There are many different kinds of hydronic heating loops. Generally, the more efficient and dense the radiator, the lower the entering water temperature. For example, a thick, insulated concrete slab may have as low as 100 - 120 degF entering water temperature. A staple-up loop with spreaders under a wood floor needs to be fed much hotter water to transfer sufficient heat, maybe 140+ degF. Look around on the forum, and you'll find specific information on a variety of configuration options. Energy audits, heat load/loss calculations, and engineering homework should happen before the equipment is commissioned.
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