Advice for Hot Water Demand for DHW and Radiant Heat

Discussion in 'Tankless Water Heater Forum' started by Peter Varieur, Mar 21, 2020.

  1. Peter Varieur

    Peter Varieur New Member

    Joined:
    Mar 21, 2020
    Location:
    03246
    I purchased a home in New Hampshire. I want to heat with radiant heat and also supply my DHW. I have a few issues. My shower is a walk in with 6 - side head a waterfall shower head and a hand held totaling about 15gpm. my radiant is 2100' of 3/8" pex. 1200 sq ft.
    I have been having trouble finding a unit either tankless or hot water heater that will provide enough hot water. I want to try and stay away from a boiler that will cycle on and off just to maintain the temperature. My source will be natural gas.
    I've thought of getting a 50gal hot water heater for storage and feed that with the tankless. I'm figuring this on a 15 minute shower so could eat up just over 200 gallons but not all hot because of mixing. Any input would really be apreciated
     
  2. fitter30

    fitter30 Member

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    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Here is a calculator for your hot water % just change temp to f and to gpm
    https://www.spiraxsarco.com/resources-and-design-tools/calculators/water-mixing/water-mixing
    As for as the 15 gpm can always use two heaters that talk to each other and have enough gas service to feed them. Radiate heat uses alot lower water temp 5-10* over room set point and have you looked at your floor covering temp ratings. How many loops and lengths are they. Double check tubing size.
     
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  4. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
    Take a big step back and calculate the design heat load (Manual-J or similar). Tankless units tend to be way overkill for the heat loads of code-minimum 1200' houses. There may be a combi-unit that works without short-cycling, given the amount of thermal mass you have to work with in the radiation. An "open system" running potable water in the radiant floor is an inherently bad idea, since the floor will be in the Goldilocks Zone for Legionella growth several months of the year (maybe even when the radiant heat is running for a radiant slab.)

    There are no tankless (or combi) units that are going to keep up with a 6 sidespray deluxe shower. If there is more than 4' of vertical drain downstream of the shower you may be able to get there with a drainwater heat recovery boost, returning half or more of the heat going down the drain back to the incoming water stream. (Can't get there if the shower is on the first floor of a slab-on-grade foundation house.)

    [​IMG]

    A tankless heating up a standalone water heater is re-inventing the commercial tank water heater. An HTP Phoenix model with a big enough tank and a modulating 199KBTU/hr burner has the same capacity, in a pre-engineered much better solution than a 199K tankless + tank. The Phoenix comes in 55, 80, and 119 gallon sizes, with either 76K, (for the "Light Duty" series), 100K, 130K and 199K burners.
     
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  5. Peter Varieur

    Peter Varieur New Member

    Joined:
    Mar 21, 2020
    Location:
    03246
    Yes, I've done a radiant heat system for my son but his is on a boiler. I have 10 loops at 200' and my flooring will be ceramic and carpet in two bedrooms. I'm going to router out 3/4" plywood for the loops layout, worked great for my son's
     
  6. Peter Varieur

    Peter Varieur New Member

    Joined:
    Mar 21, 2020
    Location:
    03246
    Dana, never thought of the down time for the radiant heat and stagnant water. I was doing good until I figured in the shower. I would be able to drain water recovery, do they work well? I was looking at the Phoenix, you think that would work with the drain recovery?
     
  7. Dana

    Dana In the trades

    Joined:
    Jan 14, 2009
    Location:
    01609
     
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