digital t-stat

ed m

had to switch out a t-stat went bad, replaced it with a new honeywell digital,would not work.tested the wires and only had 15 volts,very old cloth insulated wires. would the lack of voltage be a cause of t-stat not operating.steam boiler which they put a hw baseboard loop off for heat in basement.they rewired boiler in to a argo switching relay,and have powerd the up satirs t-stat of the same transformer in argo box.is it possible that because the basement bb loop,with aquastat,tstat,as well as cirulator relay,and the up stairs t-stat for steam are off one transformer this is limiting power to upstairs t-stat.
thanks for your help