Unwanted Heat in a zone?

Check for Ghost Flows…

I am sure you have heard theexpression “water takes the pathof least resistance.” If you havebeen in the heating business forany length of time, you’veprobably come across theseexpressions or possibly experi-enced them out in the field.

They usually show up as NOHEAT calls! The thermostat iscalling, the circulator is run-

ning, but there is no heatcoming out of a certain radiatoror piece of baseboard. You checkfor air but only water comes outof the vent, so you know it can’tbe an air problem.

The problem is there isn’tenough water flowing into theradiation. The reason for that isthere isn’t enough pressuredifferential ( P) across the

radiation circuit. Thiscauses the water to“short circuit”through the closercircuits or zones.

Look at figure #1. Inthis drawing, there isa boiler, a circulatorand four circuits. Ihave applied numbersthroughout the pipingcircuit. These num-bers represent thepressure differentialthat our circulatorcan develop when it ison. In this example,our circulator candevelop ten feet ofpressure differential.Note that as thewater moves through-out the piping, thepressure differential islowered until itreaches the suctionside of the circulator,where it equals zero.Also, note that as thewater moves furtheraway from the circu-lator, the pressuredifferential acrosseach circuit becomesless and less. Andsince a difference inpressure is what

causes flow to occur, you can seewhy the furthest circuit or zonein our example might come in asa No Heat call on a very coldday. There is a difference inpressure across the circuit whichmeans water is flowing there.The problem is not enough flowrate is moving through thatcircuit. On a cold day, all of theBtus will “jump off” the base-board in the first couple of feet,leaving cool water to flowthrough the remaining element.

Ghost flows can occur in ahydronic heating system for theexact same reason. A pressuredifferential exists across an opencircuit, allowing water to flowwhere it doesn’t belong. In thiscase, the complaint is overheat-ing! Here is an example of thatexact problem which I had thechance to see firsthand:

An oil company had replaced aboiler over the summer for anew customer. They didn’t spenda lot of time looking at theentire piping system, but ratherjust the piping in the boilerroom. They pulled out the oldboiler, installed the new one andattached the existing piping inthe same manner that it hadbeen piped.

When the heating seasonarrived, the homeowners calledthe company to complain thatseveral zones were overheating.

The company sent a serviceman over to the house. Hechecked to make sure the flow-control valves were working. Hemade sure each thermostat waswired properly to its respectiverelay and circulator. Everythingappeared to be in good workingorder. Next, the technician went

Boiler Facts

By George Carey

Oilheating/November 200940

upstairs and checked the roomtemperatures compared to thethermostat settings of the zonesthat were overheating. He foundeach thermostat was set at 60°Fand the rooms were measuringaround 75-76°F.

At this point, the service techni-cian called in the service manag-er to look at the job. After muchgnashing of teeth and wailing,the homeowners admitted thatmaybe the same problem existedwith the old boiler. They ex-plained that when the new boilerwas installed, they assumed theywere getting a new heatingsystem! (I have been involvedwith many steam systems wherethe homeowner assumed that thenew steam boiler was going tomake all of the system’s ills

vanish. However, this was thefirst hot water system I encoun-tered where the new boiler wasgoing to fix the existing systemproblems.) We went to thehouse to see if we could find outwhat was causing the overheat-ing problems.

Figure 2 is a sketch of what wefound when we wandered aroundthe basement, looking for clues.The system had eight heatingzones, each with its own circula-tor. Several of these zones splitinto smaller, sub-circuits. Andeach return from these sub-circuits connected into thecommon return manifold.

What was interesting aboutthis particular system and whatmade it more difficult to trouble-shoot was the main return

manifold that picked up all thesub-circuit returns. It waslocated in a crawl space betweenthe first floor ceiling and thesecond floor. All you could see inthe boiler room was the supplymanifold with all the zonepumps and flow-control valves,their individual take-offs, andthe return main that came downthrough the ceiling and back tothe boiler.

Looking at the sketch, can yousee how when one zone is callingfor heat, the water that entersthe return main manifold hasaccess to flow up the return ofan off zone?

I have labeled two returns fromone of the overheating zones Aand B. When the water flowsalong the return main andreaches point A, it asks itself,“Which way do I go?” Theanswer depends upon the differ-ence in pressure between pointsA and B. Because there is adifference in pressure betweenthese two points, some water hasto flow up the return, movingbackwards through the off zoneand back down to point B. Thiswas causing the overheating tooccur. The off-zones constantlyhad some water flowing throughthe baseboard even though thethermostat was satisfied.

The solution is to combine allthe returns from each specificzone pump before entering thereturn main. But in this case,with all the piping concealed inthe first floor ceiling, we decidedto install small ¾" spring checkson the returns of each baseboardcircuit. When we did that, theghost flows disappeared.

Always remember that thereneeds to be a pressure-differen-tial across a circuit for water toflow. If there is no pressuredifferential, then there is noflow! If there is a pressuredifferential across an off-zone,there will be flow when it is notwanted.

Any questions or commentsplease call me at 1-800-423-7187or email me atgcarey@fiainc.com.

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