Hybrid or full heat pump? The main types compared

“Heat pump” can mean several quite different systems: an air-to-water unit heating the whole house, a ground source system with a borehole, an air-to-air unit that works like a reversible air conditioner, or a hybrid that pairs a small heat pump with your existing boiler. Which suits you depends mainly on how well insulated the home is, what temperature your radiators need, and which grants you want to use.

The main types

Our database covers 1,350 heat pumps sold in the Netherlands, Germany or the UK, with test data mainly from the German BAFA list and the Heat Pump KEYMARK ^1,2: air-to-water (including hybrids), ground source and water-to-water. Air-to-air units are not part of these figures.

What our data shows by type

Manufacturers declare SCOP, the seasonal efficiency, at 35 °C and 55 °C flow ³. Medians per type in our database:

Ground source models have a median SCOP about 9% higher than air-to-water at 35 °C and about 8% higher at 55 °C. Water-to-water units come out highest, roughly 40% above air-to-water at 35 °C. The heat pumps in hybrid systems score slightly lower than full air-to-water units at 35 °C (4.62 against 4.74) and a little higher at 55 °C (3.60 against 3.53), and are quieter outside: a median of 50 against 55 dB(A). Ground and water source units usually sit indoors, so we don’t compare outdoor noise for them.

These are laboratory values for the heat pump itself under standard test conditions. What a real system achieves also depends on the ground loop or well, the design and the flow temperature, and the sizes differ too: the median ground source model is rated at 14 kW at 35 °C and the median water-to-water model at about 22 kW, against 10 kW for air-to-water.

Hybrid: a smaller step

A hybrid pairs a modest heat pump with a boiler. Because most heating hours fall in mild weather, the heat pump can supply a large share of the year’s heat, with the boiler stepping in on the coldest days and when high temperatures are needed.

Pros: lower upfront cost, radiators can stay, less risk from an undersized heat pump. Cons: you keep a fossil-fuel boiler, its servicing and its standing charges, and you still burn gas or oil for part of the year.

The key UK catch is funding. The Boiler Upgrade Scheme does not cover a heat pump combined with a fossil-fuel boiler ⁴, and Home Energy Scotland excludes hybrid systems as well ⁵. A full heat pump or an air-to-air system is usually the better route if you want a grant. See our heat pump grants guide for the amounts.

Full heat pump: it all comes down to flow temperature

A full air-to-water heat pump replaces the boiler entirely. Under the MCS design standard it must then meet the calculated heat load of the home at design conditions without relying on a backup electric heater, taking the flow temperature into account ⁶. Within each type, flow temperature makes a big difference. Median values for air-to-water in our database:

Even at 55 °C an air-to-water heat pump delivers more than three times as much heat as the electricity it uses, but every degree lower helps. MCS also advises designers to avoid high-temperature heat pumps unless the application really needs flow temperatures above 55 °C ⁶.

A simple winter test: turn your boiler’s flow temperature down to 55 °C or lower for a few days. If every room still gets warm enough, a full heat pump is likely feasible. If not, a few larger radiators, better insulation or draught-proofing can close the gap.

To qualify for the Boiler Upgrade Scheme, an air-to-water or ground source system must also reach a SCOP of at least 2.8 in the MCS calculation for your home ⁴; our SCOP and COP guide explains what that means.

Air-to-air: heating with a split system

Air-to-air heat pumps heat the room where the indoor unit is fitted and can also cool in summer. Since 28 April 2026 they qualify for a £2,500 Boiler Upgrade Scheme grant for homes in England and Wales ⁷. They don’t make hot water, so you still need another way to heat it, and heating a whole house evenly with room units takes careful planning.

Ground source: higher efficiency, bigger job

Ground source heat pumps use a ground loop or borehole, which stays at a fairly stable temperature through winter. That gives better efficiency on cold days than air source and no fan outside; compare models in our list of ground source heat pumps. The cost is mainly in the ground works. The Boiler Upgrade Scheme pays the same grant for ground source as for air source ⁷.

Which one fits?

• Poorly insulated, radiators need well above 55 °C: insulate or upgrade radiators first; a hybrid is an option but won’t get the BUS grant.
• Reasonably insulated, radiators cope at 55 °C or lower: a full air-to-water heat pump; compare SCOP at 55 °C.
• New build or deep retrofit with underfloor heating: air-to-water or ground source, depending on space and budget.
• Flat, no wet heating, or mainly want cooling: air-to-air.

Check noise too: in England, permitted development requires the unit to stay within 37 dB(A) measured 1 m in front of a neighbour’s habitable-room window or door. Our noise guide explains what distance that takes. Smaller units are also available: see the most efficient 4–6 kW heat pumps.