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If only 1 zone is acting up then you could have a faulty thermostat. It sounds like you have a programmable thermostat. I'd try setting the thermostat to HOLD and the desired temp then see if the problem is resolved. If it does then I'd replace the thermostat.
If the water temp was 230 to 300 then you have a serious control problem. How are you measuring those temps? Are you sure your clock settings on the Logamatic are accurate? What model Logamatic do you have?
In any event if only 1 zone is affected then the only explanation is either a thermostat or zone control issue. What do you have for zone control? zone valves? what brand? or a pump for each zone? if so what do you have for a relay controlling the pumps?
Ok, that makes more sense. I still need answers to my last questions. If the other zones are OK then there's a problem with the 3rd floor zone control. Since that floor has heat other times of the day the only culprit is what's controlling the flow of heat to that floor. While there is a remote chance there's not enough radiation (baseboard?) on that floor and the time you mention is the coldest time of the day, that one isn't very high on my list
What controls the pumps? are they connected to the Logamatic or a relay?
Please answer my last question
What model Logamatic?
As far as I know the Logamatic only has provisions for 1 pump. If you have 3 then they must (or should) be connected to a relay
If you start at the pump (for the 3rd floor) and follow the wiring it should connect to a relay. I need to know what brand and model that relay is. I also need a picture of the wiring connections in the relay. You can post a pic here by clicking on ADD FILES next to the SEND button
Ok, I need to see the wiring connections inside the Argo.
If you can't provide a picture of the relay wiring I'll give you my assessment based on what you've told me so far and what I've seen all to often in the past. I suspect the 3rd floor zone is not calling the burner in when there is a call for heat. So all that happens is that the thermostat turns on the pump and the pump circulates whatever temp water is in the boiler. If none of the other zones are calling then that could make the boiler water not very hot at all. Too often I've seen relays mis-wired in this fashion. It's also possible that there's not enough radiation (baseboard etc) in the 3rd floor zone to keep the space warm during the coldest times of the day and the mis-wiring would make that worse. There's no way I can tell from just your inventory of the baseboard if it is adequate. A detailed heat loss analysis would be required to make that determination.