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Need some help planning radiant system.

radintboyradintboy Member Posts: 5
Hey folks, first time radiant heater here.

The radiant system will go in an insulated slab, there will be a greenhouse built on the slab. The greenhouse will be air tight with double layer inflated poly.

The heat loss calc says I need 180,000 btu for 1400sf 70x20 slab. Roughly 150btu/ft

I had planned to use 1” pex spaced 6-12” OC.

I am a complete beginner at this, I would really appreciate any insight or guidance on slab thickness, insulation options, anything you recommend (including hiring a professional) thanks for reading!


  • Jamie HallJamie Hall Member Posts: 12,710
    If you are trying to get that much heat our of a slab you definitely need a really good professional to design and lay out the radiant system -- piping, pumps, controls, the works.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • John RuhnkeJohn Ruhnke Member Posts: 880
    You can start by going to the "Find a Contractor" section to find somebody that knows what they are doing. There are some real experts listed there. Just ask the contractor what kind of experience he has.
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
  • radintboyradintboy Member Posts: 5
    > @John Ruhnke said:
    > You can start by going to the "Find a Contractor" section to find somebody that knows what they are doing. There are some real experts listed there. Just ask the contractor what kind of experience he has.

    Would you be interested? Is this something you have expierience with?
  • ScottSecorScottSecor Member Posts: 384
    Not sure what area you are in but I can share from a little experience heating greenhouses here in northern NJ.

    Approximately twenty five years ago we were hired to replace two decent size scotch marine style boilers (about 1.5 million BTU each). The existing boilers appeared to be about twice the size of the heat loss of the roughly 200' x 250' greenhouse that was originally installed in the 1950's (and expanded two times). In addition the owner paid a competitor to replace these existing boilers about five years prior. We became friendly with the owner and he trusted whatever we suggested.

    We ordered two new Weil McLain cast iron sectional boilers with Powerflame burners. These new boilers combined output was about two thirds that of the of old boilers. The radiant piping was a hodgepodge of steel and copper with five or six large pumps, one for each zone. The slabs were wither six inch thick concrete and blacktop (yes, blacktop like a street). The walls were single pane glass, and the "roofs" were double thick poly with tiny fans blowing room air between the layers if I remember correctly. I am almost certain there was no edge insulation and I am certain there was no insulation under the slabs. The boilers were piped primary secondary and everything ran perfectly when we first installed the boilers in October.

    This particular greenhouse supplied most (if not all) of the poinsettias for our highly populated area of the state, this includes churches, smaller greenhouses, food stores, etc. These poinsettias that were grown from seeds were to be delivered or picked up within two weeks of Christmas. As such it was critical that the greenhouse temperatures were just right for the plants to be the perfect size for Christmas (the bigger the better if memory serves me). In addition, the four managers got their annual bonuses based on poinsettias sales. I gather the greenhouse was "in the red" until December came around every year.

    When the nights got much colder come November we started to hear complaints about lack of heat, steel pipes sweating, etc. On a cold December morning I arrived before sunrise, only to learn that the supply water was about 90 degrees and the return was less than fifty degrees with both boilers running at high fire! We did learn that the gas company had to up-size the gas meter and we assumed this would solve our problems after we learned the gas pressure was about one inch of water column too low during peak demand.

    A few days later, the temperature plummeted again and they could not get the building above 55 degrees on a cloudy day (roughly December 10th). After a heated discussion with the owner, my boss/father (Ken) and myself, we agreed to provide and install a third boiler - FOR FREE! We were unaware of just how much energy could be lost overnight. Looking back , we gathered the steel boilers likely failed in just five years due to the fact that they condensed most of the time. They were not piped primary secondary, and were slightly undersized. The (two) new boilers were also condensing on those cold overcast mornings, until we added the third boiler, despite the primary secondary piping we installed. Once the third boiler was installed, the problems went away and the system ran fine.

    My point here is be VERY careful with your heat load calculations. I will never forget looking through the inspection port on the two brand new cast iron boilers and noticing they were both "raining" inside. Looking back, if there was any insulation under the slab and some edge insulation the two smaller boilers probably would have worked fine. For the record, this greenhouse owner died about ten years ago and the family sold the property to developers (the boilers were still operational when they closed down).
  • radintboyradintboy Member Posts: 5
    Thanks for the insight. I am 100% looking to hire a professional to design the system, and re-calculate the load.
  • ZmanZman Member Posts: 5,737
    Where are you located? @Rich_49 is always up for an unusual design. My guess is that your heat loss calc is too high. Insulation on your slab edges may be a tricky detail with that type of structure.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rodhot_rod Member Posts: 13,196
    look into root zone heating for greenhouses. Rutgers has done research on the advantages to heating the objects, plants, instead of the air space in the room.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • radintboyradintboy Member Posts: 5
    This will be in Wichita Kansas
  • John RuhnkeJohn Ruhnke Member Posts: 880
    To far away for me
    I am the walking Deadman
    Hydronics Designer
    Hydronics is the most comfortable and energy efficient HVAC system.
  • radintboyradintboy Member Posts: 5
    Thanks for looking, anyone have recommendations on who could design something like this? I think a drawing with detailed plans would be sufficient for me to build this, or at least lay it out, photograph and make adjustments before pour.
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