Heat-pump types & systems: air, water & ground source, mini-splits, VRF & large water-to-water

1 · Source is everything

The opening guide ended on one rule that governs this entire series: keep the lift small. The best-possible efficiency of any heat pump is set by the temperature lift — how far it has to pump heat from the source up to the sink — and real machines reach only 40–55 % of that Carnot ceiling. Everything a designer or a maintenance engineer does to a heat pump is, in the end, an attempt to shrink that lift.

There are only two ways to shrink it: raise the source or lower the sink. The sink is usually fixed by what you are heating — a radiator wants ~45 °C water, an underfloor loop ~35 °C, domestic hot water ~55 °C. So the single most powerful decision left on the table is which source you draw from, because that number swings by tens of degrees across the seasons. Outdoor air in a Nordic January can be −15 °C; the ground a few metres down sits near +8 °C year-round. Same sink, same cycle, same refrigerant — but the ground-source machine is fighting a 37 K lift while the air-source machine fights 60 K. That difference is the reason this whole family of products exists.

2 · Air-source heat pumps (ASHP)

The air-source heat pump is by far the most common type because its source — outdoor air — is everywhere and free to reach. No boreholes, no water rights, just a fan blowing ambient air across an outdoor coil. That coil is the evaporator in heating mode: refrigerant boiling inside it at a temperature below the outdoor air, so heat flows from the air into the refrigerant. The penalty is that the source is exactly the thing that gets cold when you need heat most.

Split vs monobloc

An ASHP comes in two plumbing arrangements:

• Split — the refrigerant circuit is split between an outdoor unit (compressor, outdoor coil, expansion device) and an indoor unit (the condenser that heats your water or air). Refrigerant pipe runs between them, so installation needs an F-gas-qualified technician to braze and charge the circuit on site.
• Monobloc — the entire refrigerant circuit is sealed inside the outdoor unit at the factory, and only water pipes run indoors. Simpler and safer to install (no field refrigerant work), but the water pipes outdoors must be protected against freezing.

The defrost cycle

Here is the air-source machine's signature problem. When the outdoor coil runs below freezing — which it does whenever it is scavenging heat from near-0 °C air — moisture in the air frosts onto the coil. Frost is an insulator: it blocks airflow and chokes the heat transfer the evaporator depends on, so capacity and COP slide as the coil ices up. The machine must periodically defrost.

The usual method is reverse-cycle defrost: the reversing valve flips the unit briefly into cooling mode so hot gas runs through the outdoor coil and melts the ice off — which means, for those few minutes, the heat pump is pulling heat out of the house to thaw its own coil, and often firing a backup heater to cover the gap. Some designs use hot-gas bypass defrost, routing discharge gas straight to the coil without a full reversal. Either way, defrost is a real seasonal efficiency cost, worst in the damp, just-below-freezing conditions where frost forms fastest. Counter-intuitively, a harder freeze frosts less: frost needs both a sub-freezing coil and moisture in the air, and very cold air holds almost none — so once the outdoor temperature drops below roughly −6 °C there is little water left to deposit and frosting eases. The enemy on the coldest days is the sheer temperature lift, not ice.

Cold-climate heat pumps (EVI)

As it gets colder, two things go wrong at once: the lift grows (lowering COP) and the refrigerant vapour gets thinner, so the compressor moves less mass and capacity collapses exactly when heat demand peaks. Enhanced vapour injection (EVI) — also sold as economised or vapour-injection scroll — fixes the capacity problem by adding an economiser heat exchanger and injecting a second stream of refrigerant vapour partway through compression. This subcools the liquid feeding the evaporator (more heat absorbed per kg) and boosts discharge capacity, letting a well-designed cold-climate ASHP hold useful output down to −25 °C and below. It does not repeal the Carnot ceiling — the COP still falls with the lift — but it keeps the machine delivering enough heat on the coldest nights instead of handing the load to resistance backup.

3 · Mini-splits & multi-splits

A mini-split is a small, ductless air-source heat pump: one outdoor unit connected by a slim refrigerant line to one indoor "head" mounted on a wall or ceiling. A multi-split puts several indoor heads on a single outdoor unit, each serving its own room. Because there is no ductwork, mini-splits avoid the duct losses of a central system and are the go-to for retrofits and per-room zoning — heat or cool only the rooms in use, at the setpoint each occupant wants.

Inverter compressors — modulate, don't cycle

The feature that makes modern mini-splits so efficient is the inverter, a variable-speed compressor drive. An old fixed-speed compressor only knows two states, full-on and off, so it cycles: it blasts to overshoot the setpoint, shuts off, drifts, and restarts — and every restart is inefficient, hard on the compressor, and comes with the losses of pulling the system back up to pressure. An inverter instead modulates compressor speed to match the actual load, running slowly and continuously to hold a steady temperature.

Running at part speed lifts seasonal efficiency for a subtle but powerful reason: at low load the refrigerant flow is gentle, so the temperature approaches in both heat exchangers shrink, which lowers the effective lift and raises COP — a heat pump is often more efficient at 40 % load than at 100 %. Add the elimination of cycling losses and you see why an inverter mini-split's SCOP (seasonal COP) comfortably beats an equivalent single-speed unit. This is exactly the class of machine behind the demo AHU in our own platform: a compact, inverter-driven air-source unit doing per-zone duty.

4 · VRF / VRV

Variable refrigerant flow (VRF) — trademarked VRV by its originator — is the mini-split idea scaled up to a commercial building. One (or a few) outdoor units feed a single refrigerant loop serving dozens of indoor units, each with its own electronic expansion valve modulating refrigerant flow to match that zone's load. Inverter compressors track the aggregate demand across the whole building. The result is fine-grained zoning and strong part-load efficiency at a scale a single mini-split could never reach.

Heat-recovery VRF

The most elegant trick in the family belongs to heat-recovery VRF (three-pipe systems). In a large building, some zones need cooling while others need heating at the same moment — a sunny south face and a shaded server room, a core and a perimeter. A heat-recovery system moves heat from the zones that want to reject it to the zones that want it, using the refrigerant loop as the transport. The heat pulled out of an over-warm office is not dumped outdoors; it is delivered straight to a cold one. When cooling and heating loads are balanced, the outdoor unit barely runs at all — the building is heating and cooling itself, and the effective COP soars because you are moving heat across the smallest possible lift, room to room. This is why VRF dominates mixed-use commercial towers, hotels and offices.

5 · Water-source & ground-source (geothermal)

To escape the cold-air problem entirely, draw heat from something that stays warm all winter: the ground, or a body of water. A metre or two below the surface, soil holds a near-constant temperature close to the local annual average — roughly 6–12 °C in temperate and Nordic climates — because the earth is a vast thermal store the seasons cannot swing. Groundwater, lakes and the sea behave the same way. Feeding an 8 °C source instead of a −10 °C one shrinks the lift enormously, and there is no frost, no defrost cycle, and no winter capacity collapse. That stability is the whole point of going geothermal.

Closed-loop vs open-loop

• Closed-loop — a sealed loop of pipe carrying a water/antifreeze mix is buried and circulated to exchange heat with the ground. It comes in three geometries: vertical boreholes (typically 100–200 m deep, small land footprint, most stable temperature), horizontal loops (trenched ~1.5–2 m down over a large area, cheaper to install where land is available), and pond/lake loops (coils sunk in a water body). The ground fluid never mixes with the source, so fouling and chemistry are controlled.
• Open-loop (well or groundwater systems) — pump groundwater directly from a supply well through the heat pump's heat exchanger and return it to a discharge well or surface water. This gives an excellent, warm, high-flow source with no buried loop, but exposes the heat exchanger to real groundwater — so water chemistry, fouling and scaling become live maintenance issues, and abstraction usually needs a permit.

Whichever geometry, the payoff is the same: a source that is warm in winter. A ground-source machine on the coldest day of the year is running the small, comfortable lift an air-source machine only sees in the shoulder seasons — which is why its seasonal COP lands near 4–5 and holds there regardless of the weather. The model below lets you see that flat, weather-proof line for yourself.

6 · Large water-to-water heat pumps & central chillers

Scale the same cycle up by three or four orders of magnitude and you reach the industrial end of the family: water-to-water machines rated in the hundreds of kilowatts to tens of megawatts. Here the reciprocating and scroll compressors of domestic units give way to screw and centrifugal compressors — screws for the mid-range with excellent part-load control, centrifugals for the largest duties where a spinning impeller moves enormous refrigerant flow at high efficiency. A large centrifugal chiller is this machine run for its cold side; drive it for its hot side and it is a heat pump. The two are one product family, distinguished only by which stream you sell.

The water-to-water configuration

In a water-to-water machine, both the evaporator and the condenser are water heat exchangers — a source-water loop on the cold side, a sink-water loop on the hot side, each simply a heat-exchanger duty. Nothing exotic: the source loop might be a river, an aquifer, a sea-water intake, or the warm return from an industrial process; the sink loop might be a district-heating network or a process hot-water circuit. Because both sides are water and water carries heat far better than air, the approach temperatures are small and tightly controllable — which keeps the lift low and the COP high. This is the most efficient configuration in the whole family, and it is why the biggest, most demanding installations are almost always water-to-water.

District heating & district cooling

At city scale, large water-to-water heat pumps sit at the heart of district energy. A central plant lifts heat from a low-grade source — treated wastewater, sea water, a data-centre's cooling loop — up to the 60–90 °C a district-heating network distributes, feeding thousands of buildings from one efficient machine instead of thousands of individual boilers. Run the same plant for its cold side and it delivers district cooling, and the very best schemes do both at once, moving heat from the cooling district into the heating district exactly as a heat-recovery VRF does within one building — only across a city.

Industrial process heat & waste-heat recovery

Industry is full of low-grade waste heat: 30–40 °C streams from cooling jackets, condensate, exhaust and effluent that are normally thrown away to a cooling tower or the atmosphere. A heat pump can upgrade that waste stream — using it as a warm source and lifting it to a genuinely useful temperature. Because the source is already warm, the lift is modest and the COP is high, so waste-heat recovery is often the most cost-effective heat pump application there is: you are paid twice, once for the cooling you needed anyway and once for the heat you would otherwise have bought.

High-temperature heat pumps push the sink above 90 °C — and increasingly to 120–150 °C with newer refrigerants and compressor designs — to serve process heat, sterilisation, drying and steam-raising duties that used to demand a fossil boiler. The lift is large and the COP is correspondingly lower (often 2–3.5), but replacing burnt gas with a COP-3 electric machine is still a large primary-energy and carbon win, which is why industrial high-temp heat pumps are one of the fastest-growing corners of the market.

7 · Choosing — matching source, sink & load

Selecting a heat-pump system is an exercise in minimising the lift while meeting the load, at a capital cost the site can bear. Three levers, in priority order:

• The source — the highest-leverage choice. A warm, stable source (ground, water, waste heat) buys COP on every cold day and removes the defrost penalty. Its cost is capital: boreholes, loops and wells are expensive up front but cheap to run. An air source is the opposite — cheap to install, more expensive to run in a cold climate. Match the source to the climate and the run-hours: the colder and the more heating hours, the more a warm source pays back.
• The sink / flow temperature — lower flow temperature means higher COP, directly. Every degree you can drop the sink shrinks the lift. This is why heat pumps love underfloor heating (~35 °C) and struggle with old high-temperature radiator circuits (~70 °C) — and why retrofits often pair a heat pump with larger emitters so the same room heat is delivered at a lower water temperature.
• The load profile — size for the real demand curve, not the single coldest hour. An inverter machine sized to modulate across the year beats one oversized to a peak it meets by short-cycling. Where cooling and heating loads coincide, a heat-recovery system turns one load into the other's source.

One design move sits underneath all three and is worth stating on its own: oversize the heat exchangers. A physically larger evaporator and condenser run with smaller temperature approaches — the refrigerant boils closer to the source temperature and condenses closer to the sink temperature — which shrinks the effective lift the compressor fights and lifts COP for the life of the machine. It is the cheapest efficiency you can buy at design time, and its inverse is the most common efficiency you lose in service: a fouled condenser or a frosted evaporator is just an under-sized heat exchanger, and the lift climbs accordingly.

Key takeaways

• The source dominates. Same cycle everywhere — the choice of air, water or ground sets the winter lift, and the lift sets the COP.
• Air-source is cheapest and most flexible but pays a defrost penalty and loses COP and capacity as it gets cold; cold-climate EVI models hold output down to −25 °C.
• Inverter mini-splits modulate instead of cycling, raising seasonal COP; VRF scales that to whole buildings and can recover heat between zones simultaneously.
• Ground- and water-source trade capital cost for a warm, stable source — no frost, no capacity collapse, and the best seasonal COP.
• MW-scale water-to-water machines with screw and centrifugal compressors drive district heating, waste-heat recovery and high-temperature process heat — the most efficient configuration in the family. Rated to ISO 13256.
• To choose: warm source, low flow temperature, right-sized modulating capacity, and generous heat exchangers. Every one of those shrinks the lift.

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