I have two bedrooms (copper fin baseboard) over an unheated garage. The pipe supplying the baseboards runs about 20' through the garage, then up into the bedrooms from the back corner to the front for 18' of baseboard, then back into the garage for an L-shaped run back to the starting point. Overall, there's approx. 2-3 times as much pipe in the garage as there is baseboard in the rooms I want to heat, and these rooms get cold for days on end during bad cold spells (northern NJ).
I think the once-and-for-all solution would be to re-route the pipes to get them out of the garage and up into the bedrooms to the greatest extent possible - that is, put baseboard along all 3 exterior walls around the bedrooms instead of just the farthest one, a full U-shape instead of just an underscore _ .
A plumber friend of mine is concerned that this would overheat the bedrooms and affect the furnace cycling. Since the bedrooms comprise only 25-30% of the heated floor space, I wouldn't *think* this would adversely affect the furnace cycling, and since they're a separate heat zone, I don't *think* they could overheat. But I don't have any actual experience in this area. What do you think?