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Thread: Bosch Greenstar 151 Combi

  1. #1

    Default Bosch Greenstar 151 Combi

    Any thoughts on these?

    I haven't used one to date. Am considering using one for a system consisting of two zone infloor heat (concrete basement slab and garage slab) and DHW requirements, and possibly hydronic air handler in future if second story requires supplemental heat...


  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    What is your design condition heat load? What's the BTU/hr out of the proposed air handler?

    The minimum nominal output of the would be about my peak design heat load, in a climate similar to most southern Ontario locations, and I'd almost never get the benefits of modulation out of a beast that size. It's almost certainly oversized for any one of your zones, but the thermal mass of the slabs may protect it from short-cycling it to death on zone calls. At max output it's more than 4x oversized for my 99% design condition whole house heat load, and enough heat to run 3 low-flow showers continuously forever.

    All good heating designs start with a room-by-room/zone-by-zone heat loss calculation at the outside design temperature for your location. Sizing the boiler appropriately for both the peak & average loads as well as the output of the heat-emitters can make a huge difference in comfort & efficiency, and oversizing the boiler is the enemy of both.

    New construction, or retrofit?

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