Closing in on large building boiler design...

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I think the wait and see approach is tempting. Although, summers here are humid and everything you have touched on does make sense. We are closed up now, although it was a 50% gut, mostly ceilings, and now we are moving toward finishes. There must be through-wall units for venting that require little destruction, although they would need a power source.

The ceramic-core single room ductless HRVs are exactly that- through wall units (that of course need a power source.)

The plan os to strip off the asbestos and strip off the old clapboards, and reside using fiber cement. It hold paint well, only requiring new paint every 25 years (although colors do fade). One side of the building, due to the tall basement, is 34' off the ground at the eave. We would like to aim to reduce maintenance and at the same time take advantage of the stripped house to blow all cavities.

When you get it all the way down to the plank sheathing it's worth it to use a FULLY ADHERED vapor permeable weather resistant barrier such as Henry BlueSkin VP100 or Vycor enV-S or Delta Vent SA rather than stapled-on housewraps (Tyvek et al) as a much more reliable way of making the sheathing air tight.

If it's in the budget, adding R8 or more of rigid foam board over the housewrap would bring the thermal performance up to IRC 2018 code minimums (but putting some foam on the exposed brick in the basement would provide a much better bang per buck, even with a thin-brick veneer added to the cost to retain the appearnace.)

Sounds like you were spot on for outside design temp.

Dumb luck is my long suit. :)

But even without the lucky WAG I was pretty sure that it couldn't be too far off, unless you were at the top of a mountain or something.

One of the reasons I was attracted to this model boiler is the versatile turndown ratio. As you say, flexible for smaller zones. I am going with cast iron baseboard and radiant heat as the two emitters on the project. This simplifies the design (I hope) to have only two types. I've avoided fin tube or panel radiators since they don't do as well with thermal mass, have higher supply temperature requirments, and don't have the comfort of the added company of radiated heat energy.

Panel rads have a lot more thermal mass than you might think- it's not a huge step down from cast iron, despite being physically smaller and much lighter. The thermal mass of a single gallon of water is worth about as much as 80lbs of cast iron.

Doing even the simplest design math on the loads and system can steer you to the easier/better and often cheaper & more reliable solutions, as long as your input assumptions on things like R-value/U-factor and design temps are close. Without it you are kinda shooting in the dark, and when pushing into the unknown there is an all too huma tendency to over-spec the system, often to the point of lower comfort & efficiency.
Hey, wait a minute.

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