WATER TEMP WHEN HYDRONIC SYSTEM IS NOT RUNNING

You can let your boiler cool down to whatever it does without issues. The trick is for the boiler water temperature to increase above the dew of the flue gas to prevent condensation AND dry off the heat exchanger from any flue gases that did condense until the water temperature increased to above the dew point of the flue gases when running. The manufactures and the engineers say that the boiler water temperature below 140, give or take, is when condensation will occur in a cast iron boiler.

Now as far as running a boiler for 1hr 42 minutes is insane and does create another set of issues. i'm going to assume you have a big house with big radiators. This means that you most likely will never need 140 degree water to satisfy the space temperature. this is an assumption because you did not state the type of emitters your using. if this is the case you should have a system bypass installed at the boiler that will redirect some of the supply water back into the boilerso the water in the boiler will increase to prevent condensation.

you did not give enough info but enough to pretty much say what we are discussing

Yeah, that is a very long time. Like i said it could be a simple explanation. Oversized radiators, old gravity system with unbelievable amounts of water, boiler too small, etc. You need to provide more info to get a better picture of what you have. its all just an educated guess.

Pics of your system would be nice

I had an old converted gravity hot water system like that, from 1927. The oil burner was oversized for the heat loss of the house, but it would still take 2 1/2 hours to come from 60 up to 68° F on a cold morning. The boiler temperature never exceeded 150° F. there was no bypass, but the boiler was old and didn't seem affected by the condensation. Then with the huge radiators all warmed up it would stay hot for many hours.

This is normal operation for that type of system.

Chances are your Hydrostat hold-off for the circulator is not doing very much. With a gravity system design, you are likely getting thermosiphoning (gravity flow) even without the pump running.

Your new boiler needs to reach a return water temperature of approximately 135° to 140°F within the first 6 to 10 minutes of operation to prevent prolonged flue gas condensation. Every boiler will form some condensation on cold metal surfaces during startup. What matters is how long that condensation persists—you want that period to be as short as possible.

Boiler water starting at 60°–70°F (typical basement temperature) for ten minutes is not a problem, even if the burner runs for over an hour. As the boiler heats up, the flue gases will begin to evaporate any initial condensation once the internal surfaces reach a high enough temperature.

However, if the boiler water remains at or below 120°F for extended periods—such as during long morning run cycles—condensation will continue forming the entire time the burner is operating. This can significantly shorten the life of the boiler.

You should check the return water temperature at the boiler inlet (approximately 12 inches from the connection) to see how quickly it reaches 140°F. If it takes more than 10 minutes, a system redesign may be needed. If the return temperature reaches 140°F within that time, the system is likely piped correctly and should not be a concern.