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Sanity check of new radiant heat design

Hi all,

I’ve been lurking here for quite a while, absorbing as much knowledge as I can. I’m getting close to installing an in-slab radiant heating system for a large workshop and would like a sanity check on my design. There’s a lot of info in this post and I’m certainly not asking anyone to scrutinize every aspect of the design but I would appreciate any feedback if I’ve made any major errors. A few details to start:

• The workshop is 60 x 72 with an 18-foot ceiling
• The slab is not yet poured but will be 6-inch fiber-reinforced concrete with 2 inches of foamboard insulation under the entire slab and around the perimeter
• The tubes are ½” PEX and will be stapled to the foam
• R-19 insulation in the walls and R-49 in the ceiling

I downloaded the trial version of LoopCAD to design the system. A drawing of the shop and layout is attached along with the design report for the system. The room at the bottom left of the drawing is a 24 x 24 foot storage area. The larger room toward the top is the mechanical room. The small room next to the mechanical room is a bathroom. The remainder of the space is an open shop area.

I also attached a schematic of the heater/manifold/pump design. The main shop area has 12 circuits on one zone which are run off a single manifold while the remaining circuits are run off a smaller manifold with individual zone valves. Later, I may put an office above the storage area so I included a manifold with a couple extra circuits.

I’ll stop there for now. Thanks in advance for any thoughts or suggestions.


  • hot_rod
    hot_rod Member Posts: 22,120
    edited October 2022
    Sounds good so far. 

    Do you need a 6” slab?  That is a lot of mass to stop and start, especially with tube at the bottom of the slab 

    80 yards  for a 6” pour,
     53 yards for 4” pour
    54 ton difference  @ 4000 lbs per yard

    What is concrete running per yard there?

    Unless you are running very heavy equipment on the slab?

    Ideally the tube wants to be 2” below the surface

    While it doubled the tube cost, 6” on center helps with start up, lower supply temperature required and more consistent floor temperature 

    Any rebar or 6x6 mesh planned?

    What is the heat load number from LoopCAD?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • ScottSecor
    ScottSecor Member Posts: 855
    I do not see any attachments.

    I am not suggesting our methods are the best, but the performance is pretty good. We use 2" inch foam, then "chairs" that act as supports for the six inch wire mesh, then tie the pex to the top of the wire mesh. This gets the tubing up off the bottom of the slab and closer to the surface we're trying to heat.

    Size your boiler based on the heat load calculation. I suppose you know this, but the slab/barn won't heat any faster with one million btu boiler if only a 80,000 btu boiler is required. Size your system pump based on the load and head loss, same concept as boiler sizing (too large of a pump won't heat the space any better or faster).

    The reason I mention this is we see people oversize boilers all the time. Most of the time this results in more expensive equipment, typically the new boiler short cycles and some of the components are short lived.
  • OhioBarnBuild
    OhioBarnBuild Member Posts: 10
    edited October 2022
    Let's try the attachments again...

    Thanks for the comments and questions. I do have answers for them but I've got an early day tomorrow and it's late :) so I'll get back to you tomorrow afternoon.
  • GroundUp
    GroundUp Member Posts: 1,907
    The heat exchanger, system circ, "secondary" circ, and piping are all too small for what you are trying to do. The tankless WH will short cycle itself to death in short order due to the small zones and undersized circs. The flanges specified to connect the HX are an interesting twist, but serve no purpose- unions are a more proper selection.
  • hot_rod
    hot_rod Member Posts: 22,120
    Looks like DHW and heat from one unit?

    How much DHW do you need? I would use a 120 Combi boiler. it is designed for exactly what you are doing.
    No HX required, more control options, an official, stamped, heating appliance.

    If you could go with a 4 or 5" slab, there is the $$ difference between a tankless WH and a boiler :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • OhioBarnBuild
    OhioBarnBuild Member Posts: 10
    Thanks, everyone, for the feedback and questions.

    The reason for the 6-inch (technically 5½ inches) thick slab is to handle the weight of a medium-sized box truck, around 12,500 lbs, and other heavy vehicles. The engineer who spec'd the building and slab gave me the option of using fiber in the concrete or 6x6 mesh. After reading about some of the challenges with mesh (falling off support chairs, etc), I chose the fiber route.

    LoopCAD lets you choose the depth of the tube in the slab so I tried it at the bottom and at two inches below the surface, as suggested. The only difference I could find in the output report is putting the tube two inches below the surface would let me drop the supply temperature from 120°F to 114°F. Is there more that I’m not seeing? Clearly there’s energy savings there but I’m not sure how long it would take for the energy savings to cover the cost of the materials to raise 4,200 feet of tubing up 3½ inches (months?, years?)

    Couple things on the circuits. On the drawing, the bathroom looks like it’s a separate zone but it’s actually tied in with the main floor using a zone valve on the smaller manifold. Adding the extra bit of tube in that room to the main floor area really goofed up LoopCAD’s tube layout for the main floor. I wasn’t planning on heating the storage area as much as the main floor (maybe 20°F lower) so I put it on a separate zone. All that being said, would it make more sense just to run the entire building as one big zone? It would definitely save some valves, thermostats, etc.

    The water heater I show on the drawing is definitely oversized. I originally planned for a much larger living space, a couple bathrooms with showers, laundry, etc. I changed my mind on that but didn’t reduce the size of the water heater to better match the load from heating and one bathroom with a shower. Per @hot_rod’s recommendation, I did some searching and found the Rinnai i120CN. Looks like a nice design and I see your point about having everything together but they are definitely proud of them. I’m not seeing a lot of reviews for this one, though. Is there another that is highly regarded? My system parts are actually coming in below what I budgeted so the extra $$ for the boiler is not out of the question even without the thinner slab.

    Thanks again.
  • hot_rod
    hot_rod Member Posts: 22,120
    You get one chance to do the tubing with a slab. It does take more time and work to elevate the tube, here are so numbers to compare. I don't know if the calcs were run for 6" slabs.

    With elevated tube:
    less downward loss
    lower SWT
    higher btu output
    faster response

    Over the life of the building the better performance of the tube elevated and 6" spacing may pay multiple times over.
    I used the sheet wire and thicker gauge #4 wire
    On my shop, about 1200 sq feet my son and I tubed it in about 4 hours. I elevated the mesh on small foam blocks that I cut from the slab insulation.

    I used sheet mesh 8' wide 16' long 6X6 # 4 wire.

    I see more and more of the radiant pros spray foaming instead of sheets, price works out and you get a better vapor barrier and no broken sheets from walking over them.

    The graph shows 6 vs 12" spacing


    All the boiler brands offer combis Lochinvar, Viessmann, NTI, US Boiler, Triangle Tube, IBC, many more
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • OhioBarnBuild
    OhioBarnBuild Member Posts: 10
    Thanks @hot_rod for the article link! I wish I had seen it before I started on my design. It answers so many of the questions I had and has some great tips, like how to route the tube near the control joints, that I hadn't seen before. I definitely see your point about having the tubing closer to the surface.

    My last question, at least for now, is about elevating the mesh on the foam blocks (a very clever idea). How did you keep the crew pouring the concrete from bending the mesh by stepping on it during the pour?
  • hot_rod
    hot_rod Member Posts: 22,120
    Yes the mesh will sag some between the chairs or blocks. I use the heavier wire gauge and it doesn’t bend easily 

    6x6 # 10 wire is standard. #4 or 6 is a lot heavier wire
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • OhioBarnBuild
    OhioBarnBuild Member Posts: 10
    Been away from this for a few days but now back at it.
    @hot_rod, just so I'm certain I understand your comment about the mesh sagging, are you saying the heavier gauge wire will spring back if someone steps on it between the supports?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,828
    edited October 2022
    I noticed the DHW heater doing both the space heating and the DHW. I saw only one mention of that above by @hot_rod. Bob "Hot_Rod" Rohr's comment seemed to just be a brief mention of the unrecommended use and nothing that strongly discourages that type of system.

    1. How much water will be in the heat exchanger and the approach piping that will be stagnant over the summer?
    2. Will the Taco 007 bronze have enough head pressure to force the flow switch to turn on the burner? I believe the flow switch is variable in the amount of gas the burner will receive. The more flow the more hot water is needed so the burner will have a larger flame when the faucet is fully open, compared to a faucet that is only open to a 2 GPM or a 1 GPM flow rate.
    3. The Taco 007 bronze will not change the flow switch speed in order to add more heat or less heat as the heating load changes
    4. That heater is designed to have a maximum water temperature of 125° or 130° while a space heating heater is designed for hotter temperatures if needed.
    5. There is no circulator design involved with a tankless DHW because it is designed as an open system where the hot water goes down the drain... not circulated back to be reheated.
    6. The tankless DHW is designed to increase the water temperature 50° 60° or as much as 80° based on entering water temperature of 55° or 40° or even as low as the high 30s°.
    7. The return water from the heat exchanger can be as high as 80° or 90° entering the "COLD" water inlet. that will play havoc with the sensors and software that controls the burner operation.

    I just feel that if you are going to do space heating, you should use an appliance that is designed for Space heating. Going cheep on the Boiler and then adding additional devices to "outsmart" the appliance never goes as well as you think it should. There are many DIY systems that are installed that fail to reach expectations, and end up here for a way to solve the problems that result from this "Great idea" that failed to provide the comfort you need.

    Just my 2 cents.

    And, as mentioned by everyone else above, Get the tubing right because you don't get a second chance on that.

    Mr. Ed
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 22,120
    Been away from this for a few days but now back at it. @hot_rod, just so I'm certain I understand your comment about the mesh sagging, are you saying the heavier gauge wire will spring back if someone steps on it between the supports?
    Yes, even the #10 gauge wire mesh will spring back. Unless you bend it very tight it tends to spring back up between the chairs.

    Ever try to unroll a coil of wire mesh? Never turn your back on it!

    it wants to coil back up unless you flip it and roll tightly the opposite way

    In fact the roll mesh can actually pop the tube up out of the pour unless you get it flattened 
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • OhioBarnBuild
    OhioBarnBuild Member Posts: 10
    Thanks for the info. I plan to get WWM sheets so I don't have to deal with uncoiling the roll. I haven't worked with WWM much, but it's been enough to know I don't want to mess with trying to straighten it out from a coil.
    Mr. Ed, when I originally designed the system, I didn't know there was such a thing as a combi boiler so I was going through a lot of gyrations trying to make something work. I realize now there is a lot of stuff on the web about hydronic heating that is outdated and/or overly complex. Now that I've found this site and based on the recommendations above, I'm planning to use a combi boiler which will definitely simplify things. Because picking a boiler seems to be a small project by itself (size, design, brand reviews, etc), I'm going to focus on the tubing layout and installation process first and will start a separate thread if I have questions about the boiler selection (which I'm sure I will).
  • leonz
    leonz Member Posts: 1,126
    edited November 2022
    In my opinion,

    for total the amount cubic area you wish to heat being 77,760 cubic feet you would be better off using steam to heat it heat by using dry steam to forced air garage heaters.

    Every time you open a door you are going to lose heat, the heating system is going to work continue to replace it.

    A larger heating area requires less steam pressure for the same heat transfer rate, and because of this the steam pressure in an oversized heat exchanger will be lower for the same heat load.

    As the steam pressure is less, and the heat exchanger LMTD (logarithmic Mean Temperature Difference)
    will also be less.

    One way to understand this more easily is knowing that the Empire State Building uses steam heat that is under 2 PSIG for the entire building, the entire building!

    Dry steam will heat an area faster and heat it longer as the heat exchanger will be hotter and shed heat more slowly.

    As you are in Ohio, Anthracite Pea Coal will be easy to obtain and you could have an AHS S260,
    AHS S500, stoker Boiler to make all your steam and domestic hot water as well as it sounds as if you are also intent on using this building as living space as well.

    Anthracite coal burns cleanly with no smoke.

    It would be easy to heat with steam heat as well using the correctly sized header pipe, main vents, take off pipes, and steam to air garage heaters and radiators.

    Dry steam travels at 40-50 feet per second, the thing to remember is the larger the pipe the slower the velocity, the drier the steam.

    Based on simply calculating the volume of the well insulated open building to determine the BTU needed it is almost 200,000 BTU.

    Looking at this as a novice, dry steam would be a better way to heat the building if you had one common header in the ceiling fifty feet long with four steam to air garage heaters spaced at 25+- and 50 feet
    You would be able to plumb in a riser pipe and header for heating future living space when the boiler is installed and add the risers later as well as have a domestic hot water coil in the steam boiler if desired when th eboiler is installed


    Edited today 10/28/2022

    I have another thought on this design if you still want heated floors and perhaps heated entrance aprons.

    A high strength sand mix concrete would be ideal for this as the sand mix concrete would create an even greater thermal mass even if you choose steam to forced hot air garage heaters.

    You could make you hot water heating system a gravity fed hot water system by assembling the hot water header pipe along one wall at the top of the wall where the ceiling meets the wall and connect the floor and apron loops to the header pipe at the same time and avoid using circulators entirely and use a single thermostat for the entire building.

    All you would need for the drop pipes feeding the hot water to the floor are orifice discs sized for the connecting fittings attaching the Pex to the header pipe at the top of the wall.

    The orifice discs would all be the same size to assure the hot water heat reaches all parts of the floors and the pex for entrance aprons-if pex is installed in the aprons at the same time providing slow even heat.

    If you used a coal stoker for heat and domestic hot water you would need to install a dump zone to dump excess heat and you could use a small garage heater and circulator using black iron pipe from the boiler to the garage heater and back to the boiler sump header.

    The return lines from the floor would go to the mechanical room with no special requirements.

    If you used a coal stoker you would be able to heat your building and make all your domestic water with a domestic hot water coil in the boiler using a mixing valve to regulate the hot water temperature alone or feeding a water heater.