A smaller heat pump can be more profitable than a bigger from the same model series

There is a common belief that when selecting heat pump, one of the most important factors is to select a pump big enough for removing most of the need for additional heating. As I will show, it might well be that an intentional selection of a smaller heat pump, forcing you to do a lot more additional heating, may be the most price effective selection.

To show this, I have selected to do the calculations on three alternative heat pumps from NIBE (the Fighter 2020 series). The calculations provide support for selecting other alternatives than I would originally have expected. The reason for this seemingly illogical result is the fact that COP-factor tends to be affected infavourably by bigger pump sizes, thus negating one advantage through other dependent disadvantages.

Even the smallest heat pump in the series is good, and better than my old pump

The smallest and cheapest heat pump in the Fighter series (the 2020, 8 kW) is a good alternative when my current heat pump breaks. It should be weaker than my current heat pump (FlasH VP900, 9kW), but has a better COP factor and produces heat down to minus 20 instead of minus 10 as my current pump does. In fact, that pump, on paper, produces more heat than my current at all temperatures. It is hard to understand that my old pump was called 9kW. But I also know that there are some differences in the measurement of the Fighter series (circulation pump and defrosting is not included in the figures provided by the Vendor). I have not compensated for that, since my main intention is the comparison between the three Fighter series heat pumps.

The new heat pumps serve the house need better than my old heat pump

The below diagram shows my old heat pump, and the three heat pumps that are to be compared in the coming calculations.

In the diagram is also included the energy need for my house at different temperatures. That curve is a theoretical curve, derived from data collected during the years. However, it is probably a good estimate of the heat need, since the crossing line between the current heat pump and the house need is at a little more than one degree Celcius. The practically derived break point which I use when heating, where the heat pump no longer can alone support the house with needed energy is in reality 1.7 degrees. The difference might be due to many reasons, one of them being the heat pump producing a little less than theoretically at that temperature, since it is now and then stopping for derfrosting.

Already here we can see that a modern heat pump of a lower size can outperform an older heat pump that is bigger. This is expected by most people and come as no surprise. Yet, it is in fact giving an easily accepted proof of the statement I make: A smaller heat pump can save more money than a bigger heat pump, if the COP is better. This can be true even if the bigger heat pump might save more of the additional heating needed at lower temperatures.

Interestingly enough, changing to the 2020 8 kW pump will make very little difference regaring at what temperature the heat pump alone can heat the house. The diagram below shows the cross over to occur at zero degrees, or only about one degree lower than my cyurrent heat pump. Since the fighter 2020 diagram does not include defrosting, the difference is even less. Even using the 2020 10 kW pump, only a few more degrees are gained before the heat pump no longer can handle the heat itself.

The small heat pump is better, but there is no good profit from replacing the working old pump

In order to do the comparison, which is depending on temperatures, I have elected to do the first level of comparison decade by decade. This, in order to get an overview over a long enough time to say that it is likely to include both cold and warm years.

Decade	Average yearly saving with 2020 8 instead of VP900 (KWH)
1870	1785
1880	1800
1890	1707
1900	1592
1910	1480
1920	1490
1930	1308
1940	1580
1950	1473
1960	1462
1970	1322
1980	1420
1990	1132

Looking at all years from 1874 to 1999, the 2020 8 kW pump has performed better than the VP900 pump (from the diagram above, it is obvious that that would be the case). In average over all those years, the saving has been 1500 kWH per year. The worst decade of them all, the saving was 1800 kWH per year.

Now, let us translate this into money saved by buying a new heat pump. The pump itself costs around 45000 SEK, plus installation, let us assume an investment of 50000 SEK to get the pump installed and working.

The average payback time from buying the pump would then be 33 years. In the most favorable decade, the payback time would be 27 years. So even if the pump is better, changing to it before the old pump is broken will be a waste of money seen over a long period of time

The bigger heat pump (10 kW) is even better, but profitability is doubtful

Now, let us look at the bigger heat pump, the 2020 10 kW. Instead of comparing it to the old pump, we compare it to the 2020 8 kW pump, since that pump is our best alternative so far. First notable thing is that it is better than the 2020 8 kW pump most years, but not all years. The difference is small, but it is still worth noting that 9 years of the last 125 years, the pump needed more energy to heat the house than did the 2020 8 kW pump.

Decade	Average yearly saving with 2020 10 instead of 2020 8 (kWh)
1870	828
1880	793
1890	629
1900	583
1910	391
1920	437
1930	216
1940	627
1950	400
1960	548
1970	303
1980	426
1990	152

On average, it saved 480 kW more energy per year than the 2020 8 kW pump did. During the most favorable decade, it even saved on average 830 kWh per year. During the least favorable decade, it saved only 150 kWh. Interestingly enough, the additional heating is higher than the above figures, the yearly average saving of additional heating is 1100 kWh:

Decade	Needed additional kWh Electric Heater Fighter2020 8	Needed additional kWh Electric Heater Fighter2020 10
1870	15601	5516
1880	26732	10413
1890	25106	11384
1900	18495	5658
1910	14297	4563
1920	16743	6101
1930	8692	1793
1940	28585	14973
1950	14509	4682
1960	17149	5111
1970	11053	2764
1980	17886	7656
1990	5823	1294

Due to the loss of energy caused by the worse COP, the average remaining saving goes down to 480 kWh as mentioned earlier.

Translated to money, the rather small additional investment of 2000 SEK will give an average saving of 480 SEK, or a payback time of below 5 years. There is a lot of uncertainty in the profit. If the investment is done in a colder period, the payback will be faster.

But if the investment is done in a warmer period, it will have a long payback time. In fact, if using data from Hammer Odde and only looking at 1990-2005, almost half of the years where worse with the bigger heat pump, and the average saving is 50 KWH per year, making the payback time of the small investement 40 years.

Year	Average yearly saving with 2020 10 instead of 2020 8 (kWh)
1990	-215
1991	13
1992	-179
1993	-26
1994	53
1995	44
1996	777
1997	75
1998	-5
1999	-20
2000	-149
2001	34
2002	-101
2003	278
2004	57
2005	108

This is making a decision problematic. We could turn to statistics and evaluate the likelihood of the investment being profitable in less than 10 years. We could also start including the problem of efficiently running the bigger pump, which would be likely to make the payback time even longer. However, my intention is to later add some data on the savings using the heat pump also for tap water heating, and to wait with a final conclusion until then.

Finally the biggest pump in the series, the 14 kW pump, least sure investment

The biggest pump in the series is 14 kW, and cost yet another 2000 SEK extra. Comparing it to the 10 kW pump, it would over the long time series save an additional 215 kWh per year compared to the 2020 10 kW.

Decade	Average yearly saving with 2020 14 instead of 2020 10 (kWh)
1870	344
1880	381
1890	336
1900	216
1910	134
1920	215
1930	30
1940	478
1950	131
1960	189
1970	82
1980	244
1990	34

The most favorable decade would save on average 480 kW per year, and the least favorable decade would save only 30 kW per year. If we also look at 1990 to 2005 at Hammer Odde, it would on average cost an additional 30 kW per year.

Year	Average yearly saving with 2020 14 instead of 2020 10 (kWh)
1990	-49
1991	-34
1992	-59
1993	-39
1994	-20
1995	-37
1996	40
1997	-37
1998	-30
1999	-31
2000	-53
2001	-42
2002	-51
2003	8
2004	-6
2005	-57

Thus like with the upsizing from 8 to 10, we find a possible business case and a lot of uncertainty. The average payback of the investment would be 10 years, but with the possiblity to become only 5 years, and also to become a complete loss.

Safest profit is from the smaller pump

Considering the price of the pumps, the safe profitable alternative turns out to be the smallest one. Intentionally dimensioning the heat pump so that the electric heater will work a lot during cold winter days, but relying on those days are fewer than the days where the COP for that pump is having the advantage. As long as there is almost no price difference to the more expensive pumps, it is possible to make a profit by going up a step. The first step up is likely to have a profit, although it also puts some extra demands on the heating system (coping with a larger pump).

And in all cases, the investment is not worth doing until the old heat pump breaks down.