i often see people talk about balancing a system and i wonder what that means?  does every zone have to run at the same pressure?
the same delta t?  please excuse my ignorance-i came hear to learn.

balancing flow through the loops to the desired levels. what a "desired level" is depends on the design strategy being used.

It sometimes means, matching the heat delivered by radiation to the heat loss in each room proportionally so that the system runs perfectly as a single zone without over/underheating any single room within the zone. This can be hard to do by design alone, without the ability to tweak flows in-situ after the installation. And if the loops aren't separated room by room, it can be impossible to get right in some situations (hence the growing popularity of room-by-room micro-zoning, with all of IT'S inherent issues.)

thanks for the replies. so constricting the flow to some loops in order to get more radiation elsewhere in the zone?

what about pressure differentials between supply and return? are they meant to be exactly equal or slightly (1-3psi) higher on the supply?
i've noticed my zones that supply the rads seem to have the same psi coming and going where as my in floor seems to vary on the supply and return.

you lose pressure over the length of any loop. in a radiator loop there isn't much drop in many cases so you wouldn't see it. in a radiant loop which is generally longer and perhaps smaller diameter the drop is generally more noticeable.

so balancing is always about flow. it's just a question of what the goal of your design is for flow. on one end of the scale it's just to correct problems (this room is a little too cool, this one is a little too warm). on the other it's the full constant circ single zone balancing that dana mentioned.

thanks rob. so i guess a small pressure differential is normal and not a problem.