, the valve was fully open. All was right with the world. Bob’s your uncle up there in Canada.
Every boiler is designed to operate at a certain flow rate. Of course, that’s in the lab and we live in the real world, so let’s say it is designed to operate within a certain flow range. When it is within range, the water temperature rises slowly. Slow and steady wins the race.
When it is outside the range on the high side, it isn’t too evident because it still operates, but at a lower than expected heat absorption rate. In other words, the water passes through too quickly to get as hot as it should. The boiler satisfies the building load, but at a higher energy cost.
When it is outside the range on the low side, the boiler cycles on and off, as it quickly reaches the high temperature setting. If it is really far off, the boiler might not be able to satisfy the building load, since it may be off as much as it is on. Anyway, it is a lousy way of operating a boiler.
When these three boilers fired up, it didn’t take long for the outlet water temperature to reach its limit, maybe two minutes. By design, they shut down the burner on a soft lockout and kept the boiler isolation valve open, allowing the system water to keep passing through the boiler, cooling the outlet water temperature sensor. When it dropped below the high temperature limit setting plus the differential, it started the ignition sequence. On for two minutes then off for two minutes, repeat…
The boilers were doing exactly what they were supposed to do in case the flow rate was outside the design range on the low side. At that point, I did a walk about with the installer. We looked at the brand-new pumps since it was looking like a flow issue. It was a pair of very nice high tech variable speed pumps. I was feeling pretty good about having the ability to control flow with the touch of a button. But I didn’t sell the pumps, so I didn’t want to mess around with them too much.
The installer said he just turned them on and that the pump rep hadn’t done his startup yet. The fact that something isn’t done is pretty typical of boiler startups. At that point I wanted to wait to finish up my boiler startup until the pumps were running properly. Maybe the flow rate would come up to the range required, or maybe it was something else.
We continued the walk about to inspect the rest of the piping in the boiler room. The return from the system split to the three boiler isolation valves connecting to the boiler return connections while the three supply lines from the boiler combined into the common supply pipe heading to the new in-line air separator/expansion tank then on to those pumps. All’s well so far.
I asked the installer where the water goes when the three boiler isolation valves are closed. He pointed up to the ceiling to a full-size piping connection running between the return and supply mains. “Just the way the engineer drew it,” he said proudly. It did look very professional and we had already decided to come back later, so I drove away, already distracted by a few call and texts.
It wasn’t until I was upside-down later that day that the implication of the full-size bypass hit me. One of the classic lines in the hydronic world is “water is lazy.” Water flows along the path of least resistance. Yes, it will flow through paths of higher resistance, but the majority of the flow doesn’t want to work hard.
In this case, the majority of the flow was zipping along the full-size return to supply connection, bypassing the smaller-sized boiler connections, even with the isolation valves open. The rate of temperature rise through the boiler was indicating only about 25% of the flow required to be in the required range. Again, it could be the adjustment of the new pumps, but if the installer is right, the engineer drew it up wrong. He left too wide of a path to bypass flow.
As of this writing, since the boiler’s installation guide shows something different when piping full flow with boiler isolation valves, I am leaving it in the hands of the local rep and the engineer. I just showed up for a startup, and as sometimes happens, got hung up. Didn’t realize it until I was upside-down.
Patrick Linhardt is a thirty-seven-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.