If I've understood you correctly then what you want to do is create new records each day, one for each boiler so the operator does not need to create the records by selecting boilers individually?
If so then you will need some code which perhaps runs off a button to create those records. What that code looks like depends on your table structure. Will need to see your table structure to be more specific but in simple terms it would be something like
Code:
dim rst as recordset
set rst=currentdb.openrecordset("SELECT * FROM tblBoilers")
while not rst.eof
currentdb.execute("INSERT INTO tblReadings (BoilerFK, ReadingDate) VALUES (" & rst!BoilerPK & ", Date())")
rst.movenext
wend
me.readingsubform.requery
rst.close
set rst=nothing