FNUG,

May be undersized. An undersized boiler can perform just fine in moderate temps, but fail to keep up when it gets a lot colder. If it's not too small for the space you're heating, you may have some heat exchange issue, such as a flat plate heat exchanger the wrong size or set up wrong.

Hard to say what your problem is without any details and probably difficult to diagnose even with more detail. Could be a wide range of things, including a combination of factors.

Since it's warming back up over the next week, see if it starts performing well again. If so, that's a big clue.

Given that actual boiler output is completely unknown about all you can do is check the basics.

Determine actual water temp in the boiler by checking with another thermometer or gauge.
Make sure circulation is good.
Check temp differential between supply and return. Should be less than 20* under normal circumstances and flow rates.

Other than that, as Eric said if it catches up next week in when it's in the 20's it's pretty obvious that it doesn't produce enough heat to keep up when its cold.

I have no idea what the title of this thread means.

But anyway, am I reading your post right that you have 160-180 water leaving your boiler, but it's only 120 when it gets to your coil? If so there is something desperately wrong there. What do you have for underground piping?

I had PM'd Bob some yesterday, figured I would post it here for everyone to ruminate on.

He has a water temp of 163 at the boiler, but the outlet fitting at the boiler is only 136. So it sounds like either a water flow issue or some thermometer wonky-ness. He bumped up the aquastat 10 degrees to see if the boiler temps can get higher. Might have to move where the aquastat reads to get good hot water to circulate to the house.

Bob, if you check this thread, did the aquastat change help things at all?

F'd new uneducated guy?

Is this like an OWB? If so where are you taking the water from? If you have it so the supply is pulling from near the return you will get this type of issue. That's why OWB's take water from lower in the tank, and return it higher and tube it to the front. My brother misunderstood which pipe to use and was trying to take water from the top ports. Which in theory should get the "Hottest" water. But it was causing a lock in the tank and he was just pulling the expended water back into the supply again. Swapped them around and everything worked as it should.