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Need advice- open radiant system


I have posted this over at green building talk .com but I'm getting the feeling that this site has more users. Sorry for the cross post, but I'm just a layman trying to understand my heating system and keep everyone safe.

I recently purchased a home in Westchester County, NY. Long story short, the heating system is set up as follows:

AO Smith Promax Boiler, Model #: FPSH 75l 270

Control board: Taco zvc404-exp

Recirculation pump relay: Argo ar822-II

Recirculation pump: Taco 008-bf6

Radiant floor heat using PEX tubing

I have attached a pic.

I had a plumber stop by recently who looked at the system and said that it is illegal, because apparently the boiler services both domestic AND the radiant heating system. According to him, the water for the heating system "contaminates" the domestic hot water and can get people sick. Naturally I was a bit concerned, not really knowing much about this stuff, and feeling like perhaps I had been ripped off. So I've been doing some digging online, trying to understand this issue.

From what I've found it seems that I have a "open loop" system that mixes domestic and heating system water in a single tank. I can see why that would be "sub-optimal", if there is concern about stagnant water, and harmful bacteria, coming back into the domestic water supply. It doesn't appear to be illegal, however. Perhaps someone could correct me. It looks like in MA you have to install a pump to circulate the radiant heat water every 6 hours.

The townhouse was built new in 2004. One thought I had- perhaps this is just a "newer" type of installation that the plumber hadn't seen before. From what I can tell, the builder generally did not cheap out on the house. I have seen comments about "just add a heat exchanger and close the loop". On the other thread, someone suggested installing a Taco X block heat exchanger, although my system has 4 zones and runs at 75k BTU, so I'm not sure that will suffice. I'm also not sure how much efficiency I would lose by adding a heat exchanger.

Anyone have any thoughts or opinions on the best path forward? I like the idea of isolating the heating water, but don't really like the idea of adding a whole new boiler for $15k - $20k.

Thanks in advance for any advice.



  • Zman
    Zman Member Posts: 7,433
    Open system

    Unfortunately this type of system is legal in most areas.

    Legionella and other water born illnesses are rarely tested for for in this country , it is no surprise that they don't find many cases. Pneumonia is very common , it presents itself just like legionella. It always amazes me how many hospital acquired cases there are yet no testing. Countries that do test have much tighter standards.

    To answer your questions.

    A heat exchanger and associated piping will solve your water quality issue. You will not lose performance or efficiency.

    Your builder did "cheap out" on the heat system. You have an appliance designed to heat domestic water heating your house. I am surprised it has lasted this long.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Ironman
    Ironman Member Posts: 7,062
    Plumber is Right

    What your plumber told you is correct.The A.O. smith is a water heater, not a boiler.

    The biggest concern is that your system is a hotbed for legionella.

    What you refer to in the MA code is an exception that applies to an AHU that has 15 ft or less of enter-connecting piping with the water heater. Your floor probably has thousands of feet.

    This type of system has been outlawed in Europe for years and is becoming so in most places in the U.S. Eventually, it will be in all of the U.S.

    I doubt that your builder realized the issue involved. He probably just relied upon his pluming contractor to give him a radiant system at the best price. Obviously, the plumber doesn't realize the issues, though he should have.

    In addition to the big issue of legionella generation, your gonna have many system related issues, maybe significant ones, depending on the components used and how it was done.

    Your pic didn't post, but that, with as many others that you can post, will help identify other potential problems.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • kcopp
    kcopp Member Posts: 4,089

    because its a water heater the efficiency is more than likely  only 70%. You could do much better w/ a boiler.
  • zoominoff
    zoominoff Member Posts: 7

    Hi Carl, Bob and kcopp,

    Thanks for the responses so far. I have tried to attach a pic again.

    kcopp- like I said, I'm a laymen, but my impression was that this AO Smith Promax was considered a "high efficiency" unit, as it has the PVC heat exhaust which can only be used for low heat emission, which you only get from high efficiency units. But I could be wrong.

    I talked to another HVAC who has helped me with an older system in the past, and who I respect. He agreed that this setup was not a good one, and recommended a "plated heat exchanger", which he said might work (and would be the most economical solution), but that I might need to "add a storage tank as well or you will have to switch to something along the lines of a navian combo boiler they're wall-hung and have a internall coil / heat exchanger for domestic water."

    Does anyone have any thoughts on the "plated heat exchanger" recommendation? From what I can tell online, most plated heat exchangers seem to be designed for higher BTU's, whereas the Taco X block recommended by someone else might not be up to the task.

    The plated heat exchanger option seems like it would indeed be the easiest fix for this issue. I see that they are only in the $200 - $400 range, so it's confusing that they wouldn't have simply installed one in the first place, if it gives you such a huge benefit (isolating domestic from heating water).

    Thanks again for all the responses, this is very helpful.

  • zoominoff
    zoominoff Member Posts: 7
    A few more pics...

    Here are a few more pics...
  • zoominoff
    zoominoff Member Posts: 7

    Here is a diagram of my understanding of the plated heat exchanger solution recommended by a few people. Do I have the right idea?

    And would I need another pump to push water through the heat exchanger?


  • Ironman
    Ironman Member Posts: 7,062
    A lLittle More Complicated Than That

    Yes, you could add a PROPERLY sized heat exchanger. You would also need an additional circ that would have to be bronze or stainless for the domestic side. I'm assuming you could re-use the cast iron circ presently on there which is improperly installed whith the motor vertical. That should have been the bronze or stainless one on the open loop. You'll probably need a new one though as that one may not last. You also need a fill/backflow assembly and a relief valve. The present expansion tank may be re-usable if properly sized. But if you do that, you may need another one back on the domestic side.

    There is also the issue of what's gonna control the different temps to the slab and staple up. You'll also need to raise the temp on the water heater to about 140* or more. That would then require a mixing valve to temper the domestic.

    And, you'd still be heating everything with a water heater that is 70% efficient. That's right, 70%. It has PVC because it's power vented and dilutes the flue gasses with a large amount of air at the draft inducer, not because it's a condensing appliance.

    I would really consider a combi boiler at the very least if you can swing it. Your present water heater is expensive and probably will go out sooner than you expect. If you keep what you have and add a heat exchanger, plus the other ancillary parts, and then the water heater goes, you'll have spent more than if you installed the combi. And you'll have a much less efficient and inferior system. A properly sized mod/con with outdoor reset would probably save 30% or more on fuel.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Ironman
    Ironman Member Posts: 7,062

    I see one other potentially major issue: you need to identify what type of pex tubing was used in the floor. Look to see in it's labeled "o2 barrier" or "oxygen barrier" anywhere on it.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • zoominoff
    zoominoff Member Posts: 7
    edited December 2013

    Hi Bob,

    You are absolutely right on the recirculator pump. That's actually my fault. The old pump actually died last week, so I bought a new one for the plumber to install. He told me it was the wrong kind, but I needed the heat working and asked him to install it anyway. I have a new stainless steel one being delivered on Monday. So this cast iron pump is a complete loss. Expensive lesson learned for me.

    OK, thanks for clarifying the PVC piping, and efficiency. I had read about "mod/con" boilers, and assumed because of the PVC that this AO Smith was one of those.

    Your advice on the combi boiler sounds like a great idea. My water heater is 10 years old, so in theory it should still have some life left in it, but I like the idea of fixing this issue properly. Any chance you could recommend some specific models?

    Also, can you clarify exactly what you mean by "combi boiler"? I've been doing some digging, and it seems there is confusion with the different types out there. Although if you can give me some specific models, I guess that would be enough.


    Also, I will check the PEX tubing as you suggested. What will it mean if it is indeed labeled "oxygen barrier"?

    Thanks again for the help, this is incredibly useful. I will be passing this on to all of my neighbors, who most likely have the same setup as I do.


  • Ironman
    Ironman Member Posts: 7,062
    Combi Boiler

    A combi is a mod/con that also does domestic from and internal heat exchanger. Bosch, Triangle Tube, Navien, Lochinvar and others make them. They are less in cost than using a mod/con plus an indirect, but you get less domestic and/or space heating capacity. Their life expectancy is somewhat less also. They usually are better suited for apartments and smaller homes. You get what you pay for.

    The ideal would be to use a properly sized mod/con and an indirect to heat your domestic. Mod/con: modulating, condensing boiler. Usually about 95% efficient. The lower the operating temp, the higher the efficiency. The better ones are down fired and have a fire tube heat exchanger. Lochinvar WHN, Triangle Tube, HTP Elite FT, W/M 97 plus some others. The Dunkirk/Utica SSC has a similar designed water tube which is very good.

    A good competent hydronics man is the key: he's 98% of the equation.

    If your floor does NOT have o2 barrier tubing, then you have another issue that will effect how this is addressed.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • zoominoff
    zoominoff Member Posts: 7

    Hi Bob, Thanks for this information, that's really helpful.
This discussion has been closed.