Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Tankless Water Heaters & Radiant Heat

Bart Vaio
Bart Vaio Member Posts: 56
Exercise due diligence before ordering anything from Navien, get feedback from contractors and wholesalers that have tried to work with them.


  • Hydro102
    Hydro102 Member Posts: 1
    Tankless Water Heaters & Radiant

    Any feed back on using tankless water heaters for a combined hot water and radiant heat system. Small single bath straw bale new home radiant is fairly low temp 130 degree max and low load of about 35,000 btu/hr. We are trying to sell customer a Mod-con boiler but customers budget is very tight and is trying to save any where they can.
    I know maxium flow rate is about 3 gpm through wall hung and we will use safety mixing valve on potable hot water. Takagi has a piping diagram on their web site and was wondering where contactors have used it successfully and where they have gotten into trouble.

  • claysdad0818
    claysdad0818 Member Posts: 5

    You'll need to isolate the radiant from the domestic. In the off season when you don't have flow through the radiant tubing, it becomes a great environment for bacteria to grow (legionella). You can use a dedicated on-demand water heater for the radiant and one for the domestic, just keep an eye on the minimum flow vs. the btu output of the water heater. Hope this helps.
  • mark ransley
    mark ransley Member Posts: 155

    Is he saving or loosing, my Takagi is 82% efficent, there is one thats 94% Ng, but a boiler is designed for this and can be 96% efficent.
  • kpc_40
    kpc_40 Member Posts: 62
    try an..

    Embassy bms... not a mod/con has both in one package... less money.
  • Mike Dunn
    Mike Dunn Member Posts: 189
    Try a

    Prestige Excellence.

    Here is a link for some literature on the Excellence

    http://www.triangletube.com/PDF/Prestige Cond_Boiler/Excellence_Lit.pdf

  • Mark Custis
    Mark Custis Member Posts: 539

    Just saved a job for a new customer using a tankless for both needs.

    We used the manufacture's digram, with a twist. Most floors we do run about 85*F input, (think 1/2" at 9" OC) at about 2GPM per loop.

    The tankless company wanted me to three way mix the DWH taps, or he drew the diagram wrong. Having lived through the discovery of Leagonella, I like my DHW hot.

    We took the print and added a braized plate heat exchanger between the floor and the DHW, as instructed. The tankless would let me do what ever I wanted up to about 140*f with out a reprogram. Sooo

    We put a potable pump from the potible side of the HX back to the cold inlet to the tankless, with enough head to prove flow to fire the tankless. We put a three way manual mixing valve so that the mixing valve controlled the floor tempurature through the heat exchanger.

    Floor water never sees the DHW side. The floor is happy and the customer has warm feet. I set the tankless to run at about 125*F based on its logic.

    Down Side: Tankless warranty is halved if used for heating.

    Just an idea. I have pictures and pdf cads.

    Good Luck

  • You are better off using a tank water heater than a tankless in a heating application.

    efficiency will be very similar between the two, and the tank heater will be much cheaper.

  • However, I will note that 35kBTUs/hr is not so low that a mod/con doesn't make sense. that is, if this is a cold climate, you are not doing the clients any favors by putting in a less efficient heat source.
  • FATBOY040575
    FATBOY040575 Member Posts: 17

    Look into the Navien tankless, 98% cond. pvc vent with rec, combine this with the heating box and you are golden.
    The heating box can allow upto five t-stats, will do your dhwp, and has the exp. tank, air elim, flat plate h/ex, system pumps ect....
  • heatboy_11
    heatboy_11 Member Posts: 6
    Ever check the ...........

    ......pressure drop through one of these units? Figure 20+' at 4 GPM. That's a 009 just to drive the HX. Awful lot of money to put into a throw away heat source.
This discussion has been closed.