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

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.