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So..here goes. Been installing boilers for many years and have always known and was taught not to reduce supply and return piping diameter at new boiler to an existing monoflow loop. If its an 1 1/4 "loop, I'm keeping my circ/zone valve 1 1/4". However I've been working for a new company the last year or so and the boss man thinks it's ok just to always reduce to 3/4" I know this is incorrect and not good practice but they have gotten away with it in the past. im just looking for a technical explanation as to why this is incorrect other than "its just not right". If someone could help me out that'd be great