I do maintenance on a small apartment building. Seven apartments are heated from a single boiler, with temperature control by Taco zone valves. While painting an unoccupied apartment I noticed it was warm, even though the thermostat was turned down. I felt the baseboard heaters in several rooms and they were warm, but not as warm as when the thermostat is on. I went to the heater in the basement and felt the copper pipe leading down into the zone valve, and it did not feel warm. The pipe at an adjacent zone valve heating the apartment immediately above where I was working was quite hot. My question is, how did the empty apartment with the thermostat turned down have warm pipes in the baseboard heaters? Even though the pipe to the zone valve did not feel warm, could there be leakage through the valve?