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Community and Q&A

Open system = Legionella as well a host of other of issues

lookloan | Posted in General Questions on

I was reading a post on which is a website someone here referred me to.  I came across this article where someone was doing a under sub-floor pex radiant system and man poster went off on him on the health issues involved with Legionair’s desease as well as other health issues.  It seems the water has to be over 120 degrees to kill off this bacteria but as water goes through the pipes it lowers recreating the problem. Talk about blue and green bacteria growing stuff in the PEX so I am wondering if any of this is familiar with people here. 

It may be they are also using this open system for heating the floors and the same water for household use from a single water boiler. Below is a short cut and paste – can someone please explain what they are talking about and is it safe to have floor radiant heat?  Seems hot water going to an air handlers and radiators for many decades – maybe it has something to do with open vs a closed system of water use.  Here is part of the post 
“Open system = Bad News”

Open hydronics systems like that one are illegal in most places and are breeding ground for Legionella as well a host of other issues.

The tankless water heater is not designed, controlled or approved for space heating. It also will have no warranty if installed like that and probably won’t last 3 years.



  1. Expert Member
    AKOS TOTH | | #1

    You have to be more careful with open systems. I would not do one on well water, chlorinated city water is much less likely to cause issues.

    Key elements is high temperature water heater and no stagnant water.

    For staple up, this means you don't want plates (they'll overheat your floor), suspended pipes or go with ultra fins.

    You want your water heater at 150F, above the pasteurization temperature of bacteria. If you are using a tankless, for Rannai this means a different controller, for the Takagi units some jumper switches and menu pushing.

    To keep the water from stagnating, best is to feed your hotwater supply through your flow field. You also need a timer to circulate the water for at least 15 min in each day even in the summer.

    Instead of an open system, the simpler one might be to go with something like this:

    Just have to make sure your heat load is bellow the coil BTU limit.

    The other budget way of dealing with open system is to put in a 2nd water heater isolated from the house water with a proper backflow valve just for your space heat but connect it to the slop sink. This way it is still considered an open system but you are separating it from the house.

    Either way, make sure the water heater is rated for combi heat application, not all are.

  2. Expert Member
    BILL WICHERS | | #2

    You can get biocides to add to the water in open systems to make sure nothing grows in them. This is done in commercial cooling towers all the time where there is no way to “close” the system (cooling towers are like a waterfall with a fan blowing through it).

    Closed systems are safer, but even then you usually want to treat the water. Even with city water, the chlorine or chloramine will gradually leave the system. The usually water treatments are biocides and corrosion inhibitors to keep the system safe.


  3. Yupster | | #3

    As someone who designs hydronic systems professionally, I'm a hard no on open systems (meaning potable water mixed with heating water in this case) for residential systems. They present too much risk for very little benefit. Just incorporate a brazed flate plate heat exchanger and separate your potable water from your heating water. Then you don't have to worry about finding potable water components for your hydronic system, legionella risk is 100% eliminated for you and future occupants who might operate the system differently.

    Radiant floor heating is 100% safe, just don't mix your heating water with your potable water.

    1. lookloan | | #5

      Hey Yupster - thanks for the input.

  4. lookloan | | #4

    Thanks guys. I have well water on this house build. You may have seen me post a lot with questions lately as I broke my ankle two days after Christmas in a couple areas so I've had a lot of time to sit and think about my latest house build since building came to a stop. I apologize for having many questions and hopefully others are helped from these questions.

    I have found the hardest part of any house I build are the plumbers. When I say this, before I can say another word, the stories come out of the same experience. They want top dollar and complain - 0ne guy said I wish you told me your ceilings were 12 feet tall as I would not have wasted my time coming here. I replied there are stairs to the attic and a sub-floor so he didn't need a later. Didn't matter - said the job was too big.

    I like Uponor and no one will use it - they all use Vega - even though I have the tools, they don't want the job if I don't go with Vega so finally I don't mention Uponor and say Vega is fine, and you still don't hear back from them so you have to chase. Last plumber said drains, vents and pex rough-in for 2 1/2 bathrooms will take him a week and it's an easy job. Still disappeared. Then they all want to discuss HVAC rather than plumbing and want the whole job when you can't even get them to do the plumbing job.

    On this sub-floor job, I'm wondering if I am better off just doing Schluters electric underlayment under the tile. I'm figuring it's $2500 for good plates (not the crap .016 thick ones) $1000 for PEX, then the plumbing where you can't get a plumber then the water issues so it sounds like I need a separate water heater, which I don't mind as I prefer to be safe.

    I have a LP (1000 gallon tank) backup generator so I was hoping to have the furnace heat the floor Pex for the low side of 12 foot ceilings and electric ducted mini-splits in the attic as a supplement. So if electric was a problem, I had LP as a backup during the winter with a nice new wood stove if really cold to supplement. So of course I am complaining from sitting for 7 weeks so thank-you for listening.

  5. this_page_left_blank | | #6

    There's a lot of misunderstanding surrounding open systems. The arguments against it only apply to incorrect designs. The claim that they are illegal "in most places" is definitely false. There is a risk of legionella and other bacterial contamination present in all domestic plumbing systems, especially those using a tank. The question is, what is the increased risk with an open system vs a conventional domestic hot water tank? The answer is zero.

    In a correctly implemented system, the fresh water is drawn through the heating pipes and into the hot water tank. Every time you draw hot water, those lines are flushed. There is no greater chance of stagnation in those lines than in the hot water tank itself. There's also no need to run hotter water in those lines. Just set your hot water tank to 140F and use a thermostatic mixing valve for the hot supply to the heating pipes to 120F, or whatever temperature the design calls for.

    Check out this diagram for a visual on how this works:

    I don't know much about the tankless option, but I'm led to believe there are actually ones that are rated for combination space heating and domestic hot water.

    The advantages of this vs a closed system is that it's simpler and has fewer things for you to buy. This is only relevant in a low heating load home.

    Having said all that, I think hydronic in floor heating in general is rarely the best option. Most of the claims associated with it (higher comfort, warm feet, etc.) are complete nonsense. I'll take an air source heat pump over the in floor hydronic heat any day (I have both).

  6. lookloan | | #7

    Hi Trevor - Thanks for the feedback. Did you use heat plates under the floor or is your built into the floor like a slab or light cement? One this build I was thinking of doing 24x24 large tiles through out rather than 3/4 oak wood floors and figuring the hydronic heat would take the cold edge out of the floor

    1. this_page_left_blank | | #8

      My house is a two story slab on grade. The ground level has hydronic in the slab, the upstairs has heat plates under the floor.

      If you build a well insulated and sealed house, the temperature the floors will need to be in order to maintain room temperature will be pretty low. My house actually only has the outer 12" perimeter coverege with heating pipes, and that was enough to heat the house (in zone 6). When the heat had been running for a while, you could feel it a bit on that floor area, but if it was spread out over the whole area I doubt it would be noticeable. Note that if your house needs enough heat that you will feel a difference in the floor, you will need an additional boiler (or water heater) anyway. Don't forget the shoulder seasons, which could be almost half the year, where you're not actively heating the house much and the floor will still be "cold" when you prefer it tempered.

      I would never choose tile over wood, except in wet areas. Aside from the cold feeling of tile, it's very hard and unyielding. We had unfinished concrete for about a year, with the original intent of polishing it. That year of walking on concrete was enough to convince us to put a floor covering over it, and it was the hardness rather than the coolness that made that decision.

  7. lookloan | | #9

    It sounds like a radiant floor with PEX is safer having it's own closed system. Not sure if a closed system means two boiler. Does anyone know of some companies that make and sell pre-made kits with the Taco pumps, , zone controller that open and close based on zone calling for hot water - basically the entire plumbing is done and you simply mount the unit to the wall that needs to be hooked up to a boiler and pex? My quess is these would connect to radiators as well. I ask this because it's hard enough trying to get a plumber to do rough in so I figure it saves a step.

  8. Expert Member
    Dana Dorsett | | #10

    I'm not crazy about open systems, but they're legal in my state (MA) provided they meet a minimum duty cycle of circulation in ALL radiation, even during the summer to mitigate against stagnation & pathogen issues.

    Water temps of 120F or even 140F don't kill legionella instantly, and radiant floor water temps are in the Goldilocks zone for rapid growth of legionella if a colony gets established. At 120F legionella reproduction slows, but it doesn't die. At 140F an established colony will eventually die off, but it takes 160F+ to have a reasonble shot of killing off incoming legionella. And that's just one potential pathogen.

    >"The tankless water heater is not designed, controlled or approved for space heating. It also will have no warranty if installed like that and probably won’t last 3 years."

    While most tankless radiant solutions being marketed online are hacks prone to longevity issues (mostly due to over-pumping and overfiring), it's possible to design reasonable systems around tankless water heaters. (I've been using a tankless water heater as a hydronic boiler for 11-12 years now, which according to my arithmetic is way more than 3. It is NOT an open system.)

    1. lookloan | | #11

      Dana - where is MA are you (if it is OK to ask) Besides GBA, are you in a building related vocation?

      1. Expert Member
        Dana Dorsett | | #13


        I don't currently make any part of my living in building.

    2. this_page_left_blank | | #12

      Not to beat a dead horse, but the concerns in your second paragraph apply equally to a simple domestic hot water tank alone. The potential for stagnation is far greater in the pipes to infrequently used fixtures than in radiant pipes that flush every time time you draw hot water.

      The hot water system in your house is not intended to remove pathogens from the incoming water supply. If they are at a high enough level to be harmful, then it has to be addressed some other way. After all, nothing is being done to the cold water. The only goal in the hot water system is to inhibit growth. If you keep your tank at 140F, you are doing a good job. 32 minutes at 140F effectively kills all Legionella. For decades, the vast majority of the developed world was maintaining their DHW at 120F. Even today, I suspect more than half still use that temperature.

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