Well i'll confess to doing this before (laminate flooring throughout upstairs that homeowner didn't want to lift) tee'd into 15 mm bathroom rad pipe directly above the boiler. I would usually take it back to the 22 mm circs but with a little balancing magic its worked ok for the last 6 yrs. i've even seen one with the 22mm F&R tee'd into a 10mm rad (i schitt you not) with 7 rads and thst was also working ok.
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