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Open System Conversion

dftc Member Posts: 3
I am planning on changing my closed hydronic heating system into and open system so it can also supply my domestic HW.  I plan on having the supply water run through the hydronic piping so the water doesn't get stale.  I will have to have the intake water run through a Taco RMB-1.  Will running water through the pump cause any damage to the pumps in the RMB?

Thanks in advance.


  • Ironman
    Ironman Member Posts: 7,119
    Definitely Not a Good Idea.

    Are you trying to poison yourself and your family? You cannot run potable water through pipes have been in a closed hydronic system.

    And there's a reason that the hydronic loop is closed: to keep oxygen and minerals out. You'll also destroy the hydronic system by doing this.

    I don't understand your last sentence: "Will running water through the pump cause any damage to the pumps in the RMB?"

    If your planning on using the RMB, then why would you think the potable needs to go through the hydronic loop? The purpose of the RMB is to hydraulically Isolate the open loop from the closed loop. It's also limited to 50k btu's.

    Please post a drawing with a description of what you're wanting to do, including your heating source, and someone will point you in the right direction.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • dftc
    dftc Member Posts: 3

    In response to the first reply, attached are the two approaches I'm thinking of using.

    If these are horribly wrong, please let me know.  The tankless is a Takagi and if there's a way to have a signal sent to the RMB to start the circulators it would resolve the stagnent summer water problem.

  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840

    You and your family leave the house empty for 2 weeks during the summer and go on vacation to Disney Land. The house sits empty. Legionella, Pontiac Fever and numerous other water borne bacteria set up house and make them selves comfy and multiply. By the MILLIONS... And the RMB is going to do what to keep them from doing this? Turn on and mix things up? Guaranteeing equal spread and opportunity?

    IF you are on a city water system you would have to have chlorine at many times the normal required concentration to even put a dent in the bacterial population, and even IF you had the concentrations available, simply turning on pumps isn't going to move any of the disinfectant into your distribution system.

    So, you and your family come home, and there are numerous people on this plane that have some variant of the flu. Your immune system is depressed. You start feeling achy. All you want to do is get home so you can crawl into bed and get some badly needed rest. But before you do, you decide to hop in the shower, and get some vapors going to breath in and try and break up some of the chest and head congestion you are experiencing. As you stand in the mist of your shower head, breathing deeply, you are inhaling deadly Legionella bacteria, which set up camp in the microphages of your lungs, and soon, you have bacterial pneumonia. If you happen to be so lucky as to have someone to take you to the doctor, and the doctor actually diagnoses the bacterial pneumonia, and prescribes broad spectrum anti-biotics to kill almost every bacteria (beneficial or not), you MIGHT survive.

    Otherwise, I hope your life insurance is paid up.

    What you are proposing is illegal in some states, but not in all, at present. Eventually, the codes will catch up to reality (usually requires someone getting the living crap sued out of them over a wrongful death lawsuit) and it will then be illegal to do everywhere. It is illegal to do this in Europe.

    If you want to learn more on this deadly subject, go to www.contractormag.com and use their search engine and search legionella pneumophilla.

    Spend the extra money and put in a new boiler and the appropriate method of DHW production and you won't regret it. Do it the way you are proposing, and you will be kicking yourself in the butt for years to come.


    (survivor of Legionairres Disease)
    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • dftc
    dftc Member Posts: 3
    Thank you and that was well written

    I appreciate your reply and fully understand your concerns and the issues involved.  I have another question and that is, why would the water in the water tank not be growing Legionella?  They are typically held at approximately 120 degrees, which is the prime breeding range for this bacteria.  If turned off during the vacation the bacteria will simply go dormant and reactivate when the temperature is raised.  The thought process behind the circulation pump being activated when domestic hot water is supplied is to keep fresh water circulating through the heating system.

    Please let me know if you have any further thoughts.

  • Gordan
    Gordan Member Posts: 891

    Even if we were to sweep the health issues under the rug through feats of sophistry, why do you think that an open system is preferrable? For heating, a closed system is clearly superior. A constant supply of fresh water comes with a constant supply of dissolved air and minerals, both of which can lead to flow issues, corrosion and embrittlement of elastomeric seals and components. All of that for what? What is the perceived benefit?

    Taco makes a version of the RMB-1, called XPB-1, which includes a flat plate heat exchanger separating the heat source circuit (which, in your case, would be potable water) from the load circuit. As far as installation is concerned, it's a mirror of the RMB-1 but it allows you to keep your heating system closed. It only costs about two hundred bucks more.

    This still means that you would be using your tankless for a heating application, which has efficiency and longevity implications, but you seem dead set on that, so who am I to argue. At least it's a tradeoff between short term cost and long term cost.

    I struggle to see the tradeoff in the open vs. closed question. You seem to be very enthused about an open system, so tell us what you'd be giving up by going with a closed system. Were you harboring notions of some kind of radiant cooling plus DWH preheating during the warm season? How will you prevent condensation on your emitters if you're circulating cold water through the system during the cooling season?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,840
    edited February 2011
    Fighting an uphill battle all the way...

    The bacteria survives in temperatures as low as 55 degrees F, and thrives between 77 and 113 F. If you turned your tank to vacation LOW when you go on vacation, it doesn't go dormant. It thrives and multiplies. That is how I contracted it. Turning a tank type water heater to the pilot position at my second home, during periods of un-occupancy. And then running it at a low temperature during occupancy (weekends) in an effort to conserve fuel...

    Myself and Dave Yates have been on this "TURN IT UP" campaign for many years. This is the one place where "Turning It UP" makes sense. The bacteria can survive temps as high as 180 degrees F. It's not a matter of IF you've been exposed, its a matter of your physical condition and the quantities that you are exposed to. The CDC did a study a long time ago, looking for the antigens associated with ingesting the bacteria. They found that 95% of those tested had been exposed to the disease. It is in the water (omni present) so it makes sense that we have all been exposed. It's when you inhale them in large quantities and your immune system is depressed that it becomes a major issue. The higher the holding tank temperatures, the lower the bacteria count.

    Dave and I and others are saying "TURN IT UP" and the respected authorities (utility companies and water heater manufacturer lawyers) are saying "TURN IT DOWN".

    Who do you think has a larger voice presence in the industry? Why do you think they want people to turn down their water heaters? They use the guise of energy conservation, but in reality it has to do with the litigious society in which we live.

    Many excellent European hydronic heating manufacturers refuse to do business in the US with their products because they fear the presence of lawyers in our country, and the real possibility of getting sued for stupid reasons.

    I know that Dave performed an economic analysis of keeping a tank at 120 versus 140, and it didn't make squat for additional costs. Something like a buck a month. To me, that is cheap insurance against exposure.

    Keeping "fresh" water circing through the system guarantees that the bacteria gets plenty of oxygen to guarantee cellular success. In a lot of cases, even if the fluid is not circulated, but plastic tubing is present, the bacteria gets more than enough oxygen diffusing through the walls of the tubing. Even oxygen barrier tubing allows enough oxygen in to insure healthy growth.

    Thank you for taking the time to educate yourself, and anyone else reading this thread. It comes down to education, and once educated, most people will make the right decision.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Hot water heater temperature question.

    I currently run my hot water heater (indirect) such that the water comes out of the nearest tap at about 120F. I do not suppose the hot water heater tank is much hotter than that. If I am persuaded that I should raise the temperature to 140F, I would need a thermostatically controlled mixer valve, such as a Taco one like this:


    Other than the expense of having it installed, and the slight increase in price of the gas to heat it, there remains what to me is a more serious question. My house is 60 years old, and most of the hot water pipe is embedded in the concrete slab. The only practical place to install that mixer valve would be very near the output of the water heater. That would leave all the copper tube in the house to run at a maximum of 120F (and probably lower if the use of hot water stopped, as it would most of the time). So I would have at least a 30 foot run of tubing to, say,  my kitchen sink or relatively cool water where the bacteria could grow. While I understand that copper tubing can inhibit the growth of bacteria, I doubt it would be very long before a protective coating of goodness-knows-what would form on the copper and protect the bacteria, not what I would want to happen.

    I have lived with 120F water heaters for over 30 years and seem to have survived. But I keep getting older and who knows if my immune system keeps up.

    Since the problem of bacteria growing in the distribution piping would remain, is there really much benefit to running the water heater hotter and mixing it down right after it leaves the tank?
  • Gordy
    Gordy Member Posts: 9,537
    edited February 2011

     You have to look at it as the volume of water you are treating, and the area of the surface you are treating. Ideally the highest level of protection would be a mixing shower control valve. This would insure the supply piping is getting a dose of high temp water. What I believe is you will never kill all the bacteria, but you want to minimze it as best you can.

     We have all lived with it, and as Mark has experienced, its just a matter of the right variable to come in to play to make it happen. If you can control some of the variables your odds ofcontracting go down.

    Stay healthy

    Lower the bacteria count in the system

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I think you are right.

    I would love a mixing temperature controlled shower valve. I have such a thing in my darkroom, where the piping is all exposed.


    To make any change in the shower, I would have to rip out the 3-valve system in there now. And to do that, I would have to have a lot of ceramic tiles removed. I asked a plumber about doing that, and he said when all was said and done, I would have to have the entire bathroom retiled, since there is no way to match the tiles I already have. I am not in love with the tiles I have, but would want them to match. The pipes are not accessable from behind.

    If this house had been designed, there would have been hot water pipes (140F) to the diswasher, recirculating warm water piping (120F) for the kitchen sink (where it takes well over a minute to get hot water) and my darkroom. And shock protectors, or whatever they are called, near the diswasher and washing machine.

    I guess I will have the mixer valve put in near the hot water heater on the theory that I will be discouraging bacteria growth in the 36 gallons of water there, and forget, for now, about the water in the pipes. But that will have to wait until summertime.
  • Gordy
    Gordy Member Posts: 9,537
    edited February 2011
    Same boat

     With valve replacement JDB. I have glass tile, large pieces. My crane mix valves are almost 60 years old, and hanging by some rebuids by me. Can't get parts any more.  Gonna have to do it though in the future.  Don't make them like that anymore.  I was lucky enough that a recirc line was plumbed when house was built. Its gravity, and works quite well.  For what ever reason the shower valves when all the way hot do not disperse 140* water must mix in the valve to dilute.

  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    open systems are garbage

    both for bacterial issues and possible longevity issues, never mind it breaks every code in the book to convert a closed system to a potable system... once a product has been used in a closed system, it can never be used in a potable system.

    secondly, the takagi is a poor choice for a radiant heat source.

    third, if you have to do this, do it with the XPB at least, which has the heat exchanger. same number of pumps, miniscule difference in efficiency, keep the system and your family safe.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
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