Last fall, I taught my first online course dealing with designing hydronic heating systems. The course was titled “Mastering Hydronic System Design.” It was a collaborative effort between HeatSpring Learning Institute, BNP Media’s CE Campus, and myself.
If you’re using a calculator, spreadsheet, or other computational tool, you should use the most accurate numbers possible. It’s just part of being a professional.
The following formula has been around the North American hydronics industry for a long time: Btuh = 500 x gpm x delta T. It can be used to estimate the rate of heat transfer into or out of a device that has a stream of water flowing through it at a known flow rate, and with a measured temperature change between the inlet and outlet of that water stream.
In newer modulating/condensing boilers, many of their heat exchangers had to sacrifice two very desirable qualities in the interest of being small and light.