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Lost in the design process...
Quick intro: New to forum, not a pro (that will become obvious!) and building a small separate rental unit onto our property. In serious need of some "heating help" as I don't want to mess up this aspect of the new build. Building permit was issued with a heating design as part of it (architect hired HVAC engineer). Builder and his plumber now suggesting a bunch of changes (which are permitted so long as inspector approves). I have concerns mostly about "short cycling" (loads are light/small unit), reliability/longevity of equipment, and having sufficient DHW as there is a long term rental in the unit (upstairs) and a secondary "AirBnB" type short term rental unit as well (downstairs). 2 full baths basically, but lots of use and certainly concurrent use of both baths.
Current Design: two (main) zones in-floor heat on the main floor (slab)...one zone is single car garage other zone is approx 400 sq ft living space (AirBnB suite is on main floor). Upstairs suite is approx 800 sq ft, with heating/cooling via dedicated hydronic air handler and AC unit. Additionally main floor living space has supplementary cooling control provided by small mini-split...wanted AirBnB unit to have separate AC control basically...typically summer rental. Heating load design for Air Handler: 21,000 BTU/h @ 140F&3GPM 525 CFM. continuous run HRV fitted to air handler. The radiant design shows a "Low Loss Header" off the main boiler loop. Downstream of the low loss header are three smaller loops. One for the main floor heating with its own pump and thermostatically controlled mixing valve. Other loop has no mixing valve but a pump and loop for the air handler. Last loop includes some kind of a heat exchanger, pump, and mixing valves etc with a separate "glycol" loop system for the garage floor. DHW is directly off the "combi" boiler for entire building, both upstairs and downstairs separate unit.
Equipment originally specified:
IBC DC15-96 "combi" boiler plus piping, valves and pumps (see above).
Main concern with the current design is the DHW which is coming off the Combi boiler (only outputs 2.1 GPM) and the potential for short cycling as the loads are so light for heat, plus cycling for DHW. There is room in the utility room for an indirect tank.
Looking at switching boiler to HC series IBC (smaller unit HC 13-50) and adding an indirect tank (say 30 gallon or maybe 40 gallon). Wondering if the air handler can simply take its feed (directly) off the DHW loop set at 130F or so and thus the tank would also buffer this load? Searching for smaller boilers like the HC13-50, only options I am finding are the Viessmann 200-W (B2HB 19-68) and also Navien NHB-55. Not sure if there are other options out there worth considering on the boiler side. Building is located in Ontario, Canada and I am learning that not all boilers out there are necessarily available in Canada but the above noted options are.
Any thoughts and suggestions on the design and/or equipment choices for this would be most appreciated!!
Thanks for reading my long note!