NTI boilers

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ISOBoiler

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Just got a new bid and was hoping you could provide some feedback. I'd asked for a quote for retrofitted radiant in the basement joists - there are narrow-plank oak floors above. The contractor suggested stapling to the joists rather than using plates to avoid nails into the flooring and because the temperature that could be run would be lower. Does that make any sense? It seems like I'd want to run as low a water temperature as possible.
 

Leon82

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the staple up can make noise when warming up .

the plates have more surface area to transfer heat to the floor
 

ISOBoiler

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I guess, would a suggestion to install radiant that way indicate a red flag for the contractor overall? Pictures of prior installations look clean, at least.
 

ISOBoiler

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And, one more question - does a radiant floor increase the possibility of short cycling, since it seems like a typical radiant zone is around 300 sq ft - assuming 15 btu/sq ft, that's only about 4500 btu, which is much lower any boiler will modulate to.
 

Dana

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To get 15 BTU per square foot out of suspended tube or staple-up radiant requires water temperatures well above the condensing zone. To deliver that much heat at anywhere near condensing temps would require aluminum heat spreaders. With 3/4" plywood or plank subfloor and 1x hardwood flooring you have about R2.5.

At R2.5 for the flooring stackup, with extruded heat spreaders and 8" o.c. spacing you can get 15BTU /ft^2 out with ~115-120F average water temp. With better grade stamped sheet metal heat spreaders you can get there with 120-130F AWT, either of which is in the condensing zone. But with any type of staple up or suspended tube you're looking at 140F+ AWT to get there.

See the nomograph on the last page of this extruded heat spreader handbook.

Also see the nomograph on this page.
 

ISOBoiler

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Well, the 15 btu/sq ft was just a guesstimate on my part, but overall it sounds like the suggestion to staple to the joists is a contractor red flag. Any thoughts about the likelihood of short cycling given that the zones seem to need to be small, even with proper installation of radiant?
 

Reach4

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It seems like I'd want to run as low a water temperature as possible.
I wonder if you could get condensing by running hotter water through the floor, and then pass that same water through an efficient radiator in the room? Just musing. That doesn't seem to fit small zones, I guess. It is probably common for some installs I suspect.
 

ISOBoiler

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I guess I'm just curious how any radiant floor is going to keep from short cycling unless there is a buffer tank. It looks like Uponor recommends a maximum length of 300 ft for 1/2 inch tubing. So assuming you're talking about 10 btu/sq ft applications, it seems like none of them would be under the 8000 or so btu minimum modulation numbers. Unless maybe I'm confused about how radiant is zoned.
 

Dana

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If you can guarantee that zone calls over lap (which you sometimes can), helped along by having a buffering thermal mass (such as a high-mass hydraulic separator like a Boiler Buddy) it's possible to serve up small-load zones at condensing temps without short-cycling the boiler. Only when you present it as a zero mass and small load does it look as bad as this. As long as the whole house load is large enough relative to your minimum modulated output, a perfectly dialed-in outdoor reset curve guarantees that no zone calls are satisified quickly, thus guaranteeing very few short-cycles due to the timing overlap.
 

ISOBoiler

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Dana, I've got some updates and was hoping I could get your thoughts. Unfortunately, there are no contractors in the DC-area with HTP or Lochinvar expertise (at least that I've come across). The primary brands are NTI, Triangle Tube, and Viessmann. Given that none of the Triangle Tubes go down below 16k, I've eliminated them from the running. While the NTI water-tube boilers go low, they make me nervous given their very low mass and the potential for durability and maintenance issues. So that leaves the NTI TFT154, which goes down to 15k and has 3 gallons of water mass, the Viessmann Vitodens 200 B2HA19, and the Vitocrossal CU3A-26. One contractor said that the Vitodens and the Vitocrossal would be the same price to install, given that the CU3A could be piped direct. No one has been willing to pipe the NTI direct (and the company generally discourages it, as you saw). So, given the design day heat-loss of ~45k, the 40F heat-loss of ~15k, and a smallest zone of 9-10k output at 120F supply temps, I'm wondering what you would choose. Do you think the extra mass on the Vitocrossal and the ability to pipe direct outweighs the 12k minimum modulation on the Vitodens? The Vitodens also seems to have a temperature sensor that attaches to their LLH - wondering how much of an advantage that is and whether it really helps to low return water temps. I also wonder, given the size of the heat exchanger on the Vitocrossal, whether it will prove to be more durable than any of the lower mass boilers, though of course don't have any evidence to that effect. I spoke with the Viessmann rep and unfortunately it sounds like the model-26 Vitocrossal is actually the worst seller of the line - they don't have any plans to bring the smaller Vitocrossal units with up to 7:1 turndown to the US in the near future. I think that's a shame, since a high-mass boiler that goes down to 8500 btu/hr seems like it would be a real game-changer!
 

ISOBoiler

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Just wanted to bump this up. Dana, would love your opinion about the best choice out of some 100%-satisfying options.
 

Dana

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There is only so much "design by web forum" that makes sense, and at some point you're just going to have to take the discussion to the local contractors who would be installing & supporting the equipment. It is indeed unfortunate that the smaller Viessman's won't be imported or supported here any time soon.

The 13.5 gallons of buffering in the Vitocrossal CU3A-26 and the 17,700 BTU/hr minimum modulation output means that it won't short cycle like crazy even with fairly small zone loads, and is likely to run fewer cycles than the TFT154 with only 3 gallons of thermal mass for ~15K of minimum output. If local support is equal go with the Viessmann.
 
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