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New build help? 200 SqFt In-Slab hydronic heating (school house)

kwiggit Member Posts: 1
edited August 2021 in Radiant Heating
We're preparing to build a 10' x 20' school house in our backyard and I'm running into some issues designing the in-slab radiant heating system. Cooling will be via a ductless mini-split system with a heat pump but it's not able to keep up the room at 70 degrees in the winter when it's -10 degrees outside (we're in Zone 5 climate). So, we plan to install radiant heating in the slab.

The problem is that at 200 SqFt, boilers are oversized for our application and tankless water heaters seem to be questionable. It seems as though the best method for this application would be using a 40-gallon water heater. We will have natural gas run to the school house and definitely want to use gas instead of electricity for our heating needs because natural gas is far less costly around here (in CO).

I've reached out to Radiantec and they continue to ignore me. I've submitted drawings and details to them twice and called them 4 times over the past 8 days and they continue to tell me someone will call me back within a day or two. I now think I should avoid them because their customer service (in my experience) has been horrible.

I have a background in construction and have no problems assembling the system. But, I do want to build the proper system and am hoping some of you could assist.

Here's a drawing of the school house from the outside: https://www.screencast.com/t/BQyjMTJx

Here's the floor plan: https://www.screencast.com/t/04ldhLwvsfZ

I had told the architect that we'd go with a wall-mount tankless water heater but after a few calls to Rinnai (I love Rinnai products), I discovered that it's really not right for this application.

Is someone with tremendous knowledge willing to help determine system components and a diagram? One note on the natural gas water heater- I want it to be high efficiency so that the air intake and exhaust are piped to the exterior so that I don't have to add open venting in the room.

Thanks in advance!!


  • mattmia2
    mattmia2 Member Posts: 5,533
    Telling us what your heat loss is would help, ideally both at design temp and on a mild day. A tank type water heater is an option but a small mod con boiler with a 10:1 turndown ratio and some setting to limit the max output to make it heat more slowly and possibly a buffer tank might be a better option. A mod con boiler would have most of the controls you will need built in if you pick the right model. A water heater will require a lot of extra trim to make it work sort of like a boiler. Running just a little glycol so it won't burst if it isn't heated for some reason might be a good idea too.
  • Zman
    Zman Member Posts: 7,330
    This will be a ton of work and money for the handful of days that the heat pump won't run. Electric cove heaters would be a more affordable option. https://www.radiantsystemsinc.com/

    That being said, yours would be a good application for a small tanked water heater. You want to start with a heatloss calc https://www.slantfin.com/slantfin-heat-loss-calculator/. I am pretty sure yours will come in <6,000 BTU's

    A 40 gallon, 36k BTU heater will probably get you there. Keep in mind, you will need to derate for altitude @~4% per thousand feet above sea level.

    You might count your blessings, Radiant Tec did not call you back...
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • kcopp
    kcopp Member Posts: 3,990
    As much as I love radiant floor heat (and it is great in a slab) it seems a bit over kill for this application.
    My opinion.
    Could you really hit the air sealing and insulations details and drop you heatloss down to little or nothing?
    A 10x20 building would not be hard to do. Do a blower door test and find out what those numbers are....
    A minisplit would easily heat that sealed envelope.
    Exterior foam, insulated slab and well sealed windows and doors.

    Hot_water_fanLarry Weingarten
  • GroundUp
    GroundUp Member Posts: 1,406
    I'm glad to hear that Radiantec is too preoccupied with suing people like me for defamation to even call their potential customers back- it seems my goal is near. With that said, as much as I love radiant floor heating (it's literally my entire business model), it may be a bit counterintuitive for this application when a mini split is already in place. As others have mentioned, design load is likely to be under 6k and probably closer to half that if well insulated and sealed- meaning $2-3 on the coldest day of the year for electric heat whether forced air or radiant. If radiant is something you're stuck on, then by all means a simple water heater will do the trick. NG may only be a fraction of the cost to operate, but the upfront cost is easily 5x what a simple electric system could be and payback is never. I do a lot of small radiant systems like this in workshops, playhouses, warming shacks for skaters/hockey players, and even doghouses. Numbers are always penciled out to the clients and 10 times out of 10, they end up going with electric due to the cost factor being still less than half over the lifespan of the water heater. Something like yours works great with a simple 6 gallon WH tank, and swap the element to whatever size is necessary to heat the space at design temp. Perhaps an off the shelf 6G tank with a factory installed 2000W element from the big box store? Then either you could assemble the system yourself or purchase a plug and play piping kit from a supplier like myself to make it quick and painless.
  • psb75
    psb75 Member Posts: 570
    Do not deal with Radiantec!
    Build a tight building with an air to air heat exchanger. Especially important in a classroom in 'these' times.
    Is there a 60MBTUH mod/con boiler with a 10:1 or 8:1 turn down? Better than a water heater.

  • Hot_water_fan
    Hot_water_fan Member Posts: 537
    Skip the radiant, it doesn’t work well at this scale. Way too much labor and equipment costs for a building so small. Either install backup electric or get more insulation. Make sure you insulate under the slab.