Hydroponic zone valve won't work

Hi all,

I have a two-zone hot water baseboard heating system in my 100+ year-old home. Up until about a week ago I had no problems. All of a sudden I lost functionality of one of the zone valve. It normally responds to the thermostat but now only works if I adjust it manually. The problem is that there are no marks on the zone valve to tell me what temp I am setting it to work at. Consequently, my lower level got up to 84 degrees before I shut the d**m furnace off to save oil. I readjusted it and now it's at about 73 degrees in the lower level. There is a wheel on it that has an arrow pointing in one direction indicating "manual" adjustment and I'm not sure how far to go with it because there doesn't seem to be any consistency to the temp when I play with the adjustment. It seems to have detents as I turn it but I'm not sure if that just means I'm starting over by rolling over the limit of the adjustment or if it means I'm selecting a new range. Any ideas on how to manually adjust these?

Yes, I will be contacting the oil company to come and check it out but in the mean time, I need to find a way to regulate the temp. Here in Maine in winter it takes forever to get someone to come over because they are so busy.

Good day!

Mike