The refrigerant cycle in four steps<\/h2>\n
A refrigerant that boils at a very low temperature circulates in a sealed loop inside the unit. The process repeats continuously:<\/p>\n
- \n
- Evaporator:<\/strong> the refrigerant absorbs heat from the air, ground or water and evaporates \u2013 even when it is below freezing outside.<\/li>\n
- Compressor:<\/strong> it compresses the vapour, sharply raising its pressure and temperature. This is where most of the electricity is used.<\/li>\n
- Condenser:<\/strong> the hot vapour gives off its heat to the water in the heating circuit or hot-water cylinder and turns back into liquid.<\/li>\n
- Expansion valve:<\/strong> it lowers the pressure and temperature of the liquid so it can pick up heat in the evaporator again.<\/li>\n<\/ol>\n
You know the same principle from your fridge, which pumps heat from the inside to the coils at the back. A heat pump moves heat from outdoors into the house, and in summer many models reverse the cycle to cool the rooms.<\/p>\n
COP: how much heat you get per kilowatt-hour<\/h2>\n
Efficiency is described by the coefficient of performance (COP) \u2013 the ratio of heat delivered to electricity used. With a COP of 4, one kilowatt-hour of electricity plus three kilowatt-hours of ambient heat give you four kilowatt-hours of heat. The lab COP applies to test conditions; for your bills the seasonal figure (SCOP), which covers a whole heating season, matters more.<\/p>\n
As a rule of thumb, a unit with a COP between 3.1 and 5.1 is considered efficient, while below 2 it is not economical. Two temperatures affect the COP most:<\/p>\n
- \n
- the source temperature<\/strong> \u2013 in hard frost an air-to-water heat pump is less efficient than at +7 \u00b0C;<\/li>\n
- the heating water temperature<\/strong> \u2013 the lower it is, the less the compressor has to work, which makes underfloor and wall heating ideal.<\/li>\n<\/ul>\n
Types of heat pumps<\/h2>\n
By heat source there are three main designs for heating houses. The figures below are average annual performance factors quoted by the manufacturer Bosch and are only a guide.<\/p>\n
\n\n
\n \nType<\/th>\n Heat source<\/th>\n Average annual factor<\/th>\n Key points<\/th>\n<\/tr>\n<\/thead>\n \n Air-to-water<\/td>\n outside air<\/td>\n about 3.5<\/td>\n cheapest and easiest to install; placement matters because of noise<\/td>\n<\/tr>\n \n Ground source (brine-to-water)<\/td>\n collector 1\u20132 m deep or borehole up to 100 m<\/td>\n about 4.0<\/td>\n steady efficiency all year; more expensive to install<\/td>\n<\/tr>\n \n Water-to-water<\/td>\n groundwater from two wells<\/td>\n about 4.5<\/td>\n highest efficiency; demanding permits; not allowed in water protection zones<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n There are also air-to-air units (air conditioners and ventilation systems) and hot-water heat pumps that only heat domestic water.<\/p>\n
Monobloc or split?<\/h3>\n
A monobloc has the entire refrigerant circuit in the outdoor unit, which is hermetically sealed; only water pipes run into the house, and they should be short and protected against frost (often with antifreeze). In a split system part of the circuit sits in the indoor unit, so it copes better with frost, but the refrigerant lines must be connected by a certified technician. Some newer monobloc units use the natural refrigerant propane (R290).<\/p>\n
Heat pumps and radiators<\/h2>\n
The most common question from owners of older houses is whether a heat pump can run their existing radiators. Often it can, with conditions. If the house is at least moderately insulated and the radiators are large enough, water at 40\u201345 \u00b0C is sufficient. For less well insulated homes with radiators there are medium- and high-temperature units that reach 60\u201365 \u00b0C, and two-stage models up to 80 \u00b0C \u2013 but at a lower COP.<\/p>\n
![\"Radiator](\"https:\/\/temha.net\/wp-content\/uploads\/2026\/10\/kako-deluje-toplotna-crpalka-250558-1024x680.jpg\")
<\/figure>\nSlovenia’s ENSVET energy advisers warn that if a house has less than about 8 cm of wall insulation and under 12 cm in the loft, the building envelope should be upgraded first. Cheaper units switch on electric heaters in severe cold, which pushes up consumption and cuts the savings.<\/p>\n
Choosing the output and placing the unit<\/h2>\n
The output should match the house’s heat loss, which you can take from the design documents or energy performance certificate, or estimate from past use of heating oil, gas or firewood. For domestic hot water add roughly 200\u2013250 W per person. An oversized heat pump switches on and off too often, so inverter units that adjust their output to demand are recommended.<\/p>\n
When siting the outdoor unit, keep in mind:<\/p>\n
- \n
- as far as possible from bedrooms and neighbours \u2013 if a neighbour is closer than 3 m, use a noise barrier or shrubs;<\/li>\n
- a sheltered spot, because wind blowing against the fan outlet reduces airflow and efficiency;<\/li>\n
- a concrete base with a gravel layer so condensate from defrosting can drain away;<\/li>\n
- a short distance to the indoor unit (ENSVET advises 10 m at most).<\/li>\n<\/ul>\n
Savings and grants<\/h2>\n
According to Eko sklad, the Slovenian environmental fund, running costs with a heat pump can, under optimal conditions, be as much as half those of a fossil-fuel boiler. In Slovenia, Eko sklad offers grants and low-interest loans for installation, with a higher grant when an old boiler is replaced (Eko sklad heat pump leaflet, in Slovenian<\/a>). Elsewhere, check the current national or local support schemes and their conditions before you buy.<\/p>\n
Frequently asked questions<\/h2>\n
Does a heat pump work at \u221215 \u00b0C?<\/h3>\n
Yes. Good air-to-water units keep working in severe cold, but their COP drops. Some cheaper models switch to an electric heater, so check the low-temperature data before buying.<\/p>\n
How much electricity does a heat pump use?<\/h3>\n
It depends on the house’s heat loss and the COP. With an annual factor of 3.5, producing 10,000 kWh of heat takes about 2,900 kWh of electricity.<\/p>\n
Is a heat pump worth it in an old house with radiators?<\/h3>\n
Yes, if the house is at least moderately insulated and the radiators are large enough. In a poorly insulated house, insulate the walls and loft first or choose a high-temperature model.<\/p>\n
Which type of heat pump is best?<\/h3>\n
Water-to-water is the most efficient, air-to-water the cheapest to install. For most houses a correctly sized and well-placed air-to-water unit is a good compromise.<\/p>\n
- the heating water temperature<\/strong> \u2013 the lower it is, the less the compressor has to work, which makes underfloor and wall heating ideal.<\/li>\n<\/ul>\n
- Compressor:<\/strong> it compresses the vapour, sharply raising its pressure and temperature. This is where most of the electricity is used.<\/li>\n