Please help me decide on boiler vs tankless heater

[Tim Pape]

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...Is it better to have a boiler working a lot less than capable or a tankless that is running 95% of capacity much of the time?

I have a Navien tankless for domestic in my main home and love it and have had no issues whatsoever. Open to other brands though.

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

Any advice/recs 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 for the heat source and domestic water (one full bath and three utility sinks), but I need to check on the BTU capacity.

Thank you!
-Tim

 

[Dana]

The 95K heat load @ 55-60F number seems suspiciously high even for an uninsulated building in Marshal VA, unless there are large truck bay doors opening & closing frequently.

Tankless water heaters can't handle the flow requirements of heating systems such as yours and have to be plumbed primary/secondary handle it, but Navien's boilers have the same issues. HTP's cheap stainless mod-con UFT series fire-tube boilers (or another fire-tube boiler-these just happen to be cheap and popular in my area) are probably a better bet. (The Westinghouse WBRUNG series are the same boiler, different paint.) The UFT-100W (Westinghouse WBRUNG100W ) will deliver the 95K in condensing mode, and has a huge 10:1 turn-down ratio. Usually these boilers can be pumped direct, and can handle a large range of flows without damage or short-cycling on zone calls. Even the 120K and 140K versions are usually under $2000 for the boiler, and very straightforward to install, but there's no reason to up-size from the 100K version.

A combi boiler can probably work here but if you really need the 95K on the heating side it's probably not going to work. How much hot water does this place really need? (not much)

The other thing about condensing tankless heaters is that they are designed for much colder incoming water temperatures and high delta-Ts. It's one thing to hit 95% combustion efficiency when the incoming water is 50F with an output temp of 120F, another when the incoming water temp is 90F. Though related, they are really different beasts. (I say this even though I'm currently heating my own house with a Takagi water heater. )