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Adding Underfloor Radiant to Existing Hot-Water Baseboard System

NTL1991 Member Posts: 103
When the current baseboard hot water heating systems were installed in my 3-family house, to replace the old whole-house steam system, it seems as though they overlooked the kitchens.

The kitchens are very small, only 10.5' wide by 6' deep. When they were renovated, they were stripped down to the subfloor, and tiled. I'm assuming they didn't install a baseboard in the kitchens because, for one, there were no radiators from the original steam setup in the kitchens, and secondly, adding a baseboard behind the stove (on the left wall), or refrigerator (on the right wall) would cut away from the walking space in the area.

My idea (of course, this comes after the heating systems have been installed for 2 years now...) is to add underfloor radiant heat using PEX in the 1st floor kitchen (my kitchen). The 12" O.C. floor joists are easily accessible from the unfinished basement.

My current hot water system is one-zone series loop which heats the ~950 square foot apartment. About 85% of the basement piping is 3/4" oxygen barrier PEX and the rest (mostly for vertical drops through the floor) is 3/4" copper.

My questions are:

-How would this new radiant piping be tied into my existing 180 degree hot-water baseboard system?

-How would the temperature be reduced down to the proper temperature that radiant heat works at?

-What's the best method of securing the PEX to the bottom of the floor? Do I need heat transfer plates? Will I need to use foil under the PEX?

Nick, Cranston, RI


  • Ironman
    Ironman Member Posts: 7,379
    Radiant Floor

    The best approach would probably be to create a separate zone (and loop) for the floor. It would have its own pump and I would recommend just using a thermostatic mixing valve to get your water temp down to the 100deg range (give or take a little). IMO, plates are a must for radiant floors. You'll loose at least 30% efficiency without them. Insulate well with reflective barrier batts to minimize downward heat loss.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    Look at Ultra Fin.

    Saves having to do a separate low temp distribution system or mid system mixing/pumping station. If you are already running the boiler at high temp for baseboard, setting a mixing/pumping station is not going to save you any thermal energy, but will increase human comfort by significantly influencing the MRT, with little to no additional operating costs. Make sure the insulation gets done right. your work depends on it.

    Just sayin' :-)


    There was an error rendering this rich post.

  • NTL1991
    NTL1991 Member Posts: 103

    Very informative replies; Thank You!

    Seeing as the whole apartment is only on one loop, I'd hate to go through the process of installing another circulator and the near-boiler piping just for a room with only about 60 square feet of floorspace. It seems as though the Ultra-Fin is the type of radiant product I should be looking at. Also, now I know insulation is a must with these systems, or I'd just be heating my unfinished basement.

    Thanks Again,

    Nick, Cranston, RI
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