Im getting a subquery returned more than 1 value error. Please Help!

The error is occurring in the where clause that I spaced out down below, Its dealing with the State_No I know for a fact. I can't figure out how to run it in the Query anzlyzer to relate to match up with another table

PLEASE HELP!!!!!!

This is the error I'm getting in the DTS package i ran and query analyzer:
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Update HBC_Boiler_Inspection
set Safety_Valve_Cap = '" & rs1 ("Safety_Valve_Cap") & "', "
strSQL = strSQL & " Hydro_PSI = '" & rs1 ("Hydro_PSI") & "', "
strSQL = strSQL & " Hydro_Date = '" & rs1 ("Hydro_Date") & "' "

strSQL = strSQL & " where Boiler_ID = (Select ID from HBC_Boiler where State_No = " & rs1 ("State_No") & ") "try

Boiler_ID IN (Select ID from ...

caution: this will update all the boilers selected in the subquery

rudy