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How to effectively exercise stagnant water in open system

Hello all,

I am switching out a leaky hot water tank in a small cottage that is currently using a single domestic tank for space heating as well.  Tank has side ports that run to a manifold with four PEX runs that serve the wall blower units.  A simple control strategy is set up right now with the Tstats wired into a transformer and a small switch relay serves the zone actuators and the single circ pump. The homeowner doesn't want to incur the expense of making the heating side into a closed loop.  As a intermediate upgrade, I would like to adapt the system to exercise the water periodically to prevent the buildup of stagnant water in the heating loops. 

I was considering using a Taco SR504 switch relay with a plug in PC610 exercise card, however I have never used the plug in card before. 

The big question I have is how do I effectively set up the controls to exercise the water through the entire system?  Does the PC610 allow for the zones to open and close?  And if they all open, what is to keep the system from simply circulating through the shortest loop and leaving the other three to stagnate?  Does this control cycle through the different zones as well?

Any feedback or different strategies that you might suggest?


  • Paul48
    Paul48 Member Posts: 4,469

    you saying the way the system is configured now, it can only run one zone at a time?
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    edited April 2012
    By the time you wire and purchase the controls

    would it not be cheaper to install a flat plate heat exchanger and an extrol 30? also tell them how they will be wasting fuel exercising the system all summer and risking their family's health. On dead or very sick relative is way more expensive then making a closed loop.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Steamhead
    Steamhead Member Posts: 15,512

    and if they won't go that way, don't work on it. If someone gets sick, they'll come looking for whoever worked on it last...........................
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • icesailor
    icesailor Member Posts: 7,265
    Exercise in futility:

    PEX: Heat PEX isn't NSF approved for potable water. Water PEX isn't approved for heat applications.

    In Massachusetts, this system must meet all requirements for a potable water system. That means that whatever the heat emitters are, they must meet potable water code. Meaning that the coils in the heating emitters are Type L copper.

    Just turn the thermostat up and you will have flow through the water heater, Sending all that stagnant water full of who knows what into the tank to be consumed by the owner.

    YUM!! I love the astringent taste of Copper Sulfate in the morning in my coffee.
  • kinglerch
    kinglerch Member Posts: 14
    How do legionella survive in open systems?

    I don't know much about legionella, but I do have open loop heating in my house. It is essentially 5 feet of copper that goes to a radiator in the air system, and then another 5 feet back to the water heater. It's been this way since the house was built 20 years ago.

    I'm curious with all this talk about "legionella amplifiers" in open loop heating systems, how exactly do legionella survive in this open loop with no air? If the system is copper, and full of water all the time, can legionella grow in this environment? Doesn't it require oxygen for mold and other things to grow?

    But the same holds true for the probably 500 feet of copper in my house. What about the guest shower I rarely use or the sink that is almost never used? Why is this 10 feet of open heating such a health hazard when it can be heated to 140 degrees or higher, while there are hundreds of other feet of rarely used water in the house that are not heated at all?

    If what you are saying is true, wouldn't someone using the guest shower for the first time in a while have to flush it out completely before each use?
  • Gordan
    Gordan Member Posts: 891
    edited November 2012

    The problem with this and any other emotionally loaded discussion is that people seem to gravitate toward categorical statements. You make some good points, such as identifying the potable water distribution system in your house as a possible (and possibly bigger) contributor to the likelihood of contamination, but I don't think that anyone would disagree with that. What you seem to miss is the underlying goal, which is to do what is practical to make your plumbing system less hospitable to the nasties. This typically involves minimizing places in your plumbing system that go stagnant for a while, especially for months-on-end. Thus the criticism that open systems, in general, receive from plumbing safety advocates.

    As far as your particular open system, it seems like it would be no worse (or at least not significantly) than a short loop to a flat plate heat exchanger; both involve some modest length of piping, a circulator, and a heat exchanger (in your case, water-to-air) that the potable water has to traverse and both would presumably be stagnant outside of the heating season. But it would be no better, either. Both are far better than hundreds of feet of non-oxygen-barrier plastic tubing, which is what people usually use the phrase "open system" to describe, although the truth is that any heating system that uses potable water as the primary heat medium is open to a degree. But that degree matters - a lot. In any open system, including the ones using a short-looped heat exchanger, I'd still be mindful of the fact that the return portions of the heating system will be considerably cooler than the supply and may, in fact, continually be in the ideal range for legionella incubation.

    As for stagnation and oxygen availability, putting aside oxygen-permeable plastic plumbing, I'm not so sure that the issue of stagnant sections of the plumbing system has as much to do directly with allowing legionella to flourish as it does with providing the conditions beneficial to the development of biofilm, which is then in turn beneficial to harboring legionella and other microorganisms.
  • kinglerch
    kinglerch Member Posts: 14
    I guess my point was

    that there are tons of posts here decrying "open systems" as breeding legionella because the water is not used often, when in my house probably 30% of all pipes have "stagnant" water. I just don't use certain sinks and showers that often. But my understanding is that for solid (copper) pipes, that the exclusion of oxygen makes it very difficult to grow baddies such as legionella. And the cries of "open systems will kill you" seems to leave out these facts.

    I was not thinking of "open system" as you had described, with hundreds of feet of flexible tubing. With the oxygen and space to stretch, that's exactly the way water gets rancid and full of bad things...a little like a previously open jar of tomato sauce will expand with mold until it cracks the glass.

    But leaving that part aside, I would imagine my situation (many feet of unused or rarely used copper pipe in the house) is very common. And to say that this is an imminent health hazard requires some explanation of exactly how this could occur.
  • BobbyG
    BobbyG Member Posts: 79
    non barrier pex tubing oxygen gateway

    when I think of open systems and legionnaires disease is with non barrier pex tubing which allows oxygen to permeate through walls.  I think that is the biggest difference between your system and what I dislike about radiant open systems....

    I feel the argument of your situation versus the radiant non barrier pex tubing systems are totally separate and should be addressed separately.
  • JharrisSeattle
    JharrisSeattle Member Posts: 26
    edited December 2012
    Sprinkler timer

    Open systems are extremely common here in the Seattle area. For the homeowners who are not willing to pay the added expense of adding a hx we have typically put timers on the zones to exercise. A multi-zone sprinkler timer control works wonders for this application and is inexpensive as well. This will let each zone exercise independantly and is a fraction of the cost of a sr504.
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