I have natural gas furnace (which is connected to radiators in rooms), but i keep failing to understand output rating, for example 20kW vs 24kW.
For example, if on both devices output temperature is set 70°C, where does the difference kicks in? Is it only heat exchanger size different, or there is something else i am missing?
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Its going to be the fuel burn rate that’s different
If you have a room/building that is 20m x 20m x 3m that you need to heat from 10C to 20C then you need to add 3 kWh of heat. The 20 kW furnace can do it in 9 minutes, the 24 kW furnace will do it in 7.5 minutes. If you’ve got a bigger space and lose a sizable amount of heat through the walls/windows then the furnace can struggle to keep up, your two options are close enough it won’t be significant but this is why there are different sizes
Its aiming to keep the water its pumping out through the heating system at 70C but if you start pulling heat out of it fast enough then it may not be able to keep up and even running continuously the output temp could drop below 70C
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