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In-slab radiant heat source wall mount Boiler vs tankless heater myth?

papetk Member Posts: 9
First post so thank you all for having me here. I just finished building a detached shop here in northern VA. It's two sections, a 32x48 with 9' ceilings and 12" poured concrete walls, and this opens up to an attached 40x40' with 2x6 framing and a 14' - 18' vaulted ceiling.

I poured the slab which is a continuous 2900 sq ft and broken up into the two zones above. I have 12 loops of 1/2" pex 10" apart in the 5" slab under a system designed by Radiantec.

What I am greatly struggling with is the heating source. Radiantec wants me to use a rebranded Takagi 199k BTU tankless condensing heater with a copper main exchanger and stainless secondary. My HVAC guy wants me to use a 199k BTU Navien tankless modulating boiler that will only heat as much as needed and therefore be less taxing on the unit. Navien is stainless on both exchangers. Boiler vs hot water heater...

The floor system requires about 95k BTUs from a heat source based on calcs from Radiantec.

Any advice/reccs are greatly appreciated. This is going to be a auto restoration shop so I'm only looking to heat the place to 55-60 degrees max.

The other option is the Navien combi-boiler which would allow me to use this for the floor and domestic hot water, but I think it may be borderline on the BTUs required for the floor.

Thank you!


  • Zman
    Zman Member Posts: 6,565
    You have a heating application, you should install a boiler. If you had a DHW application, a water heater would be a good choice.
    You will save money and greatly reduce overshooting by utilizing outdoor reset.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Snowmelt
    Snowmelt Member Posts: 1,276
    edited August 2018
    both will work, they both modulate down I go with the navien though the takagi is more maintenance, I think the navien is also a water heater and will modulate like the takagi.
    bath should be handle that load, I have a tk3 doing a snowmelt application
  • STEVEusaPA
    STEVEusaPA Member Posts: 4,970
    Why 199k if you only need 95K?
    delta TIronmanSolid_Fuel_Man
  • papetk
    papetk Member Posts: 9
    It is a modulating system so the thought process (of my HVAC guy) is it is a lot less taxing on the system and allows future proofing for any additional heating space needs
  • ScottSecor
    ScottSecor Member Posts: 386
    I think the point Steve was trying to make is that in the worst case scenario (design day, I'm guessing about 20 degrees outdoor temperature?) you need 95,000 btu's. On almost every day of the heating season you'll need less than that to make the building comfortable. In a nutshell this is why we prefer our heating only boilers to closely match the load (output, not input).
  • ScottSecor
    ScottSecor Member Posts: 386
    In my experience with our customers asking us to up-size the boilers for the future additions, it almost never happens and they end up paying twice, Once for us to provide and install larger equipment than is needed, and the boiler short cycles often and wastes energy doing so for the life of the equipment.
    Jean-David Beyer
  • KC_Jones
    KC_Jones Member Posts: 4,653
    papetk said:

    It is a modulating system so the thought process (of my HVAC guy) is it is a lot less taxing on the system and allows future proofing for any additional heating space needs

    For that to be a valid point you would be planning or at least thinking about doubling the size of the heated space, are you thinking that?

    Also is the 95k the output of the system or the heatloss of the building, those are 2 very different things.

    32 btu/sqft for modern construction being kept at 55°-60° seems rather high to me.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
    delta T
  • plumbbob
    plumbbob Member Posts: 19
    the ncb 240 output is 108,000 on the heating side and 199000 output for domestic hot water
  • papetk
    papetk Member Posts: 9
    I appreciate all the input! Please correct me if i am wrong. I am being told that a modulating boiler will throttle down to the actual need (or the temp set) regardless of actual high end capability and therefore it will never short cycle. Is this true? Then it is just a matter of whether I'd want to pay for a boiler and only use 50% of the capability (which I don't want to do).
  • Snowmelt
    Snowmelt Member Posts: 1,276
    Yep it will always be in low fire
  • papetk
    papetk Member Posts: 9
    Thank you! I will check out the HTP UFT-120W and also the Navien 110K BTU boiler.
  • Gordy
    Gordy Member Posts: 9,514
    edited August 2018
    95k heatloss sounds awful high for a 3200 sf 2x6 construction building with a design day of 20 degrees. If that design day is correct for your area. What's the roof r value?
    Many garage doors, Many windows?
    Is the slab insulated? Slab perimeter?

    I think I would revisit another heatloss calc. You may get by with a UFT 80.
    Rich_49rick in Alaska
  • Ironman
    Ironman Member Posts: 6,367
    I've been doing load calcs for almost 40 year and I've never seen any house (even old, leaky farm houses) need more than 32 btu's per sq. ft. With tight modern construction, it's not unusual to see houses that only require 12 btus per sq. ft.

    A tight modern shop that's gonna be kept at 55 - 60* should be in the same ballpark as a new home; unless you plan on leaving the doors open constantly.

    Please don't trust radiant wreck's info or sizing advice.

    DON'T OVERSIZE THE BOILER. The modulation of the boiler is there to match its firing rate to the load which varies with outdoor temperature; it is NOT there to compensate for oversizing. ANYONE that recommends oversizing a boiler doesn't understand hydronics. Getting this through to thick-headed "technicians" is one the biggest problems in this industry. With old cast iron boilers that have a lot of mass, some oversizing could be done without much penalty. Low mass, high efficiency boiler are very unforgiving in this area.

    At 2900 sq. ft., you can expect about 72.5k btus output from the floor at a reasonable supply water temp. Why would you choose a boiler that's larger? You can't put more btus into a system than what the emitter (your floor) can extract.

    You need to do an accurate heat loss calc like a Manual J. SlantFin has a free app you can download.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • papetk
    papetk Member Posts: 9
    Thank you for the note! I just had a more thorough heat loss calc done and it looks like I'm at a tad under 60K BTUs. Half the area is has 12" bare concrete walls but that area has a second floor above it that is heated. The other half is predominately 2x6 construction with R19 and R30 in the 14' vaulted ceiling. This is the only area that has openings, a 10x20 insulated garage door and a 3' man door. All slab has 2" rigid insulation. Based on all the info I have gleaned, I am now looking at an 80K vs a 110K mod con. Sounds like even the 80k is the best choice? And then it comes down to there are many. I have been very happy with my Navien in the house, but a lot of folks are talking about HTP and Laars.
  • papetk
    papetk Member Posts: 9
    Also, do I need to be worried about the mechanical package/design from Radiant Wreck?
  • Ironman
    Ironman Member Posts: 6,367
    I'd recommend the HTP UFT080.

    I have gone behind and seen several radiant wreck jobs and the kindest thing I can say is that I was very impressed - very negatively impressed!

    I don't know what they spec'd for you other than their typical wrong choice of an oversized Takagi, so I can't speak to the rest. If you post the info, we'd would have a better picture.

    I used to have my business in Chantilly about 30 years ago, but I escaped northern Va asylum and the warden still hasn't caught me. I'm down in the valley near Staunton now.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • NY_Rob
    NY_Rob Member Posts: 1,370
    +1 for the HTP UFT-80W.

    Reasonably priced, US based support and parts... and it can be piped direct vs. Primary/Secondary which requires a second pump and more complicated piping. In addition to wall mount- HTP has a floor standing cabinet/base for the UFT's now so it can be easily installed in almost any location.
  • GroundUp
    GroundUp Member Posts: 1,175
    Laars Mascot is nearly identical to the UFT, so depending on what the local contractors and suppliers carry I would say either one would be a fine choice. As for Radiantec, I have come across a few of their designs and ah.... Let's just say I have no idea how they're still in business. They do some odd things over there
  • papetk
    papetk Member Posts: 9
    Bob, you're a lucky man to escape northern VA. I originally from central Massachusetts and came down to VA on a PCS with the Air Force. I haven't been able to leave yet, but someday I will go back to New England. Northern VA does nothing for me except pay the bills!
    Not happy to hear all the bad news on RadianTec, but at least am not going with their Tagaki, and yes, they wanted me to get the 199K BTU model. Sounds like the 80K is a much better choice.
    As for their design, I have 2 zones and 6 loops per zone. They spec'd out two pumps and then a primary loop booster pump.
    Reading reviews online (always dangerous) there are a lot of negative things about the HTP, but just as much for the Navien, so not very helpful.
  • Ironman
    Ironman Member Posts: 6,367
    Online reviews about HVAC products are almost worthless because the customer always blames the appliance when in reality it was the incompetent contractor almost every time.

    We've installed a good number of the UFT with great results as have several of the men on here.

    I also have other good brands such as Viessman, Lochinvar, ECR, Bosch, etc. But at this time and with your situation, I believe the UFT is gonna give you the best bang for your buck.

    You need to take a trip down here to the Valley to see what Va is really like. Norther Va is just the overflow section of the national asylum known as D.C. :)
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • papetk
    papetk Member Posts: 9
    Pouring through literature on both and some quick items of note. The Navien allows antifreeze (glycol) but the HTP strictly says no antifreeze or even ones approved for hydronic systems. That may be a show stopper for the HTP. I found that odd. The Navien has a minimum supply temp of 104 while I can't really find a spec for the HTP other than 40 as the min.
    Absolutely agree on DC. I live an hour and a half outside of there and I have been there once in 10 years.
  • papetk
    papetk Member Posts: 9
    Scratch that...unclear language. It says you can use specific antifreeze in a separate section
  • hot_rod
    hot_rod Member Posts: 14,786
    I think a lot of the boiler brand mod cons share the same components, hX, inducer fan, gas valve, etc.
    Controls vary a bit.
    Lochinvar is another option, comparable to the HTP line up as far as sizes and modulation. HTP has been at it as long as any and as long as you have availability and support you should be fine.

    I doubt you will find any brand with 100% positive reviews.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,257
    Yes, Viessmann is pretty much the only brand who makes all of their own stuff, which is both good and bad.

    I've had good luck with the Weil-McLain ECO boiler which has the same burner and heat exchanger as many others.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
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