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Thread: hot water in zone when T-Stat not calling

  1. #1
    DIY Junior Member
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    Default hot water in zone when T-Stat not calling

    I have a 4 zone hydronic heating system. 4 taco pumps off a supply manifold. 3 zones are copper piped fin tube baseboard. The basement was added and has flexible rubber pipes and runs buderus panel radiators.

    The second floor zone just started running at about 72-74 regardless of what the T-stat is calling for (66 for example). First I swapped out the T-stat, which didn't make any difference. I turned the Tstat to "off" and overnight it dropped to 68 and the fin tube is just a tiny bit warm.

    In a possibly related problem, the basement zone has one radiator which stopped working a 6 weeks or so ago. We found we could get it to work a little bit by bleeding it. The radiators on that zone are below the high point in the supply and return lines, and I worry that there's air in that part of the system. After the initial bleed, the radiator stopped working again, until I bled several quarts of water from it last week. The water was coming out without bubbles FWIW.

    My suspicion is that there's a bubble in the high point of the rubber tubing.

    My questions.
    1) could the problems be related? (resistance on the basement supply leading to backflow to the 2nd floor zone?) It's not happening to the 1st or attic zones).
    2)Does the bubble in the basement pipe sound plausible, and if so, how to remove it? Drain the zone? There is an air removal device mounted on the system over the expansion tank.

    3)If they're not related, could there be a problem with the 2nd floor zone circulator? How would I diagnose that? (other than just replacing the cartridge)


    Thanks so much for any advice

  2. #2
    In the trades Dana's Avatar
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    Are there any check valves isolating these zones? If not, pressure differential from one active circulator creates some amount of reverse flow in the other zones, pulling from the return manifold toward the supply manifold. The amount of this flow is determined by the relative head of the active zones and inactive zones. If a zone has fairly low head relative to the active zone, can get enough induced flow to put some heat where it's not wanted when the overall heating load is fairly light (as it has been recently in Boston), even if it's pulling say, 110-120F water from the return manifold. Sometimes convection forces alone are enough to induce flow.

    A system schematic would be useful here.

    It may or may not be related, but EPDM tubing has a checkered history in the trade- when bleeding the apparently vapor-locked zone, was there any black gunk (or black water) evident?

  3. #3
    Moderator & Master Plumber hj's Avatar
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    IF the pump is not running, then it is NOT the problem, but without being there to do our own tests to see what could be inducing circulation we cannot be of any real help. A pump circuit is like a race track. The water goes into the pump is pushed around the circuit and arrives back at the pump. Unless something else can replicate that pattern, the only other possibility is convection. Hot water rising in the piping can do it since you probably do not have "flow control" valves on the zones. Air will settle in ANY high points of the system and when it does it can only be removed by pressure purging or air vents at the high points.

  4. #4
    In the trades Dana's Avatar
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    Quote Originally Posted by hj View Post
    IF the pump is not running, then it is NOT the problem, but without being there to do our own tests to see what could be inducing circulation we cannot be of any real help. A pump circuit is like a race track. The water goes into the pump is pushed around the circuit and arrives back at the pump. Unless something else can replicate that pattern, the only other possibility is convection. Hot water rising in the piping can do it since you probably do not have "flow control" valves on the zones. Air will settle in ANY high points of the system and when it does it can only be removed by pressure purging or air vents at the high points.
    But in his case the race tracks overlap, they're not separate- all pumps are pumping to and from manifolds. Without check valves, the pressure difference between manifolds created by the active pump induces a reverse flow through the inactive loops.

    The air-locked (or possibly gunked up) loop isn't related to heat showing up in the zone not calling for it. Bad EPDM tubing has been responsible for all sorts of issues like sticking valves, gummed up pump cartridges, etc. Even though it's allegedly been "fixed" since the worst stuff of the 1990s, I'd never install it in my place.

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