The steam radiators in our 150-year old home are not heating up very much. Only two or three fins get hot on the large one in the kitchen and the one in the upstairs hall doesn't get hot at all. The valve on the one in the kitchen was replaced about 5 years ago. The one upstairs just hisses a little. Could the pipes be plugged with mineral deposits, like some of our water pipes are? What can we do to fix this problem? Upstate NY gets too cold in the winter to not have a system that works right.