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Potable radiant versus non pressurized

HousedocHousedoc Member Posts: 66

A customer I visited yesterday wants a solar,wood and propane system to heat domestic hot water and radiant heating for their master suite. In other words, solar will heat 80 gallons of potable water via flat plate collectors and an internal heat exchanger. He wants to supplement the solar hot water with a wood fired boiler located in the yard and top off any deficit with a propane Rinnai tankless water heater. So it's solar, then wood then propane when the wood fire goes out. At the same time, he wants to provide floor warming in the proposed master bedroom, bath and closet. Radiant distribution can be done with quick trak, on top of the subfloor.

I envision the solar storage tank receiving cold water supply. Then preheated water would feed a second tank, an HTP with dual heat exchangers. The bottom exchanger would be connected the the non pressurized wood boiler and the top would be parallel to the Rinnai. This lives me with a question.

Which is the grater sin. (a) potable water in a radiant floor warming system or (b) non pressurized water in a radiant floor warming system.

I hate using the Rinnai this way, but he already has it. I am just trying to give him the best set up he can afford.


  • ZmanZman Member Posts: 6,312
    Have you???

    First of all don't mix potable water and radiant, period.

    Non pressurized radiant will only work if the boiler is higher than the floor.Why not use a plate exchanger on the wood boiler?

    Have you done a heat loss calc on the space? How about the domestic water needs? My guess is that the unless you have a huge solar panel, you will not generate enough energy in your winter months to supply more than domestic water. I would suggest looking at these numbers carefully as it would be much simpler to do DHW solar and a separate heat system. There have been several posts about using a Rinnai this way, It looks ugly. A combi mod con would be a good choice.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HousedocHousedoc Member Posts: 66

    Zman, I assume you are saying don't use potable water in a radiant heating system... What is the disadvantage of doing such over a small area? I have a bathroom and one bedroom. It is about 450 sq ft. The heat loss won't be more than 5,000 btuh. Hardly worth a boiler, especially for just 2 people.

    The solar connect is token to earn an additional state tax credit. I know there will be no meaningful solar contribution to heating the floor.
  • Mark EathertonMark Eatherton Member Posts: 5,837
    Legionaires disease...

    WILL flourish in the system during non use periods. There is no safe way to do this other than complete isolation with a heat exchanger. None.

    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.
  • kcoppkcopp Member Posts: 3,597
    there are some....

    jobs where you can "talk" the customer into doing it the right way... other no way to get them to see the proper way to do it. Run away.....if your gut says to.

  • HousedocHousedoc Member Posts: 66


    Wouldn't a control circuit to energize the long periods of non use reduce the likelihood of stagnant fluid? Honeywell makes a thermostat with that function built-in.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    edited July 2012
    I am just a homeowner.

    My boiler has a pump exercising feature in the controller. It will run the selected circulators for 10 seconds every time 48 hours expires since the circulators last operated.

    For keeping the circulators from seizing up due to inactivity, I imagine this is just fine.

    I see several problems with using this feature to overcome the dangers of combining domestic hot water and hydronic heating systems without suitable separation.

    1.) 10 seconds is nowhere near enough to flush the water from the entire circuit(s) of the hydronic heating system.

    2.) Since my circuits are different lengths, have different flow rates, and so on, I have not even figured out how long it would take to ensure that all these would be adequately flushed. It would surely be many minutes. And this would assume that all zone valves, if any, and all TRVs, if any, were fully open.

    3.) Would a cold water flush, or a domestic hot water temperature flush, be enough to clean out the entire system? If not, I would need the heat source to be turned on and temperature raised to a temperature sufficient to sterilize the entire system. This could easily require 160F or even more. And if I did this for long enough, and frequently enough (how often is frequently enough?), I would not wish to live in my house during the non heating season. It went up to 100F here last week. The last thing in the world I would want is to be putting 160F water through my heating system at such a time. I would not need more than 120F water in there when it is 14F below design temperature around here, much less 144F above design temperature. Also, I would not wish to pay the gas (or oil, or electric bill) to do this. Would I need a super-sized air conditioner to remove all that heat?
  • ZmanZman Member Posts: 6,312
    The customer's always right.....

    Unless of course they are wrong. Then it is your job as a professional to tell them. If they don't want to hear it you will have to politely walk away.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEISWEI Member Posts: 7,356
    the problem is temperature

    If you could guarantee that the fluid stayed at or above 140F this might work, but 140F and floor heat really don't mix.  Isolate the radiant heat, use an ODR-controlled mixing valve and all is good.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,665
    If you could guarantee that the fluid stayed at or above 140F this might work,

    Not in my house. The hottest I would run my heating system is 120F in the slab and 135F in the baseboard, and this only when the outdoor temperature is below 6F. I would absolutely not wish to allow this to happen during any time when the outdoor temperature was more than about 6F outside. How would I keep the house cool if there were 16,000 BTU/hour going into the house from the boiler, say in the summertime?

    You are right: the domestic hot water just has to be isolated from the heating water.
  • Charlie from wmassCharlie from wmass Member Posts: 4,158
    Not allowed by Massachusetts plumbing code.

    fan coil units less then 50 feet round trip of piping from water heater to and back are only cases domestic is allow for space heating, and I still will not install it because when someone gets sick and the code gets changed I do not want to be holding the bag. what you want to do can be done safely with proper heat exchangers, heat dumps, heat storage and more. There are dual coil indirects that would take care of the solar and wood heating the domestic with the Rinnai being fed from that to kick in if the water temp drops. The radiant needs to be none domestic water, so feed this from a seperate indirect dedicate to non potable water and you have the barrier between the wood furnace and the radiant tubing.

    Without talking specific $ amounts I can say it's not going to be cheap to do it right. There is reason green is the color of being ecologically present and money.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Mark EathertonMark Eatherton Member Posts: 5,837
    Circulation won't cut it...

    Unless the water is above 140 degrees F., and must be maintained at that temperature for many minutes in order to guarantee sanitization. Guaranteed discomfort in the middle of the Summer, to say nothing of wasted energy. Simply circulating water only guarantees a source of food and oxygen to the flourishing bacteria. Other than having a heat exchanger to isolate the heating fluid form the potable hot water, there really is no safe way to do this.

    Chlorination has to be 100 times stronger than the usual minimum requirement before it has any kind of impact against the bacteria.

    Although allowed by numerous building codes, it has been proven to be unsafe, and has been outlawed by many intelligent AHJ's that can see through the smoke and mirrors set out by numerous DIY purveyors, and home builders associations who are looking out for THEIR interests, and not the best interests of the consumer.

    It is not allowed in Europe, for obvious reasons.

    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.
  • HousedocHousedoc Member Posts: 66
    Got it

    Okay guys, I got it......pour some Clorox in the well, put a timer on the circ pump and install a window unit in the bathroom to compensate for a hot floor in the summer.

    Or, install a heat exchanger like everyone says. Hmmmmmmm. Decisions, decisions...
  • Mark EathertonMark Eatherton Member Posts: 5,837
    Alternatives to hydronics...

    Maybe you should consider electric radiant. For small, occasional use spaces, like a bathroom, it makes good sense. I prefer the use of a screen type of mat, so the possibility of losing the whole circuit due to nails, etc, are avoided. Look at STEP warm floors.

    Some times, ya just gotta bite the bullet, and move on.

    Thanks for doing the right thing.

    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.
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