Heat flow visualizer

It seems that many of the problems homeowners come here with involve mismatches between boiler output, radiation, and/or building heat loss. This morning there's yet another post here from someone whose boiler is short cycling. Various pros here asked the usual questions and made the usual suggestions, all good.

That got me thinking about how to visualize heat flows from the boiler into the building, and the image of three buckets flowing into each other in series popped into my head. I'm sure I'm not the first person to draw this analogy, but since it helped me make sense of how heating systems work, I drew some sketches to illustrate various problems that people come here with.

The boiler is illustrated by the first "bucket," which is a BTU "reservoir" whose output rate R1 is represented by the diameter of the output pipe at the bottom of the bucket.

The thermal mass of the water and radiators is illustrated by the second bucket, and the radiation output rate R2 is represented by the diameter of its output pipe.

The building is represented by the third bucket, with its heat loss rate R3 represented by the diameter of its output pipe.

The water flowing between buckets represents the flow of heat (BTU's). If BTU's flow into a bucket faster than they flow out, the temperature (water level) in the bucket rises.

The size of the thermal mass bucket in the middle of the series matters, because a small thermal mass "bucket" will fill very quickly, with high water temps causing short cycling. Conversely, a large thermal mass bucket (like a gravity conversion system) will fill very slowly, keeping water temps low.

Common problems seen here at HH happen when R1> >R2, when an oversized boiler heats a small thermal mass too fast, resulting in short cycling, and when R3 > R2, meaning the building loses heat faster than insufficient radiation can emit.

In the case of gravity conversion systems, the thermal mass "bucket" is relatively large, meaning the "water" (temperature) level rises slowly. If the boiler is correctly sized to the building heat loss, R1 and R3 are roughly equal, and the large thermal mass and large EDR (R2) means the water temperature stays low.

But commonly we see gravity conversion systems with massively oversized boilers. This is the case in my 4-unit condo building. Here we have R1> >R3, where the boiler output rate is 2-3 times the building heat loss rate.

All analogies are imperfect, and I'm sure I'm not the first one to think of the bucket analogy, but it helped me make sense of the various flow rate imbalances that often show up as problems here at HH.