May 2013
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Who’s That Knockin’ On My Pipes?

Hot water reset control to the rescue

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor



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Distech Controls

As I’ve done in past columns, I once again have decided to talk about the old house that I live in. I was born and raised in this house, and quite frankly I never left! You have to understand that this house was built before the Great Depression, to be a rectory for a church that they were to build on the corner lot. Well, the church never got built, and so my grandparents were fortunate to purchase the house in the early 40’s. The house consists of two full apartments, and a basement running the full length of the house. My grandparents raised my father and uncle on the first level, and for years rented out the second level.

When my father got married, he and my mom moved into the second floor apartment, and proceeded to raise our family (me, my younger brother, and my kid sister). Eventually we moved downstairs, and when my brother got married, he moved into the upstairs apartment and proceeded to raise his family. When he moved out and I got married, I moved upstairs and proceeded to raise my family, which I still do to this very day. For the record, my mom still lives on the main level, and has become a wonderful built-in kid-sitter!

So back to the point of the story…when I first moved (back) upstairs, I noticed that in the winter the hot water heating system made a considerable amount of noise, especially at night when I’m trying to get to sleep. The hot water system consists of a hot water boiler, a zone pump for each level, and mostly baseboard radiators, with a few old-fashioned radiators scattered about the house. Anyway, I noticed that the hot water pipes would “knock”, meaning that every 10-15 seconds or so, and not consistently throughout the night but periodically for a stretch of time, I would hear a thud. I took me a long time to figure out, but I eventually theorized that it was due to the expansion and contraction of the pipes as the space thermostat would cycle the pump on and off. The pipes would expand when the pump was called to operate, and then contract when the pump would stop. I confirmed my theory and concluded that the pipes, upon expanding and/or contracting, would rub against the structure of the house (floorboards, studs, etc.), and make the thud noise.

As I became more and more sensitive to the phenomenon, I noticed that, in the dead of winter, I seemingly wouldn’t be bothered by it all that much. But when winter would draw to an end, I’d start to hear it again. I came up with an explanation as to why, and acted upon it. First the explanation…

On the coldest days, when the heating system is at “full load”, the boiler operates close to 100% of the time to maintain a 170-degree hot water temperature setpoint. The pump serving my apartment runs constantly, trying to maintain our space temperature setpoint. Once the pipes have expanded to their extent, the noise goes away, and for the most part stays away, on these coldest of cold nights and days in the middle of winter. The pump runs continuously to maintain setpoint, maybe shutting off occasionally during the day when there might be a bit of a solar load helping the cause. Of course during the day (if we’re home, which is seldom to never) there’s so much ambient noise that nobody can hear the pipes contracting and then expanding again. Once night falls, the solar impact goes away, and we’re back to full-time pumping, in the (sometimes futile) effort to reach and maintain our space comfort level. So in bed, we don’t hear the pipes contracting and expanding, because they’re not. With me so far?

Now for the solution. I concluded that if I could get the pump to run more often, then the pipes would go through the expansion / contraction cycle less often. Well, the pump runs plenty often in the deep freeze of January, in fact almost continuously. How then, do I get the pump to run more often in the milder months of winter, without overheating the space and wasting energy? Then it dawned on me: hot water reset control! Something that I’d been implementing for years in my professional life, designing hot water control systems with electronic single-purpose temperature reset controllers early on, and with DDC-based control systems in more recent years. So I got to thinking, how to go about this?

Reliable Controls For my application, the design and installation was pretty straightforward. I consulted with our service manager, and he was generous enough to “sell” me an old electronic hot water reset controller that had been sitting on the shelf for years. Still worked though. My boiler is a single stage, on/off type boiler with a central pump controller that ties the pumps, space thermostats, and boiler together to operate as one tidy system. Without going into detail, I was able to integrate the electronic reset controller into the existing scheme of control and set it up so that, when the outside air temperature is -10 degrees, the hot water temperature setpoint is what it always has been: 170 degrees. When the outside air temperature is 50 degrees, the hot water temperature setpoint is 140 degrees. The relationship is linear, meaning that, if you were to consider this in graph form, with the x axis being hot water temperature setpoint and the y axis being outside air temperature, you could plot the two points (170 /-10, and 140 / 50) and draw a line to connect them. At any outside air temperature, you could then graphically find what the corresponding hot water temperature setpoint would be. For instance, if you “split the middle” between the two extremes of outside air temperature, which is 20 degrees (halfway between -10 and 50), you would find the hot water temperature setpoint to be….anyone…anyone?? That’s right, 155 degrees! The relationship between OA temps and HW setpoints is also often referred to as a “reset schedule”.

So how it did work? Well, aside from increasing our energy efficiency and improving the accuracy of our space temperature control, it also seems to have worked rather well for the main reason that it was implemented, and that was to help me sleep! In practice, as in theory, the pump runs more continuously during the milder weather, as the “cooler” water takes longer to satisfy space temperature setpoint. And the more the pump runs continuously and the less frequently it cycles on and off, the less expansion / contraction of the pipes, and the less amount of thudding. I’m still playing with this and trying to determine the best reset schedule, trying to walk the line between minimizing annoying noises at night and keeping my family comfortable. Until my wife and kids tell me that they’re cold, I’ll leave it the way it is, and enjoy a good night’s sleep!

Tip of the Month: The underlying moral of the story, as it relates to controlling HVAC systems, is this: hot water temperature reset control is not only a good scheme of control to implement for energy efficiency, but also a really good strategy for accurate temperature control.


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