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@EdTheHeaterMan and others here have been very helpful and patient in helping me understand the heating system in our 100-year-old 4-unit condo building heated with two Weil-McLain WGO-5 hot water boilers and old cast iron radiators. I've also found a number of articles by @RonBeck very helpful.

Some of the most illuminating points I've found were made by Ron Beck are in this article he wrote about outdoor reset:

"cast iron radiation, contrary to popular belief, is low-temperature heating, especially in modern homes...Back in the day, they may have needed 170 degree water. With single pane, wooden frame windows, and zero insulation in the walls and ceilings, homes had HUGE heat loads. Now, insulate that same house and install new windows. What do you have? About half the heat load, at most. Nonetheless, the cast iron radiators haven't shrunk, so you'll be overheating the home or short-cycling the boiler....

I've been asked many times about what water temperature cast iron systems require. My answer is always the same, "it depends." There is a lot of mass and high water volume in cast iron systems. Therefore, if it's and insulated home, the water temperature doesn't need to be very high...

In cast iron applications where the home has some insulation, I like to see low boiler temperature, minimum water temperature, and possibly the high boiler temperature reduced. Do the homework. Do a heat loss, calculate the water temperature required at design temperature with the amount of installed radiation. Once you have your design water temp, change the outdoor design temp to 60F on the heat loss and calculate the minimum water temperature. You may be surprised."

https://www.usboiler.net/outdoor-reset-doesnt-work.html

Sorry for the long quote, but it's all so relevant to our heating system. It's like Ron wrote that after visiting here and analyzing our building.

Just as he said, we have a massive amount of radiation, roughly twice what we need for our 45,000 BTU/hr heat load at zero degrees. But because we have so much radiation area (EDR=480 sq ft) and so much thermal mass (over 1,000 lbs water and 4,000 lbs cast iron), the thermal mass takes a long time to heat up. So boiler installers look at all the radiation and thermal mass, and oversize our boilers based on those factors, and not on our modern, vastly reduced heat load, just like Ron said.

As a result, we can heat a 4,800 sq ft 100-year old building with water that rarely gets above 140 degrees. The average radiator surface temp at boiler shutdown is 120 degrees. And there's so much radiator surface and boiler capacity that the boilers run only about 1/3 of the time during a zero-degree day. The rest of the time, the system is slowly radiating all the BTU's that were stored in all that thermal mass.

So I began to wonder, at the end of a typical 45-minute boiler cycle, how many BTU's have been radiated, how many end up stored in the water, how many end up stored in the cast iron of the radiators, etc? Most/all of them eventually end up radiating into the house, but I found it instructive to see how they were distributed throughout the thermal mass at the end of a 45-minute boiler cycle.

So here are some relevant numbers:

Gross boiler output in 45 minutes: 105,000 BTU

At the end of the 45 minute cycle, those BTU's have either been radiated away or stored in the water, cast iron radiators, or the boiler mass itself. (I've lumped the piping mass in with the water mass for simplicity).

Here's our thermal mass breakdown:

System water volume: 135 gallons (85 in rads, 40 in pipes, 10 in boiler)
Water weight: 1125 lbs
Cast iron radiator weight: 4000 lbs
Cast iron boiler weight: 600 lbs

Using our typical water and radiator temperatures, in which the radiators and water end up around 125 degrees, from a cold start of 68 degrees, I made a little spreadsheet to see where those BTU's ended up, based on masses, delta T's, and specific heat capacities of water and cast iron:

9,000 BTU (9% of total) radiated (based on avg water temp during 45 minute cycle)
9,000 BTU (9% of total) stored in boiler cast iron
23,000 BTU (22% of total) stored in radiator cast iron
64,000 BTU (60% of total) stored in water

105,000 BTU output total

So what surprised me is the finding that, based on our average water temps over the 45-minute cycle, less than 10% of the heat produced has been radiated into the building. Over 90% of the heat remains stored in the thermal masses of the water, piping, radiators, and boiler itself. And while some of it gets convected up the flue, most of it slowly radiates into the building after the boiler shuts down.

These numbers made me realize that the thermal mass in our 100-year-old system is like an aircraft carrier. It's a huge mass, but because our building heat loss is so low, we don't need the aircraft carrier to go very fast. Only a few miles an hour. So we can put a small engine on that aircraft carrier, run it for 45 minutes, and at the end of that time the aircraft carrier is moving only a few miles an hour. But that's all we need, and now we can shut off the engine, and let that massive aircraft carrier coast for several hours until we have to restart the engine.

Sorry for the long post that isn't really a question, but I found it interesting that Ron Beck's quote above describes our heating situation perfectly. I had come into this house assuming that our old cast iron radiators were high-temperature devices, and with the help of Ron Beck and EdTheHeaterMan have come to understand that, in fact, they are now low-temperature devices because of how our 100-year-old house has evolved and modernized over the years, and that sizing new boilers based on installed radiation and/or water volume instead of the "modernized" heat load will lead to oversized equipment.