A brine/water heat pump extracts heat from a closed antifreeze loop — the brine — connected to ground collectors, boreholes or solar absorbers, and delivers it to a hydronic heating system. Its standard rating point is B0/W35: brine entering at 0 °C, heating water leaving at 35 °C, where quality units achieve COP values above 4.
What does brine mean in a heat pump system?
The brine is simply a water–antifreeze mixture circulating in a sealed source loop; no salt is involved in modern systems, though the name survives from early installations that used saline solutions. Today the fluid is typically a 25–35% glycol mix — often propylene glycol for its low toxicity — giving frost protection to around -10 to -15 °C, comfortably below any temperature the source loop will see.
The loop collects low-grade heat from whatever source it is buried in or bolted to: horizontal ground collectors, vertical boreholes, surface water, waste heat — or uncovered solar absorbers on a roof. The heat pump's evaporator chills the brine by a few kelvin; the source re-warms it; the cycle repeats.
What does B0/W35 mean on a datasheet?
B0/W35 is the standard European rating point for brine-source machines under EN 14511: brine (B) entering the evaporator at 0 °C, heating water (W) leaving the condenser at 35 °C. It is the brine-source counterpart to the A7/W35 point used for air-source units, and because 0 °C brine is a harsher source than 7 °C air, the two figures are not directly comparable.
| Rating point | Source | Typical use |
|---|---|---|
| A7/W35 | Outdoor air at 7 °C | Air-to-water units, mild conditions |
| A2/W35 | Outdoor air at 2 °C | Air-to-water units, cold conditions |
| B0/W35 | Brine at 0 °C | Ground- and solar-source units |
| W10/W35 | Water at 10 °C | Groundwater-source units |
At B0/W35 a good brine/water heat pump reaches a COP above 4 — and unlike an air-source figure measured in a laboratory's mild air, that performance holds through January, because the source barely changes.
Why is a brine source more stable than air?
Below a few metres' depth the ground sits near the local annual average temperature — roughly 8–12 °C across much of Europe — and drifts only slowly through the seasons, so the heat pump sees nearly the same source in February as in November. Air-source units face the opposite correlation: the coldest air arrives exactly when the building needs the most heat.
Stable, above-freezing source temperatures also mean a brine machine needs no defrost cycle, has no outdoor fan noise, and achieves a higher seasonal performance factor for the same compressor technology — the core trade-off examined in ground source vs air source heat pump. The price is the source itself: trenching or drilling typically dominates the installation budget.
Can solar or PVT panels be the brine source?
Yes — this is the fastest-growing alternative to digging. Unglazed PVT hybrid collectors or dedicated absorbers replace the ground loop: the same glycol brine circulates through the panels, harvesting heat from sun, ambient air, wind and even condensation, and the panels' PV side generates electricity to help drive the compressor. This architecture — one branch of the solar-assisted heat pump family — suits plots where drilling is impossible or unaffordable.
Sizing is the critical discipline: the collector field must supply the evaporator's full draw on a sunless winter day, which typically demands several panels per kW of heat pump capacity. The design rules, yield expectations and system layouts are covered in Can PVT panels be the only heat source for a heat pump? — Solimpeks, which has produced PV-T hybrid panels since 2008, designs them to work alongside its heat pumps and storage tanks.
Frequently asked questions
Is a brine/water heat pump the same as a ground source heat pump?
Every closed-loop ground source heat pump is a brine/water machine, but not the reverse. Brine/water describes the technology — antifreeze loop in, water out — while the source can be ground, surface water, waste heat or solar absorbers such as PVT panels.
What antifreeze concentration does a brine loop need?
Typically a 25–35% glycol mix, protecting the loop to roughly -10 to -15 °C. The evaporator only chills the brine a few kelvin below the source temperature, so deeper frost protection than that is rarely needed in a correctly sized system.
Why is COP at B0/W35 lower than at A7/W35 for a comparable unit?
Because the source is colder: 0 °C brine versus 7 °C air widens the temperature lift the compressor must deliver. Despite the lower-looking rating figure, brine machines usually win over the season because their source stays near 0-10 °C while winter air falls far below.
Does a brine/water heat pump need a defrost cycle?
No. There is no outdoor coil gathering frost — heat arrives via a sealed liquid loop — so no heating output is diverted to defrosting. This is one reason brine machines hold higher seasonal efficiency in cold climates.