Heat pump calculator: air-to-water, air-to-air or ground source
For an nZEB house of 150 m² on two storeys in Bucharest, Romania, with underfloor heating at 35 °C and hot water for 4 people (4,008 kWh a year for heating and 2,500 kWh for hot water), the calculation gives a 7 kW heat pump. A year it uses: air-to-water 1,627 kWh, €381 (SPF 4.0); air-to-air 1,711 kWh, €400 (5 splits and a separate hot-water heat pump); ground source 1,753 kWh, €410, with 2 boreholes of 100 m (200 m). The same house with a gas boiler pays €460 a year.
At 1.25 lei/kWh for electricity and 0.35 lei/kWh for gas (€0.234 and €0.065), a heat pump costs less a year than gas only if its SPF is above 3.3. Here the saving against gas is €50–€79 a year, too small to pay back €7,140–€13,960; against direct electric heating (€1,520 a year) air-to-water pays back in 6 years. Semper Supra’s rule from its projects: in a passive house with heat-recovery ventilation, air-to-air splits are enough; in an ordinary or nZEB house, an air-to-water heat pump with a low flow temperature; ground source has the best efficiency in the cold but the longest payback and is rarely worth it for a single house.
Result: Air-to-water heat pump
| Space heating demand | 4,008 kWh/yrnZEB house, cost calculator; heating limit 12.2 °C |
| Design load | 5.1 kW34 W/m²; house at 23 °C, outside −15 °C, no gains; plus 1.00 kW for hot water |
| Heat pump size | 7 kWdelivers this at −15 °C; at +7 °C about 14.0 kW |
| SPF, with hot water | 4.00heating only: 4.69 |
| COP at +7 °C · at the design temperature | 6.5 · 2.4at −15 °C outside, water at 35 °C |
| Electricity a year | 1,627 kWh/yrheating 855 + hot water 772 + heater 0 kWh |
| Electric back-up heater | 0 kWh/yrbelow −15 °C: 0 days in the typical year (daily means); 7 hours below −15 °C |
| Cost a year | €381 a yearelectricity at €0.234/kWh |
| Current heating | €460 a yearGas boiler: 7,074 kWh a year, hot water included |
| Saving a year | €79 a year |
| First cost, installed | €7,140pump €5,680 + cylinder €1,460 |
| Payback | 90 yearslonger than the 25 years counted |
| Net gain after 25 years | −€4,260the savings, with prices rising 3.0 % a year, less the first cost |
Air-to-water: heats the water in the floor or radiators and the domestic hot water. The cooler the water (35 °C instead of 55 °C), the higher the COP.
Noise: 58 dB(A) at 5.0 m gives about 36 dB at the neighbour’s window, below the night limit of 45 dB (Romanian Order 119/2014, art. 16). With the window closed, the limit in the room is 30 dB.
Refrigerant: from 1 January 2027, new air-to-water heat pumps up to 12 kW must use a refrigerant with GWP under 150, in practice R290 (propane). What it means for you →
Comparison: one house, three heat pumps
- Cost a year
- €381 a year
- First cost
- €7,140
- Payback
- 90 years
- SPF
- 4.0
- Cost a year
- €400 a year
- First cost
- €8,190
- Payback
- 138 years
- SPF
- 3.8
- Cost a year
- €410 a year
- First cost
- €13,960
- Payback
- 281 years
- SPF
- 3.7
Current heating (Gas boiler): €460 a year
First cost is the installed cost less the subsidy; payback is the first cost divided by the first year’s saving against the current heating. SPF = heat delivered in a year divided by the electricity used, hot water included.
COP day by day: Air-to-water heat pump
The heating SPF, 4.7, is the mean of the daily COPs weighted by each day’s heat. The house is heated on 174 days a year, those with a mean below 12.2 °C.
Capacity: the heat pump against the house
An air-to-water or air-to-air pump delivers at −15 °C about half of its +7 °C output. The 7 kW pump must give 7 kW in the cold, so at +7 °C it gives about 14.0 kW. A pump with 7 kW only at +7 °C no longer covers the house below −13 °C and switches on the heater.
The house is the cost calculator’s simplified one, the climate is the PVGIS typical year day by day, and the COP comes from Keymark certificates, i.e. from the laboratory. In real houses the measured SPF is usually lower (Fraunhofer ISE: 3.4 on average for air-to-water). The real size comes from a heat-loss calculation of your house.
How a heat pump works
A heat pump does not make heat, it moves it: it takes heat from the air, the ground or water at a low temperature and gives it to the house at a higher one, using a compressor and a refrigerant (page on heat pump refrigerants). The smaller the lift between the source and the water in the house, the less the compressor has to work. The theoretical limit (Carnot) is Twater ÷ (Twater − Tsource) in kelvin; a good heat pump today reaches about 41 % of it (air-to-water) and 49 % (ground source).
COP and SCOP: what they mean
COP (coefficient of performance) = heat delivered ÷ electricity used, at one test point, e.g. A7/W35: air at +7 °C, water at 35 °C. SCOP = the average over a whole season under EN 14825, in a reference climate (on the EU label: Strasbourg, −10 °C). SPF (seasonal performance factor) is the same ratio measured or computed for one house in one year; here the SPF includes hot water, and “heating only” is shown separately.
Why 35 °C beats 55 °C
For the same house, with water at 35 °C (underfloor) the heating SPF is 4.69, and at 55 °C (old radiators) 3.41: 37 % better, 320 kWh less electricity a year. Published comparisons give 17–24 % for every 10 K lower flow, so about 40 % between 35 and 55 °C.
How warm the water is on mild days
The heat pump does not keep the water at 35 °C all winter: on the heating curve the water has its design temperature only on the coldest day and falls linearly to 24 °C (underfloor), 27 °C or 30 °C (radiators) at +12 °C outside, as in EN 14825. That is why the COP rises a lot on mild days.
Air-to-water heat pump
The outdoor unit takes heat from the air and heats the water in the underfloor or radiators and the domestic hot water. It is the usual choice for a new or renovated house with underfloor heating or large radiators. For the house here: 7 kW, 1,627 kWh a year, SPF 4.00 with hot water and 4.69 for heating only.
Heat pump at −15 °C
The COP falls with the cold: at −15 °C outside and water at 35 °C it is 2.4, against 4.5 at +7 °C (table below, from the Keymark certificate of an R290 heat pump). It keeps working at −20 °C (its operating limit), with a COP of 2.1. In Bucharest the design temperature is −15 °C, but the daily mean of the typical year never drops below −12.7 °C, and 7 hours a year are below −15 °C.
Label SCOP and Romania
The EU label shows the SCOP for the average climate (Strasbourg, −10 °C). Romania’s design temperatures (−12 °C in Constanța to −21 °C in Brașov, SR 1907-1) sit between the average and the colder climate (Helsinki, −22 °C), so the label shows a little more than the pump does here. Keymark example (Vaillant aroTHERM plus): SCOP 4.75 at 35 °C in the average climate, 4.05 in the colder; at 55 °C, 3.44 and 3.06.
Heat pump running cost in Romania, month by month
For the 150 m² nZEB house in Bucharest: 390 kWh in January (€91), 48 kWh in July, for hot water only. The monthly table of all three types is below, in the comparison. An ordinary house with old radiators uses about 3 times as much (4,367 kWh a year).
What size of heat pump do I need?
The design load of the house is 5.1 kW (34 W/m²): the heat loss at 23 °C inside and −15 °C outside, without solar and internal gains, plus 1.00 kW for hot water (0.25 kW per person). Rounded up: 7 kW. An air-to-water heat pump delivers at −15 °C about half its +7 °C output, so it must be chosen by its output in the cold: one with 7 kW only at +7 °C is too small and switches on the electric heater on cold days (second chart). Below the bivalence point (−15 °C, as in the Keymark certificate) the heater covers what is missing; in Bucharest’s typical year it is not needed on any day, but 7 hours are below −15 °C.
Hot water with an air-to-water heat pump
A 200–300 l cylinder next to the pump, heated to 50 °C (the refrigerant works at about 55 °C): 2,500 kWh a year for 4 people, with 772 kWh of electricity (mean COP 3.2). Hot water is cheap in summer and dearer in winter.
Defrost
Between about −7 and +5 °C, with humid air, ice forms on the outdoor evaporator; the pump melts it by reversing the cycle for a few minutes, and the house loses a little heat. The Keymark point at +2 °C includes defrosting, and so does this calculation.
Outdoor unit noise and the neighbours
Romanian Order 119/2014, art. 16: outside, at the neighbour’s dwelling, at most 55 dB by day and 45 dB at night (23:00–7:00); in the room with the windows closed, 35 dB by day and 30 dB at night. A unit of 58 dB(A) sound power gives about 44 dB at 2 m, 36 dB at 5 m, 30 dB at 10 m (Lp = Lw − 20·log10(d) − 8, over a hard surface; a wall behind the unit adds up to 3 dB). Fraunhofer ISE found in 2025 three of five tested units above the night limit. The calculator has fields for the unit and the distance.
Heat pump cost with installation in Romania, 2026
Romanian prices incl. VAT in the table below. The calculation starts from €5,980 for an 8 kW pump installed and €1,460 for the cylinder (the difference between the same pump with and without a 300 l cylinder); for other sizes 60 % of the price stays fixed and 40 % grows with kW. For 7 kW: €7,140.
R290, the refrigerant from 2027
From 1 January 2027, new air-to-water heat pumps up to 12 kW sold in the EU must use a refrigerant with GWP under 150: in practice R290 (propane), already in 38 % of new certificates in 2024. Details on the page on heat pump refrigerants.
| Outside | Water 35 °C | Water 45 °C | Water 55 °C |
|---|---|---|---|
| 7 °C | 4.5 | 3.4 | 2.8 |
| 2 °C | 3.8 | 3.0 | 2.6 |
| −7 °C | 3.0 | 2.5 | 2.2 |
| −15 °C | 2.4 | 2.1 | 1.8 |
| −20 °C | 2.1 | 1.9 | 1.7 |
| What | Price | Source |
|---|---|---|
| Air-to-water 8 kW, equipment only | €2,800–4,210 | necesit.ro, 2026 |
| Air-to-water 12 kW, equipment only | €4,450–6,320 (mean €5,380) | necesit.ro, 2026 |
| R290 monobloc 8 kW (example Samsung EHS) | €4,450 | arenainstalatiilor.ro |
| 8 kW monobloc / the same with a 300 l cylinder (example Daikin Altherma 3) | €4,750 / €6,210 | sebra.ro, 16 Sept 2026 |
| Installation and materials | €930–2,240; labour alone €650–1,120 (monobloc), €840–1,500 (split) | sebra.ro, necesit.ro |
| Complete installed system 8–12 kW | €6,540–10,280; premium €9,350–11,220+ | sebra.ro, May 2026 |
| House of ~150 m², 6–8 kW, installed | €7,100–9,720 | sebra.ro |
| This calculation (8 kW installed) | €5,980 + cylinder €1,460 | cost calculator, 28 Oct 2026 |
Prices converted from lei at the BNR reference rate of 28 October 2026, 1 € = 5.3495 lei, and rounded.
Air-to-air heat pump (split)
An inverter air conditioner is an air-to-air heat pump: in winter it reverses the cycle and blows warm air into the room. Models made for heating have stronger compressors and work down to −25 °C; ordinary ones to −10/−15 °C. For the house here: 5 units of 1.4 kW each, 1,711 kWh a year, of which 833 kWh for hot water made by a separate heat pump.
A split’s COP in the cold
The table below: the indicative COP of a split made for heating (solutiiverzi.ro, April 2026) and the one computed here, with the condenser at 38 °C. An ordinary split has only 1.5–2.0 at −15 °C, or stops. Studies of real splits (E3S, 2019) found climate-adjusted SCOPs up to 25.6 % below the declared values.
When splits can heat the whole house
Semper Supra’s experience: in a passive house with heat-recovery ventilation, one or two air-to-air units heat the whole house, because the losses are small (here 18 W/m² at design) and the ventilation carries the warm air to every room. The cost calculator uses exactly that for its passive house column (passive house cost calculator); size the ventilation with the ventilation calculator. In an ordinary house the heat comes only with the air, so every room needs its own unit, and with the doors closed the rooms without one stay cold.
Air-to-water or air-to-air heat pump?
For the nZEB house here: air-to-water 1,627 kWh and €7,140, air-to-air 1,711 kWh and €8,190 (with the hot-water heat pump). Air-to-water also makes the hot water, uses the underfloor and blows no air; air-to-air is cheaper in very good houses, cools in summer and needs no underfloor or radiators. For the passive house: air-to-air €4,260, air-to-water €6,240, without underfloor.
No hot water
Splits make no hot water. The calculation adds a hot-water heat pump (cylinder with its own heat pump) of €1,640, with a COP of 3.0, as in the cost calculator.
What splits cost installed
A premium wall split: €930–1,780; installation €170–220 for one of 9–12 kBTU, €220–340 for a multi-split with 2 units, €340–560 for 3 or more, plus €10–20 for every extra metre of pipe (climatizareonline.ro, March 2026). The calculation: €1,310 per installed unit.
R32 until 2029
Air-to-air splits up to 12 kW may still use R32 (GWP 675) until 31 December 2028; from 1 January 2029, only refrigerants with GWP under 150. An R32 split bought now can be serviced for its whole life (page on heat pump refrigerants).
| Outside | Table (solutiiverzi.ro) | Calculation |
|---|---|---|
| 7 °C | 5.5 | 5.5 |
| 0 °C | 4.5 | 4.5 |
| −7 °C | 3.7 | 3.8 |
| −15 °C | 3.0 | 3.0 |
| −25 °C | 2.2 | 2.2 |
Ground source heat pump
A ground source heat pump takes heat from the ground through brine (water with antifreeze) flowing in pipes in vertical boreholes or a horizontal collector. At 60–150 m the ground is at 10–12 °C all year, and the brine comes back from the ground at about 0 °C on the design day. For the house here: 7 kW, 1,753 kWh a year (with 195 kWh for the brine pump), 2 boreholes of 100 m.
How many metres of borehole does a heat pump need?
The rule: length = heat taken from the ground ÷ 50 W/m, with heat taken = output × (1 − 1/COP). For 7 kW and a COP of 4.3 at 0 °C: 5.4 kW from the ground → 108 m → 2 boreholes of 100 m. Boreholes that are too short cool the ground year after year. A horizontal collector 1.2–1.5 m deep takes 10–40 W/m² depending on soil moisture (VDI 4640); at 25 W/m² it would be 220 m² of dug ground.
Ground source efficiency and the climate
The source has almost the same temperature all winter, so the label SCOP (5.2 at 35 °C and 4.0 at 55 °C for the NIBE S1255) is close to what the pump does in Romania; Fraunhofer ISE measured 4.3 on average in existing houses, against 3.4 for air-to-water. In the calculation here, in Bucharest the heating SPF is 4.65 for ground source and 4.69 for air-to-water: mild winters help the air-to-water pump, and hot water made in summer from cold brine pulls ground source down. In the cold cities (Brașov, Sibiu, Iași) ground source wins (city table below).
Geocooling: summer cooling
In summer the cold brine from the boreholes can cool the underfloor directly, with only the circulation pump running (passive cooling). The heat taken out of the house recharges the ground a little for winter.
Permits for a heat pump borehole in Romania
Closed-loop boreholes (ground source) normally need only a notification. Open-loop systems (water-to-water, with water pumped from a well and returned) need a water management permit (“aviz de gospodărire a apelor”) from Apele Române (Water Law 107/1996, art. 48 and 50; Order MAP 828/2019 for the permit, Order MMAP 3147/2023 for the authorisation). The basin administration decides the classification, and only an accredited firm may write the hydrogeological study.
Ground source heat pump borehole price
The 12 kW unit: €3,980–6,320, 20 kW €7,010–9,350 (necesit.ro). Drilling: from €35/m (≈ 175 lei/m with the U-pipe, bentonite and pressure test, forajesiputuri.ro, 2026); Bosch quotes €50–60/m for boreholes and €40–50/m for trenches. An installer gives the unit from about €7,000 and the ground works another ~€10,000 (sibotherm.com, February 2026); complete systems €15,000–25,000 (solutiiverzi.ro). The calculation: unit €5,230 for 8 kW, drilling €33/m, connection €930; for the house here, €13,960.
How long it lasts
Boreholes last 25–50 years, the heat pump 20–25 years. There is no outdoor unit, so no noise at the neighbours’.
| City | Design temp. | Heat pump | Heating SPF air-to-water | Heating SPF ground source | SPF with hot water, air-to-water | SPF with hot water, ground source | Borehole | Hours below −15 °C |
|---|---|---|---|---|---|---|---|---|
| Bucharest | −15 °C | 7 kW | 4.7 | 4.7 | 4.0 | 3.7 | 200 m | 7 |
| Cluj-Napoca | −18 °C | 7 kW | 4.4 | 4.6 | 3.8 | 3.9 | 200 m | 15 |
| Timișoara | −15 °C | 7 kW | 4.7 | 4.7 | 3.9 | 3.7 | 200 m | 0 |
| Iași | −18 °C | 7 kW | 4.2 | 4.6 | 3.7 | 3.8 | 200 m | 28 |
| Brașov | −21 °C | 7 kW | 4.3 | 4.7 | 3.8 | 3.9 | 200 m | 15 |
| Constanța | −12 °C | 6 kW | 4.7 | 4.6 | 3.9 | 3.6 | 100 m | 0 |
| Sibiu | −18 °C | 7 kW | 4.1 | 4.6 | 3.6 | 3.8 | 200 m | 87 |
| Craiova | −15 °C | 7 kW | 4.6 | 4.6 | 4.0 | 3.7 | 200 m | 2 |
| Cornu de Jos, Prahova | −18 °C | 7 kW | 4.4 | 4.6 | 3.8 | 3.8 | 200 m | 3 |
Which heat pump for your house
The calculator’s comparison strip puts the three heat pumps side by side for the same house, with the current heating’s cost as a reference. The table below does the same for the three houses of the passive house cost calculator (150 m², Bucharest, hot water for 4): electricity and cost a year, then first cost and payback against gas.
Heat pump against gas, wood and electricity
With gas at 0.35 lei/kWh and a 0.92 boiler, a heat pump costs less than gas only with an SPF above 3.3. Against wood the saving is larger, and against direct electric heating very large: the second table. In a new house the heat pump also replaces the gas connection and the boiler (€940 in the cost calculator): then the air-to-water payback against gas falls to 26 years.
Rules from Semper Supra’s experience
Passive house with heat-recovery ventilation: air-to-air splits are enough, one or two units. Ordinary or nZEB house: an air-to-water heat pump with a low flow temperature, underfloor or large radiators. Ground source: the best efficiency in the cold, the longest payback; for a single house it is rarely worth it, more often in large houses, cold areas or where you want geocooling.
Hybrid heat pump
In an existing house with a gas boiler, a smaller air-to-water pump can work next to the boiler: the pump does most of the year, the boiler starts only on cold days or for hot water. It works with the existing radiators and needs less money up front; this calculator does not model it.
| House | Air-to-water | Air-to-air | Ground source | Gas now |
|---|---|---|---|---|
| Ordinary house: 13,307 kWh, 12 kW, 55 °C | 4,367 kWh · €1,020/yr €8,640 · 91 years | 3,581 kWh · €840/yr €8,190 · 29 years | 4,512 kWh · €1,060/yr €15,270 · 251 years | €1,120 |
| nZEB house: 4,008 kWh, 7 kW, 35 °C | 1,627 kWh · €381/yr €7,140 · 90 years | 1,711 kWh · €400/yr €8,190 · 138 years | 1,753 kWh · €410/yr €13,960 · 281 years | €460 |
| Passive house: 882 kWh, 4 kW, 35 °C | 987 kWh · €231/yr €6,240 · 790 years | 1,047 kWh · €245/yr €4,260 · never | 1,089 kWh · €255/yr €9,870 · never | €239 |
| Current heating | Cost a year | Air-to-water | Air-to-air | Ground source |
|---|---|---|---|---|
| Gas, €0.065/kWh, boiler 0.92 | €460 | €79 · 90 years | €60 · 138 years | €50 · 281 years |
| Firewood, €0.056/kWh, efficiency 0.70 | €520 | €140 · 51 years | €120 · 68 years | €111 · 126 years |
| Direct electric, €0.234/kWh | €1,520 | €1,140 · 6 years | €1,120 · 7 years | €1,110 · 13 years |
| Month | Air-to-water | Air-to-air | Ground source |
|---|---|---|---|
| January | 390 kWh · €91 | 356 kWh · €83 | 348 kWh · €81 |
| February | 264 kWh · €62 | 253 kWh · €59 | 262 kWh · €61 |
| March | 138 kWh · €32 | 150 kWh · €35 | 165 kWh · €39 |
| April | 71 kWh · €17 | 81 kWh · €19 | 88 kWh · €21 |
| May | 62 kWh · €15 | 74 kWh · €17 | 79 kWh · €19 |
| June | 50 kWh · €12 | 68 kWh · €16 | 72 kWh · €17 |
| July | 48 kWh · €11 | 71 kWh · €17 | 73 kWh · €17 |
| August | 45 kWh · €10 | 71 kWh · €17 | 73 kWh · €17 |
| September | 55 kWh · €13 | 70 kWh · €16 | 72 kWh · €17 |
| October | 86 kWh · €20 | 97 kWh · €23 | 98 kWh · €23 |
| November | 141 kWh · €33 | 148 kWh · €35 | 149 kWh · €35 |
| December | 279 kWh · €65 | 272 kWh · €64 | 273 kWh · €64 |
What the calculator does not tell you
- The house is the cost calculator’s simplified one: a compact rectangle, U-values per class, thermal bridges as an addition. Your house has other losses.
- The COP comes from certificates (laboratory). In real houses the measured SPF is usually lower: Fraunhofer ISE found 2.6–5.4, on average 3.4 for air-to-water and 4.3 for ground source.
- The climate is the PVGIS typical year, day by day: the coldest hours of a day are not seen, so the electric heater may start more often than the calculation shows.
- Prices move: equipment, installation and energy are those of 31 October 2026; electricity offers range from 1.04 to 1.89 lei/kWh.
- Romania’s Casa Verde for heat pumps was not open on 31 October 2026; the subsidy is a field, not a promise.
- The real size comes from a heat-loss calculation (EN 12831) or a PHPP model of your house: see Semper Supra’s services.
Questions about heat pumps
How much electricity does a heat pump use a year?
For a 150 m² nZEB house in Bucharest, with underfloor heating and hot water for 4: air-to-water 1,627 kWh (€381), air-to-air 1,711 kWh (€400), ground source 1,753 kWh (€410), at €0.234/kWh. An ordinary house with old radiators: 4,367 kWh with air-to-water.
What size of heat pump do I need for 150 m²?
It depends on the house, not the area: the 150 m² nZEB house in Bucharest needs 7 kW, the passive house 4 kW, the ordinary house 12 kW (with hot water). The output must be the one at −15 °C, not the label one at +7 °C.
Is a heat pump worth it against gas in Romania?
At the prices of 31 October 2026 (electricity 1.25, gas 0.35 lei/kWh), a heat pump is cheaper a year than gas only with an SPF above 3.3. The nZEB house saves €79 a year with air-to-water: not enough to pay back €7,140. In a new house, where the pump replaces the gas connection and the boiler, the payback is 26 years; against electric heating, 6 years.
Does an air-to-water heat pump work at −15 °C?
Yes. A current R290 heat pump has a COP of about 2.4 at −15 °C with water at 35 °C and runs down to −20 °C; its output however falls to about half of the +7 °C value, which is why it is chosen by its output in the cold. Below −15 °C the electric heater helps; Bucharest’s typical year has 7 hours below −15 °C.
Which is better: air-to-water or air-to-air?
In a passive house with heat-recovery ventilation, air-to-air: one or two units are enough and cost less. In an ordinary or nZEB house, air-to-water with underfloor heating or low-temperature radiators: it heats every room evenly and makes the hot water.
What does a ground source heat pump with boreholes cost?
For the 150 m² nZEB house: €13,960, with 2 boreholes of 100 m at €33/m, the unit and the cylinder. Complete systems cost €15,000–25,000 (solutiiverzi.ro). It is the dearest up front and has the longest payback.
Why does the water temperature matter for a heat pump?
With water at 35 °C (underfloor) the heating SPF is 4.69, at 55 °C (old radiators) 3.41: about 37 % apart. That is why a heat pump works best with underfloor heating or large radiators.
Which refrigerant should a heat pump have?
From 2027, new air-to-water heat pumps and monoblocs up to 12 kW use refrigerants with GWP under 150, in practice R290 (propane); air-to-air splits may use R32 until the end of 2028. Details on the page on heat pump refrigerants.
How it calculates
The heat demand is the cost calculator’s, for the same house; the rest is computed day by day over the city’s typical year. Prices of 31 October 2026, incl. VAT. Prices converted from lei at the BNR reference rate of 28 October 2026, 1 € = 5.3495 lei, and rounded.
Demand and load
Monthly EN ISO 13790 balance of the cost calculator (losses, solar and internal gains) → kWh a year and the heat-loss coefficient H, W/K. Design load = H × (23 °C − SR 1907-1 design temperature), no gains, + 0.25 kW per person for hot water; heat pump = the load at max(design temperature, −15 °C), rounded up.
Source: passive house cost calculator · SR 1907-1
The days
The demand is split over the 365 days: H × (Tlim − tday) × 24 h, with the heating limit Tlim chosen so that the sum equals the yearly demand. A gas bill, if given, scales H.
Source: PVGIS typical year
COP
COP = η · Tflow ÷ (Tflow − Tsource), in kelvin; air-to-water η 0.412, falling 0.55 % per degree below −7 °C; air-to-air η 0.544 at 38 °C in the condenser, 0.95 % per degree; ground source η 0.49 with the brine between 0 °C (end of winter) and +5 °C (start of autumn). Heating curve after EN 14825. Below 25 % of the pump’s output, on-off losses with Cdh 0.97.
Source: HP Keymark · Vaillant · NIBE · solutiiverzi.ro
Money
Cost a year = electricity × price. The saving is against the current heating (gas 0.92, wood 0.70, electric 1.0), hot water included. Payback = (first cost − subsidy) ÷ first year’s saving; the net gain adds up the savings with prices rising 3 % a year, less the first cost.
Source: economisi.ro (ANRE comparator) · necesit.ro · sebra.ro
Check against the certificate
| Keymark point | Certificate | Calculation | Difference |
|---|---|---|---|
| A7/W35 | 4.42 | 4.53 | +2.6 % |
| A+2/W27 | 5.04 | 4.90 | −2.7 % |
| A−7/W30 | 3.42 | 3.36 | −1.6 % |
| A−15/W33 | 2.57 | 2.52 | −1.8 % |
| A7/W55 | 2.85 | 2.82 | −1.2 % |
| A+2/W37 | 3.70 | 3.62 | −2.2 % |
| A−7/W44 | 2.54 | 2.56 | +0.9 % |
| A−15/W50 | 1.92 | 1.96 | +2.2 % |
R290 air-to-water heat pump (Vaillant aroTHERM plus), HP Keymark 2022: the +7 °C points at a fixed flow (EN 14511), the others at the colder climate’s variable flow (EN 14825). For the average climate the same calculation gives SCOP 4.80 at 35 °C and 3.43 at 55 °C, against 4.75 and 3.44 on the certificate. Ground source: B0/W35 4.31 against 4.12–4.16 (NIBE), B0/W55 2.92 against 2.90–2.97.
Limits
- The house is the cost calculator’s simplified one: a compact rectangle, U-values per class, thermal bridges as an addition. Your house has other losses.
- The COP comes from certificates (laboratory). In real houses the measured SPF is usually lower: Fraunhofer ISE found 2.6–5.4, on average 3.4 for air-to-water and 4.3 for ground source.
- The climate is the PVGIS typical year, day by day: the coldest hours of a day are not seen, so the electric heater may start more often than the calculation shows.
Sources
- HP Keymark, certificate Vaillant aroTHERM plus VWL 75/6 and 85/6 (R290), 2022 · NIBE S1255/S1256, data sheet
- Fraunhofer ISE, 77 heat pumps in existing houses, November 2025 · review of 30 tested air-to-water units (Renewable and Sustainable Energy Reviews)
- Prices: necesit.ro, sebra.ro, arenainstalatiilor.ro, greenlead.ro, solutiiverzi.ro, climatizareonline.ro, forajesiputuri.ro, sibotherm.com, 2026
- Energy: economisi.ro, from the ANRE comparator, 7 and 29 October 2026 · Casa Verde: AFM, instalatiipro.ro, August 2026
- Noise: Romanian Order MS 119/2014, art. 16 · Boreholes: VDI 4640; Water Law 107/1996 · Refrigerants: Regulation (EU) 2024/573
- Climate: PVGIS, typical meteorological year, daily and hourly · SR 1907-1, design outdoor temperatures
Calculation and text: SEMPER SUPRA INITIATIVE S.R.L. · Climate data: PVGIS © European Union, 2001–2026.