October 2010


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Hydronic Zone Controls 101

Control theory as explained through real-life experience

Steven R. Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

Last month I did not manage to submit an article for publication here, as I was busy cleaning up from the flood that ravaged my neighborhood and the surrounding communities, and dumped close to five feet of water into my basement. This month I’m back to write about it! Seriously though, it’s more a case of “out with the old and in with the new”, especially when it comes to the general subject of this column, that being my home’s hydronic heating system.

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A little background first. My home is an old house, and we do not have forced air heating or central air conditioning. For our cooling purposes, we have window shakers, and for heat, we’ve had a vintage 1960’s boiler. The boiler sat in our basement, and three zone pumps would draw from the supply side of it. These three zones are comprised of the main level, the upper level, and an addition that was built in the 1950’s. Each zone pump was controlled by a thermostat located in the zone served. Each thermostat, also vintage 60’s equipment, operated a relay, which in turn cycled the zone pump. The three relays were located in an enclosure mounted to the side of the boiler.

The boiler itself operated via a typical operating control, in order to maintain hot water temperature setpoint when any of the three pumps was being called to operate. This little bit of logic was performed with good old-fashioned ladder logic, and prevented the boiler from operating to maintain the “full-out” setpoint of 170 degrees if none of the pumps were running. In this scenario, the relay logic would “redirect” temperature control to an aquastat that was set at a much lower setpoint, in essence putting the boiler system into an “idling mode” whenever there was no call for heating from any of the three zones. This allowed the boiler to at least stay hot enough so as to prevent any sort of thermal shock when called to ramp up to operate at normal setpoint.

Good system, served us well for many years, but the flood waters brought it to its untimely (yet perhaps long overdue) demise. Though as I stated before, in with the new…so the guys came to demo the old boiler and pumps, and install the new system, pretty much like-for-like, as far as the mechanical equipment goes. I will say that the new boiler, with the same capacity and higher efficiency, is about half the size of the old one! Dinky little thing when compared to the mammoth that stood before it for so many years.

So the old equipment was dismantled and removed, along with the old temperature controls (relays, old wiring, etc.) The new boiler was installed, and three shiny new pumps were put in to match the capacities of the pumps before them. The guys reconnected all the hot water piping, along with the gas and power, and installed a new temperature control panel/system.

The control system is a residential grade, four-zone temperature control system. Seeing as we have three zones of control, this appears to be a good fit, and likely the only choice as they probably don’t offer this in a “three-zone” variety. I look at it this way; if I ever decide to add another pumping zone, I already have the capability to control. Also I figure that the redundancy is good in that if one of the three zones ever fails electronically, I can always switch that zone over to the “spare” zone, and carry on with my day.

[an error occurred while processing this directive] The system is microprocessor-based, meaning that it’s electronic in nature. It has inputs to accept the contact closures from the existing thermostats. It also has outputs (contact closures) to control the zone pumps. Upon a call for heat by any stat, the control system energizes the corresponding zone pump, and allows the boiler to fire, as by its own on-board operating control. If no zones are calling for heat, then no zone pumps operate and there is no flow through the boiler. The boiler is in turn “locked out” and prohibited from operating. This is something that I’m not used to, seeing as our old boiler would percolate even if no zones were calling for heat. I’m told that this type of operation, in the context of residential heating systems, is “on demand”, meaning that, it's there when you need it (any zones calling), and off when you don’t (no zones calling). I’ve traditionally been of the opinion that, however unfavorable it is to fire a boiler when there is no flow through it, it's somehow worse to have a boiler be allowed to heat up and cool down over and over again. The lesser of the two evils to have the boiler idle at some intermediate setpoint whenever none of the zones were calling for heat. Interestingly enough, apparently that is no longer much of a concern with the newer residential grade boilers, and comparing the physical size of our new boiler with our old one, I can begin to see why that may be the case. For starters, I suppose that there’s much less “thermodynamic lag” with the new, smaller boiler. And therefore less stress on the boiler’s heat exchanger during heat-up and cool-down cycles (just theorizing here, I’m not a mechanical engineer!).

Anyway, as I conclude this writing, we’re still in the final throes of summer, and have yet to come to rely on the new boiler system. I will say that, coupled with the piece of mind in knowing that we have a brand new boiler and hydronic zoning control system, I still remain wary of the centralized nature of the new controls. With our old system, we had individual stats controlling the operation of individual pumps. There was no intermediary; if there was a failure, it would be associated with either a thermostat or a pump relay, and it would be isolated to a single zone. Now, if there’s a malfunction with the main control panel, it could feasibly affect all zones, and render our heating system completely non-functional until the system was repaired (or until I manually jumpered the outputs to the pumps!). I’m not too concerned though, for even though the old system served us well for so many years and was extremely reliable and trustworthy, it was definitely time to jump ahead into the current state-of-the-art, and develop our trust in our new wintertime friend!

Tip of the Month: I guess given that this column is more of a story than an article, maybe instead of a tip I offer a “moral of the story”, that being, as I stated in the first paragraph, “out with the old and in with the new!”. However much of a comfort level you acquire with an old system or piece of equipment, there will come a time to upgrade, and instead of resisting or dreading it, you must embrace it, for in the long run it will only serve to benefit.



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