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Low temp radiators and mod-con efficiency
FHVs [usually] come in two varieties.
One modulates via slab sensor. Such is generally only suggested for fairly small spaces--up to about 100 sq.ft. of heated floor area--think a reasonble bathroom.
Another version modulates via room air temperature. The practical limit for a single FHV is about 10,000 btu/hr. While multiple FHVs could technically be used in the same room, you'd likely wind up with only a single FHV providing the vast majority of the heat in most weather conditions. Perhaps not too much of a problem (other than potential "striping") if you've interleaved the tubing, but otherwise you'd likely wind up with only certain sections of the floor being heated.
Because FHVs (like TRVs) can ONLY regulate flow, they allow <I>unlimited</I> panel temperature if you "crank the operator". In other words if you don't have a decent reset curve you could do the <A href="http://www.amazon.com/Demon-Seed-Julie-Christie/dp/B000A0GOFU/ref=pd_bbs_sr_1/104-6384430-4388761?ie=UTF8&s=dvd&qid=1175117955&sr=1-1">Demon Seed</A> thing and cook eggs on the kitchen floor.
For this reason, the [better] FHV valve bodies has a built-in flow-setter adjustment. This allows you to keep head loss (and thus flow) similar regardless of the tube length in individual circuits. The purpose of such is to limit the ability of a relatively small floor area to overheat if the user cranks the operator.
My offer of free design assistance still stands for any reasonable contractor or <I>highly informed</I> homeowner who wants to use FHVs, Warmboard and a Vitodens.