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In hydronic in topping slab vs hydronic fan coils?

i have a small in-law unit. about 400sqft total and maybe 300sqft available for pex. i'm in san francisco where it's maybe 45 degrees as the average low on coldest months. the building is up to current codes...r30 ceiling, r19 walls, a good amount of windows but all up to CA current codes. for domestic hot water there are 2 sinks, a shower, 18" dishwasher.

the initial plan was to put down 1" ridgid polystyrene on existing cement slab. then a topping slab of about 2" gypcrete with pex in it. i wanted an "open" or "direct" system but apparently they are illegal here. so the ideas was to use a combi.

i got several contractors ready to install a combi. one bid was for a Lockinvar Noble combi boiler 110K BTU. then i had another guy come in and tell me to scrap the radiant idea. he said floor radiant was overkill, the tankless heater would rapid cycle, it would cost more, and i'd never have the need for that amount of heat. instead he recommended i insulate the floor as above and install hydronic fan coils like this in the toe kick of counters. two of them. then have a small tankless heater for domestic water.

i went to this calculator:
it came up with a need of 1,800 BTU/hr. the Lockinvar puts out about 50 times that. is that even right? even the lowest rated hydronic fan coils puts out something like 2500 BTU.

my questions is: is he right? does it make no sense to do pex in the floor? i got radiant quotes from 3 people at least and none of them mentioned this to me.

the only advantage i see with the pex in floor is no fan. i don't like the idea of fans running both for noise (i'm an audio guy) and for circulating dust from the intake.


  • hot_rod
    hot_rod Member Posts: 14,786
    A small space, in a mild climate could present some challenges.

    Some suggest the human body give off 400 BTU/hr, 500 BTU/hr from a refrigerator, invite some friends over load the fridge with beer, crank up the amp :)

    Maybe some panel rads, even electric ones may make more sense than a entire boiler system for a small space?

    I built a 160 sq ft tiny home with radiant, powered by a small electric boiler, I downsized the element to 1500W

    What about cooling?

    I don't think you want to pour gyp over foam, it is not a very structural product?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    HVACNUT Member Posts: 3,918
    Ductless heat pump and an electric water heater.
  • Zman
    Zman Member Posts: 6,565
    edited August 2018
    Gypcrete over foam will likely crumble.
    I would go with a minimum of 3" high strength concrete.
    If you have your heart set on in floor heat, a heat exchanger off a water heater would do the job nicely.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • duffy_4
    duffy_4 Member Posts: 70
    If you go with radiant use zmans suggestion about the concrete and could use taco xblock of huh. It’s plug and play simple install for small radiant ares

  • Brewbeer
    Brewbeer Member Posts: 616
    How about electric radiant? You could set it up to heat during the night/early morning when electric rates are lower.
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
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