Harold Kestenholz
Look at the piping layout at http://www.aresinc.com/Hydronic/multcirc/multcirc.htm
to see the difference in the arrangement of the pupms and expansion tank. When the pumps pump toward the expansion tank, you get the symptoms that you describe. There a more expansive explanation at http://www.aresinc.com/Hydronic/Des/start.htm#Positive
