Thanks Ecowarm. I agree. But I'm trying to cover different zones coming on at different times. E.g. underfloor being on all day but rads upstairs on am and pm (which may be on later am than underfloor. When rads come on the bolier is already up to operating temp supplying underfloor heating and along comes cold water from rads, so bringing return temp below 40degC. And design return temp for underfloor is less than 40deg so problem when underfloor on alone.
One solution could be to use a 22mm TMV (usually used for bath HW). Positioned further away from bolier than auto diverter valve (pressure differential) so when failsafe kicks in on TMV flow goes through diverter.
Another option would be to have 3 way valve on return alowing return directly to boiler when temp in return greater than, say, 50deg. Otherwise send through a 3 port underfloor mixer valve set to, say, 55deg.
