Nothing like turning the heat on for the first time in the season (people coming over for an ExComm meeting in 90 minutes at this point) and finding no heat in the most important room of the house (family room / great room). Other zones seem to work. Eventually remembered the trick for forcing the zone value open and confirmed that it looks like the zone value. Fortunately, have been stocking a spare zone valve because that is the part that tends to fail and it is not covered by the service contract. (The house is forced hot water with four zones and four zone values.) Replaced the zone value. We will see. Hopefully it will do the trick. Unfortunately that is some time I didn't want to spend.