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What size water heater for 200sf system?

craigwilliscraigwillis Member Posts: 3
edited July 2019 in Radiant Heating
Your professional opinion will be appreciated.
What size water heater should i use on the below described system?
200sf area, heat loss of less than 9,000 btu/hr, 1/2” pex staple up w/ transfer plates, 2 loops of 150’ (300lf total), single zone, crawlspace installation, north missouri, crawlspace to be insulated, joist bays to be insulated.
See attached photo for reference.

My questions are:
What size tank style water heater should i use? Due to space it needs to be a short 30 gallon or less.
What size elements?


  • Robert O'BrienRobert O'Brien Member Posts: 3,256
    200 Sq/ft? 9000BTU heat loss? 45 BTU per sq/ft? Heat loss is incorrect and if by some miracle it isn't, you can't get 45 BTU out of a sq/ft of floor surface..... without being able to fry eggs on it!
    To learn more about this professional, click here to visit their ad in Find A Contractor.
    Zmandelta T
  • ZmanZman Member Posts: 6,336
    I agree with @Robert O'Brien .
    That being said. To covert from Btu's to watts, divide by 3.412. Keep in mind that electric water heater will only run element at a time so you only want to calculate the output of one element, not both.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rodhot_rod Member Posts: 14,418
    200 X 25 btu/sq.ft. realistic output per square foot = 5000 btu/hr

    An electric tank with a 1500W element= 1500 X 3.41= 5115 btu/hr.

    Most small, off the shelf 6 or 10 gallon tanks have a single, 1650W 120V element, 5626 btu/hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HVACNUTHVACNUT Member Posts: 3,710
    Is this an addition to your home? What's the primary heat source? Warm air?
  • craigwilliscraigwillis Member Posts: 3
    Thank you gentlemen for your replies! Mr. Rohr, thank you for answering both of my questions so directly. That is the information I was needing.
  • craigwilliscraigwillis Member Posts: 3
    edited July 2019
    This is a tiny house/cabin quality built, small but very nice. The primary heat will be the radiant floor heat. Previously was heated by a 10,000 btu wall mounted lp gas heater.
    On low would run you out, and my son keeps his bedroom window open as that heat was not good for a loft bedroom.
    And, I am fully aware that a wall mounted gas heater should not be used a primary source of heat. This will become backup only in case of power outage.
  • hot_rodhot_rod Member Posts: 14,418
    really no sense in putting more radiant "heat source" in, then the floor can comfortably distribute. Mid 20's (btu/ sq. ft.) is a generally agreed on output for radiant floors once furniture, etc is in place.

    With electric tanks, an inexpensive element change-out can quickly increase or decrease output.

    Modulating electric boilers are another option for an appliance that could adjust it's output to the ever-changing load.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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