How to save 4000 EUR on heat pump installation?

A few weeks ago, I had one design revision for a homeowner. 110 m2 of underfloor heating, new build. Just on the heat pump itself, we saved 2000,00 EUR + an additional 1000,00 EUR on removing the buffer and 2 pumps. And probably, 1000+ EUR on electricity bill in next couple of years.

The oversizing topic is always interesting, as there are some houses working really efficiently with a 20-30 % oversized heat pump. This is not a problem, especially in the UK, where the design outdoor temperature is the one with the highest defrost, so you need to take that into account. But 100 % oversizing is not a good idea.

On the other hand, in Croatia, we have lower design temperatures but also not so much defrost in those conditions. In addition to that, most of the designers are using -15 °C for Zagreb when in reality, -12 °C is in technical norms, but rarely below -10 °C (99.6 % of the year is above -9.3 °C, see ASHRAE meteo analysis below).

A detailed calculation of losses for different outdoor air temperatures in the project in question is shown in table below:

Room	Area (m2)	-15 °C	-12,6 °C	-9,3 °C	-6.7 °C
BEDROOM 1	10,64	576	537	482	439
BEDROOM 2	14,46	759	707	635	579
BATHROOM 2	10,66	624	581	522	476
OFFICE ROOM	5,14	252	235	211	192
BEDROOM 3	15,46	798	743	668	609
LIVING ROOM AND KITCHEN	41,98	1 966	1 831	1 646	1 500
TOTAL	98,34	4 975	4 634	4 165	3 795

Table 1. Display of different heat losses by room

If -15 °C is taken as the outside temperature, the calculated heat losses are 4975 W (4.975 kW), and for -9.3 °C, the heat loss is 4.165 W (4.165 kW).

If we compare datasheet for 8 kW heat pump in design, and 4 kW what I proposed, here is comparison:

Maximum is not a problem - it will be enough. Also, there is multisplit air to air and fireplace in living room. If we add thermal mass in the equation, we are definitely on the safe side.

Minimum capacity or part-load is what is the best part here:

The chart compares a 4 kW "right-sized" heat pump versus an oversized 8 kW unit, matched against the actual heat demand of a house (with a peak loss of 4.2 kW at –10°C).

The 4 kW unit can modulate output to tightly track the real heating needs throughout the year, delivering efficient comfort at lower outdoor temperatures. And what is important, modulation.

The 8 kW unit, by contrast, is forced to cycle ON/OFF for the complete outdoor temperature range, leading to inefficiencies, increased wear, cost, and poorer comfort.

Here is video about oversizing if you want to learn more about that topic:

Watch the video

Sometimes I receive feedback and comments about education that we are forcing too much of that topic. But an educated manufacturer, designer, installer, and homeowner is a win-win situation. Just on this one project, you can save yourself, your customer, and the industry in general, a few thousand euros/pounds. Payback period is instant! 😊

(P.S. Sorry, the heat pump manufacturer will "lose" some money on cheaper equipment, but every manufacturer I know is more than happy with that as they will have better installations and happier clients.)

P.P.S. This is exactly why we created heat pump education with exact steps how to design, install, and commission heat pump systems with 5 different courses. Each course is broken down into microlearning modules with short videos, animations, drawings, and interactive assessments so learners can move from theory to practice step by step.