Heatpump vs Gas break-even calculation

Energy
1

Aircon vs. Gas Boiler: Calculating the Cost Breakeven Point

Hi, I’m setting up a Homey flow to automatically switch my heating source. I want to determine the exact outdoor temperature (T) at which running my LG ARTCOOL AB24BK air conditioner becomes cheaper than using my gas CV boiler.

I’ve used the product sheet data to create a more realistic COP curve than my original guess.

My Formulas and Assumptions

I compare the heat generated in Megajoules (MJ) per Euro for both systems:

    1. Gas CV Output (MJ/Euro): MJ/Euro = (31.65 * 0.90) / 1.15836

      • 31.65 is the energy content of one cubic meter of gas (MJ/m3).

      • 0.90 is the assumed efficiency (COP) of my gas CV boiler.

      • 1.15836 is my gas tariff (Euro/m3).

    2. Aircon Output (MJ/Euro): MJ/Euro = (3.6 * COP_HP) / 0.24404

      • 3.6 is the fixed conversion factor (MJ/kWh).

      • COP_HP is the variable efficiency of the aircon.

      • 0.24404 is my electricity tariff (Euro/kWh, normal rate).

    3. Aircon COP Formula (Based on LG data): COP_HP = (0.264 * T) + 3.702

This COP formula is a straight-line approximation derived from the performance points in the LG specification sheet for the AB24BK (e.g., 7 degrees C → COP 5.55 and 2 degrees C → COP 4.23).

My Calculated Breakeven Point

By setting the two MJ/Euro values equal, I calculate the breakeven temperature to be approximately -7.71 degrees C.

Any feedback on how to make this calculation more robust would be great!

1 Like
2

for example, at 13C outside temperature:
gas: 24,59 MJ/Euro
airco: 105,24 MJ/Euro

so it is 4,28 times as cost efficient to run the airco

3

COP is not a good value to use for this.

Its a nice salespitch based on ideal situation. (Just like electric cars that have a range of 560km…, you will never make it.)

Search for the SCOP thats a little bit better….

4

I think that dynamic COP (Coefficient of Performance) formula is the correct choice for the Homey automation, not the SCOP (Seasonal COP).

• COP for Switching: You need the instantaneous COP (which changes with temperature T) to determine the exact cost at the moment of switching. Your formula \text{COP}_{\text{HP}} = 0.264 \cdot T + 3.702 provides this dynamic efficiency.

• SCOP for Averaging: The SCOP is a single, fixed number representing the average efficiency over an entire heating season (including defrosting, etc.). If you used the SCOP, your comparison would be static, making the breakeven temperature calculation useless.

5

You are right, SCOP is not working for automation.

So COP is the way to go.

But be aware that the actual eficiency will be worse than what you calculate(especially below 4c). (But its the best you got)