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Small renovation on existing boiler "best" option


I am currently in the middle of a renovation that is adding a unconditioned garage, and an office/passthrough partially below grade and a bathroom at grade. West side of garage and office/passthrough are 4' under grade and east side is at grade. Basement is not heated at this time, but I did put a design load 68 deg.

I have an existing non-condensing 125k BTU slantfin boiler running @180. Grundfos UPS 15-42 8622. Cast iron rads throughout the rest of the 1931 house. Existing house is only approx 1500 fin sqft. New office/passthrough is 265 sqft, Bath 49sqft.

I did do a heatload via loopcad Minneapolis zone -8 designed temp and also checked edr of existing iron as a rough comparison.

What I am getting is the bath @ 1198 btu/hr (bath 10' ceiling others 8'), passthrough @4027 and office @3044. The lower rooms seem higher than I would have expected due to partial below grade but they do face masonry walls (insulated R20 however) and have partial ceiling exposure.

Anyway, what I am struggling with is the design/choice between in-slab and panel rad. In any case I was going to put these new areas on a single manifold with a temp valve (either static or a taco i-valve - suggestions?).

With in slab I was looking at delta10 with water @120. I would have taco 007-ZF5-3IFC, mixing static or taco i-series I075T3R-1, then manifold. Loops as in attachments, trv on other house rads.

If I went panel I was thinking delta 20 @140. I would probably do a mixing valve either static or taco i-series I075T3R-1 and swap circulator to Grundfos alpha 15-55. Panel rads for new areas TrV on all rads in house.

As an FYI, while the current boiler is functional I know a replacement will be required within 10 years. Also, given the current BTU capacity of cast iron, I believe water in the new boiler can be run @140 without issue, given our overall home improvements. LoopCad estimates total BTU of home @ 61487.

I am struggling with the two options given the small space, the future need for boiler and the trade-offs between the in-slab and panel options. The bathroom is an oft-used space, the passageway/office not as much. I also am I bit concerned with the finality of the in-slab radiant and system adjustability.

In my "dreams" bathroom would have in-slab, passage with panel rads but that is making for a more complex system. I even thought a electric heating pad in bath floor with larger towel-warmer or smaller panel rad might be option in bath.

Any thoughts/opinions here? Anything would be helpful.

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