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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A kilowatt is a unit of power. I think it might be easier to think about in terms of a motor, if only because there are only 2 factors to consider, and both have very real consequences. Power of a motor tells you basically how much torque (think force, I’ll replace torque with force since it’s eli5 anyway) and speed you can get, but there’s a tradeoff. If you increase force, you lose some speed. If you increase speed, you lose some force. If you need both more speed and more force for your application, the only option is to step up to the next higher power motor.
Now, something about power that is a bit easier to understand with a furnace than with a motor is the units themselves. A watt is equivalent to 1 joule per second. I mean that quite literally. If you take energy in joules and divide it by the amount of time in seconds it took to deliver that energy, that *IS* power in watts. This is also why the amount of energy you used is measured in units that multiply power by time (kilowatt-hour), you’re just cancelling the time component. (And the conversion from watt to kilowatt and second to hour puts things in a nice range, but doesn’t change the fact that it represents energy. If you use 1 kilowatt of power for 1 hour, you’ve used 1kWh of energy. 3kW for 4hr, that’s 12kWh)
20 and 24 are a bit large to make a neat example, so let’s pretend you’ve got a 3kW and a 4kW option. You set the thermostat to 70⁰ in 2 identical houses with each furnace. They both kick in. Let’s say that 120kJ of energy is required to get up to 70⁰. A 3kW furnace delivers 3 kJ every second, it will take 40 seconds to reach 120kJ of total output. A 4kW furnace delivers 4kJ of energy every second, it will take 30 seconds to reach 120kJ of total output. A higher power furnace outputs more energy per second.
There’s one tricky little thing also going on behind the scenes which is usually more prevalent for motors than for furnaces and that’s efficiency. Motors will often use a gearbox to turn high speed low force motors into high force low speed, but there’s a lot of moving parts sapping energy. In a furnace, it’s a combination of several things. How cleanly the fuel burns (if gas), how much energy used to run the electronics. How efficiently the heat is transferred from inside the furnace to inside the home… And there’s something often referred to as the law of large numbers. If you can limit your losses to a constant minimum, then the higher your power output, the smaller of a percent those losses take up. 4J lost for every 40J used is 90% efficiency. 4J lost for every 80J used is 95% efficiency. So larger furnaces *can* sometimes be more efficient. It depends on many things.
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