Natural gas furnace – what exactly is kW?

Question

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?

In: 0

Answer 1

I am told problem is water flow – pipe diameter is smaller than required, which makes harder to move the water. Problem is (at least temporary) reduced by increasing speed of water pump (2 to 3).

Problem is also 1 line system (radiator is connected to previous one). I have understanding that 20kW warms up smaller amount of water, so less warm water reaches final radiators (water temperature is the same, for example 75deg).