electric hot water baseboard heaters.
do you have another heat source also if not might be cheaper to separate the rooms into separite zones then add new heat system
be careful my advice only comes from general knowledge i know about the 4 rules it takes to plumb and they are
1 poop needs to flow down hill. 2 hot on left cold on right
3 i get a pay check on Mondays 4 do not chew finger nails at or after work.