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flat plate heat exchangers

rrekih Member Posts: 5
I have a Navian tank less for doing my domestic and radiant heating.

It feeds a closed loop heating box.

The water feeds out of the tank less and into the heating box, and then for the radiant heating travels through 2 flat plate heat exchangers, (the other side of the heat exchangers is the closed loop for the radiant heat) then feeds back into the cold water inlet to the tank less unit.

Hows much heat should I be dropping through the heat exchangers?

The tank less is set at 140deg F, what should the return temp be after feeding the heat exchangers?

I understand it depends on the radiant heating water temp, of course when the radiant heat loop is cold there will be more of a drop.




  • Ironman
    Ironman Member Posts: 6,868
    How Long is a Rope?


    You've given no pertinent info other than the supply temp.

    To attempt to answer your question would require the answers to these questions at minimum:

    1. What is the heat loss for the area?

    2. Is this a slab or "staple up" floor?

    3. If staple up, are there heat transfer plates?

    4. What's the square footage of the floor?

    5. How much tubing is in the floor? What size?

    6. How many loops?

    7. What size and number of circulators?

    8. How is it piped? Pics or an accurate diagram are needed. Pipe sizes , too.

    9. What size are the heat exchangers and what are their btu ratings at your operating temps?

    10. What size is the tankless and how is it piped?

    11. What type of floor coverings do you have?

    12. How is this system controlled?

    13. If slab, how much insulation under it and around the perimeter?

    Even with the above info, anything we give would be estimated.

    A general design principal is that there should be no more than a 10-12* delta T on a radiant floor in a residence, but given the fact that your installer chose a tankless as the heat source, I doubt that much technical knowledge went into the design.

    Are you having problems with the system? If so, what kind? What return temp are you getting after the floor is up to temp?
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rrekih
    rrekih Member Posts: 5

    Thanks for you reply Ironman

    The radiant floor heating part is working alright.

    When it comes up to temp the supply side will be right about 138deg F to 140deg F and the return approx. 130 deg F to 132deg F.

    What the problem is: the supply to the heat exchangers. The supply to the heat exchangers is 140deg F. What is happening is the return will get up to 135deg F fairly quickly and the burner will shut down then restart when the return gets down to 128deg F.

    The unit is working correctly as when the return gets close to the supply temp. it will shut down.

    When this starts happening the radiant heat hasn't come up to full temp yet. Supply 120deg F return 112deg F is when this all starts.

    My concern is the heat exchanger isn't pulling enough temp. from the supply source and allowing the return to come up to supply temp.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,221
    edited January 2014
    It's never

    going to work correctly with your current setup. Do yourself a favor and put in a real boiler or a combi unit.

  • Ironman
    Ironman Member Posts: 6,868
    Two Problems


    The wrong appliance was used. Tankless heaters are not designed, controlled or approved for space heating. The controls are looking for COLD inlet water (50-55*). The appliance is designed to heat that water to 125*, or a 77* delta T. Returning 120-135* water to the inlet, instead of 50* water, is not what the control is looking for. It's gonna keep the burner off or short cycle it at best.

    There is also a very high pressure drop through the appliance's heat exchanger which necessitates a large circulator and some form of hydraulic separation from the rest of the system.


    Your plate heat exchanger may not be large enough. The typical btu ratings on these are for a boiler heating domestic water where there's a large delta T. Typically 180* in, 160* out on the boiler side; 50* in, 125* out on the domestic side. If there's very little delta T between the two sides (as in your application), then the btu capacity is greatly reduced from the rating. Typically, you need to at least double the size of the plate heat exchanger in a "boiler to boiler" application to get enough heat transfer from what it's rated at for "boiler to domestic".

    Again, we'd need more info to make a call.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rrekih
    rrekih Member Posts: 5
    More info

    This is a Navien CR210A with a Navien Heating Box.

    They are designed to run together, there is a setting in the CR210 to tell it that there is a heating box installed.

    There is a primary loop pump, (CR210 to heat box) secondary pump for radiant heat and a 3rd pump for domestic water.

    The combo has worked till now, I am concerned that there may be something wrong with the heating box heat exchangers. I descale the CR210 heat exchanger but not the heating box. I didn't think it would be necessary as there is no flame involved in the heating box.

    The question is should the primary loop still drop 10deg F across the 2 heat exchangers even though the secondary loop is creeping up in its temp.
  • Ironman
    Ironman Member Posts: 6,868

    You need to descale the plate heat exchangers as well. The flame has nothing to do with mineral deposits collecting. Heat accelerates the process, but you have heat throughout the open circuit, including the heat exchangers.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • rrekih
    rrekih Member Posts: 5

    do you think that could be the problem?

    I have used both vinegar and a product called Flow Aid

    what do you use or recommend?