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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Electric furnaces are usually rated in kw, gas furnaces are usually rated in BTU. In either case, the rating means the same thing; the total heat output capacity of the furnace. The difference is in a furnace’s ability to heat a larger space, or to heat a smaller space faster. The higher the rating, the larger the space a furnace can heat, or the faster it can heat the same space as a lower rated furnace.
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
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.
The difference is how much water/air at 70°C is can produce per unit of time.
1W (Watt) is 1 joule/second, joule is a unit of energy. So 20kW means it produces 20,000Joules of heat every second
Water have a specific heat capacity of 4.2kJ/(kgK) that mean heating up 1 kg=1liter of water one-degree celsius requires 4200Joule of energy.
This mean 20kW can heat up 20/4.2 = 4.8 degreeliter of water every second. That can be 1 liter 4.8 degrees. 4.8 liter 1 degree. 0.5 liter 9.6 degrees.
The 25kW can heat up 6.0 degrees later of water every second,
If the input water is 30C and the output is 70C you need to heat the water 40C. This means a 20kW can output 4.8/40 = 0.12 liter of 70C water per second compared to the 25 kW that can output 6/40 =0.14 liter of 70C water per second
So the difference is how much warm water/air it can produce per unit of time. How much you need depends on the size, insulation of the house, and how cold it is outside, technically is the difference in inside and outside temperature that is relevant.
So the difference will be in how cold weather can it keep your house warm, that is what is relevant for you. If you get the 20kW and it never get colder than it can handle you will not notice any difference compared to the 24kW but there will be a clear difference if it cant handle it
It would also be the time it takes to heat up the inside if the inside temperature is lower, which is a lot less relevant for a house but would be relevant if you, for example, have a diesel heater in a car that gets cold when not in used.
When I think about that, if the heater also is used to heat up warm water then it power difference can have a noticeable effect even if it is not very cold outside.
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).
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