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. Thank you! -Tim

Welcome!

I think your first step should be to accurately determine your heat loss at your design condition (55 or 60F minus the 99% outdoor temp at your location). I am not saying that Radiantec is wrong, but that you would be wise to do an independent check. There are plenty of free heat loss analysis software on the Internet that would allow you to do this and we have some on our website too. You don’t want to grossly over size your heat source for inefficiency and short cycling reasons. Normally a modulating heat source is the best solution, but your HR system is just two relatively large slab emitter zones and I don't think you care about barefoot comfort. So you may be able to get by with a simpler and cheaper heat source solution. But you first need to nail down your design heat loss.

32 x 48 plus 40 x 40 is 3,136 sf and NOT 2,900 sf? Assuming 2,900 is correct and all the 12 circuits of your 10” spaced PEX are equal length, each circuit is about 290 feet long and less than the maximum 300 feet circuit length for 1/2” diameter PEX. This is good as exceeding 300 feet in length would be problematic. I trust that you used O2 barrier PEX.

Apologize for the duplicate!

Thanks for the response! The 2900 vs 3136 is due to a cold storage room that will not be heated and has insulated walls. I will check out the heat loss calculator. The only outside openings are a 10x20 insulated garage door (R5), and a single 36" man door. R19 in all exterior walls and R30 in both ceilings. The 32x48 portion has a second story that will be independently heated. The 40x40 is cathedral ceiling (scissor trusses) with R30. The 32x48 is below grade for the most part. You are correct on the heating circuits. Each one is probably closer to 275 feet. I definitely used O2 barrier PEX...Radiantec didn't want me to, but I insisted. I also got specs and quotes from The Radiant Company, and they wanted to use 7/8" pex at 16" spacing. Right or wrong, I went with the 1/2 that Radiantec suggested only because they were much more responsive and their rationale made more sense at the time. I was also laying all the pex in February and doing tight radius bends with the 7/8s was not a welcome thought.

If it helps Radiantec calculated an energy factor of .3 for the 32x48 section and .5 for the 40x40. They spec'd a 4.5 gpm flow rate through each zone and a temp from 130-150 max. So if I am correct if both zones are calling for max heat, I need something capable of 9gpm

Where are you located or what is your design outdoor temp? What do you have for insulation under the slabs? I can run the numbers and give you a heat loss number. I would have recommended an 18” spacing (since you don’t care about barefoot sensitivity in this project) and likely 1/2” PEX or next larger size. There’s nothing bad about having a 10” spacing except that your PEX expense was higher than it needed to be. 9 GPM sounds like way too high a flow rate and 130-150F sounds way too hot? Did they tell you what delta T (difference between supply and return temps) they designed the system? I can give you better flow rate and supply/return temp numbers too as soon as I run the heat loss number. We have free software on website that you could use to accomplish all this yourself too, but this is such a simple design it should only take me several minutes to get you some numbers to consider.

I really appreciate the help/offer. Thank you! I have 2" rigid foam under the slab. I tried to attach the two PDF spec sheets they gave me but it's above the size limit for attachments here. They list a 65 degree "temperature difference" but I'm not sure if that is the return loop vs supply loop for the closed system or when I had talked about an open system (not doing that any more). They have a heating source requirement of 29952 BTUs for the 32x48 part and 52000 BTUs for the 40x40 and once heated, shows a BTU heating output of the 32x48 slab as 61440, and 64000 for the 40x40. Not sure if any of this is helping, but just wanted to get you everything they gave me. Clarification on the temps given. They say the "Suggested fluid temp is 130" and the "maximum temp" is 150.
Thanks again. I am leaning towards the boiler and just cranking down the temperature, but looking forward to what you recommend.

Where are you located so I can determine your outdoor design temp?

I am in northern Virginia, specifically Marshall.

Okay, for outdoor design temp of 16F and indoor design temp of 60F, the 32 x 48 has a total heat loss of about 12,000 Btu/h and the 40 x 40 has a total heat loss of about 22,000 Btu/h or 34,000 Btu/h for both zones. The supply temp can be 82F and the return temp would be 67F (i.e., designing for a 15F delta T). The flow rate to each 275’ circuit should be about 0.32 GPM or a total of 3.8 GPM total for all 12 circuits and would result in 4.5 feet of total head for circulator pump selection.

I wasn’t sure which zone had the doors, so I put them in both zones. A R5 door heat loss is only 1,936 Btu/h anyhow. I used R19 for all walls, R30 for the ceilings and R10 for slabs. For the 32 x 48 breakdown, the wall heat loss is 2,825 Btu/h, the ceiling heat loss is 0 (because the above space is heated), the floor heat loss is 1,622 Btu/h and the infiltration heat loss is 5,474 Btu/h. For the 40 x 40 breakdown, the wall heat loss is 5,419 Btu/h, the ceiling heat loss is 2,347 Btu/h, the floor heat loss is 2,023 Btu/h and the infiltration heat loss is 10,138 Btu/h.

Thank you very much for running this. Wow that is different than the specs Radiantec sent over. Are you saying I should GREATLY reduce the BTUs of the boiler/heater I'm looking at? With all that, even an 80k BTU is overkill...

Yes, you only need about 1/6 the boiler capacity you were originally considering...if the R-values you provided are accurate. Is R19 valid for the 12” concrete walls or do you just have a concrete wall and perhaps R19 is only for the 2x6 walls?

Yes, I should clarify that. The 40x40 is the section with the man-door and garage door. The 48x32 has no doors. the 32x48 is bare 12" concrete walls with a 28' opening into the 40x40 section. THere's no insulation in front of the concrete.

Okay, that changes things a lot. Are the 32 x 48 concrete walls above or below grade? And if below, what percentage is below?

Okay, for outdoor design temp of 16F and indoor design temp of 60F, the 32 x 48 has a total heat loss of about 51,000 Btu/h and the 40 x 40 has a total heat loss of about 20,000 Btu/h or 71,000 Btu/h for both zones. The supply temp can be 105F and the return temp would be 85F (i.e., designing for a 20F delta T). The flow rate to each 275’ circuit should be about 0.71 GPM or a total of 8.5 GPM total for all 12 circuits and would result in 12.8 feet of total head for circulator pump selection.

I used R19 for the 2x6 walls and R1.2 for the concrete walls, R30 for the ceilings and R10 for slabs. For the 32 x 48 breakdown, the wall heat loss is 43,560 Btu/h, the ceiling heat loss is 0 (because the above space is heated), the floor heat loss is 1,622 Btu/h and the infiltration heat loss is 5,474 Btu/h. For the 40 x 40 breakdown, the wall heat loss is 4,381 Btu/h, the ceiling heat loss is 2,347 Btu/h, the floor heat loss is 1,517 Btu/h and the infiltration heat loss is 10,138 Btu/h.

That looks spot-on, thank you. So that narrows down my modulating condensing boiler options. If I go with Navien, I have an 80K and a 110k option. I am also looking into HTP. Do you have any opinions on either the brand or the sizing? Modulation should throttle them down to the actual level neededm regardless right? The cost between the two sizes is only $100 or so.

I always try to avoid recommending boiler brands online. I would recommend going to a site like SupplyHouse.com and looking at the ratings provided by buyers. With your two large zones, short cycling shouldn't be an issue unless you significantly oversize and if you have modulation that will help too. You could run different supply temps to each zone as the 40 x 40 only needs 82F. It is the 32 x 48 that needs the higher 105F.

And be sure to read this too:

Sizing a Mod Con Boiler

Thank you! I am heavily leaning towards the 80K BTU Navien vs the 110K. Does that sound like a good plan?