Heat pump: heat from cold air

A monobloc heat pump puts the fan, the coil, the compressor and the whole refrigerant circuit in one sealed steel cabinet outside, and only water pipes go into the house. This one is shown at 3.5 times life size, bigger than the house beside it so you can see inside; the real thing is 1,100 mm wide, 765 mm high and 450 mm deep and weighs about 128 kg. Use the casing control to take its panels off.

At about 0.38 m³ it is well inside the 1.5 m³ that permitted development allows for a heat pump at a house in England.

A large, slow propeller fan behind a spiral guard pulls outside air in through the coil at the back and side and blows it out of the front a couple of degrees colder. Its motor speeds up as the load rises and stops while the unit defrosts.

Give the unit room: cold air that blows against a wall and curls straight back into the coil lowers the COP. Each maker sets clearances round the unit, typically a few hundred millimetres behind it and a metre or more in front.

Hundreds of thin aluminium fins on copper tubes, wrapped round the back and one side of the unit behind a guard. Inside the tubes the refrigerant is colder than the air, so heat flows into it and it boils, even when the air is below freezing.

Propane (R290) boils at −4°C at about 4.2 bar, so air at 2°C is warm enough to boil it. In damp air near freezing the fins collect frost, which the unit melts off from time to time.

A twin rotary compressor: two rollers turn off-centre inside two cylinders, each sweeping vapour into a shrinking space, half a turn apart so it runs smoothly. Squeezing the vapour from about 4 bar to 13 bar heats it to around 70°C. An inverter changes its speed to match the load, and it sits on rubber mounts inside a felt jacket to keep it quiet.

Small units use rotary compressors like this one; larger ones often use a scroll. Every watt the compressor uses ends up in the water too, along with the heat taken from the air.

The steel can on the compressor’s side. Vapour on its way to the compressor passes through it, and any drops of liquid fall to the bottom and stay there. A compressor is built to squeeze vapour, and liquid can damage it.

It matters most just after a defrost, when the coil can send liquid back for a short while.

A four-way valve beside the compressor, with a small electric coil on top that moves a slide inside it. For heating it sends the hot gas to the plate heat exchanger. To defrost it flips for a few minutes and sends the hot gas to the coil instead, so the ice melts.

You often hear it as a hiss when a defrost starts and ends. The cloud of steam that rises off the coil is the ice melting, and is normal.

A small electronic valve with a stepper motor on top. The warm liquid squeezes through a tiny opening, its pressure drops from about 13 bar to 4, and about a quarter of it flashes to vapour, which chills the rest. It comes out colder than the outside air, ready to take heat from it again.

The controller trims the opening many times a minute so the vapour leaves the coil just a few degrees warmer than it boiled.

Stacked stainless steel plates brazed together, with refrigerant between every other pair and heating water between the rest, flowing opposite ways. The hot gas condenses back to a liquid and hands its heat to the water.

The refrigerant condenses a few degrees above the flow temperature, so a lower flow means a lower pressure for the compressor to reach.

A circulating pump on the return, inside the cabinet. It pushes the heating water through the plate heat exchanger, out to the house and back, about 9 litres a minute in this house.

The water side needs a minimum flow; a unit that cannot get it stops to protect itself. That is one reason heat pump systems avoid valves that can shut every radiator at once.

The circuit board that runs the unit. It turns the mains supply into the varying frequency that sets the compressor’s speed, and reads the temperature and pressure sensors round the circuit. Its finned heatsink sits in the stream of cold air so the fan cools it.

Because the inverter can slow the compressor down, the unit turns itself down on mild days instead of switching on and off.

From the expansion valve, through the coil and back to the compressor, the refrigerant is cold and at low pressure: a mix of liquid and vapour into the coil, cold vapour out of it. This is where heat comes in from the air. The pipe back to the compressor is lagged.

From the compressor to the expansion valve the refrigerant is hot and under pressure: hot gas into the plate heat exchanger, then a warm liquid out of it. This is where the heat goes into the water.

An engineer’s pair of gauges on the service ports: blue for the low side, red for the high side. A refrigerant’s boiling point follows its pressure, so each pressure tells you the temperature at which it boils or condenses. These read absolute pressure; an engineer’s gauge reads about 1 bar less.

Water does the same: it boils at 100°C at sea level and at about 70°C at the top of Everest.

The glass column shows the four temperatures that matter. The refrigerant boils a few degrees below the outside air and condenses a few degrees above the heating water. The gap between the two is the lift, and the bigger the lift, the more work the compressor does for each unit of heat.

The ideal (Carnot) COP is the condensing temperature in kelvin divided by the lift. A real unit gets about half of it.

The weather station reads the air the heat pump works on: its temperature and how damp it is. Air at 2°C is 275 degrees above absolute zero and still holds plenty of heat. It only feels cold because we live at 21°C.

The heat pump needs a great deal of air because it takes only a couple of degrees out of each cubic metre.

Meltwater from each defrost runs into the base tray and out through the drain into a bed of gravel. Let it soak away; dripping onto a path or patio, it freezes.

In this model a fully iced coil holds 2.5 kg of ice, so each defrost sends about 2.5 litres down the drain. On a raw, damp day that adds up to tens of litres, so plan the soakaway when you design the garden.

The unit stands on rubber anti-vibration feet on a firm, level base: a concrete pad, paving on a solid bed, or a ground frame. The feet stop the compressor’s hum travelling into the ground and the house, and lift the unit clear of snow and standing water.

Avoid hanging a unit on the wall of a bedroom: brackets carry vibration into the structure.

The only energy you pay for. An armoured cable from its own circuit in the consumer unit runs to an isolator, placed where the maker’s instructions allow (for a propane unit, outside its safety zone), and in through a cable gland. The electricity drives the compressor, the fan, the water pump and the controls. It does not heat the water directly: it does the work of lifting heat from the cold side to the warm side.

A heat pump needs its own circuit and usually a type B or similar RCD, as the maker’s instructions set out. Tell your electrician early. The network operator must be told about a heat pump; on a self-build, put it on your new connection application.

In England a heat pump at a house is usually permitted development if the MCS 020(a) calculation shows 37 dB(A) or less 1 m outside every neighbouring habitable room window or door. It starts from the unit’s sound power from the maker’s data (its energy label, product fiche or the MCS product directory, never a low-noise mode), adds for walls within 1 m (the ground counts as one), takes off for distance and for a solid wall or fence that hides the unit. With a 55 dB(A) unit against one wall, the point 1 m outside the window needs to be about 4.5 m from the middle of the unit, so a window facing it about 5.5 m.

Since 29 May 2025 a unit can be up to 1.5 m³ and within 1 m of the boundary, and a detached house can have two. For a propane unit the maker’s safety zone usually cannot cross the boundary, so it may still need to sit back from it. A fence or wall only counts as a barrier if it is solid with no gaps and runs at least 1 m past each side of the unit; slatted fences and hedges count for nothing. For a new self-build, show the unit on your planning drawings: permitted development rights apply once the house exists, and a condition on the permission can remove them.

The flow and return pass through the wall in a sleeve, lagged all the way and sealed on the inside with airtight tape or a grommet so the hole does not leak warm, damp air into the wall. Outside, the lagging needs a weatherproof, UV-proof cover.

Plan the hole at the design stage: a short, straight run from the unit through the wall saves heat and pumping. The water in a monobloc runs outside, so ask how it is protected from freezing in a power cut: antifreeze, a separating heat exchanger or anti-freeze valves.

A small sensor on a shaded north wall that tells the controller how cold it is outside, so it can pick the flow temperature before the rooms start to cool.

Keep it out of the sun and away from the unit’s cold air, or it reads the wrong weather.

Insulated pipes carry warm water from the unit to the rooms and the cylinder, and back. The flow leaves warm; the return comes back a few degrees cooler, having given its heat away.

Heat pumps move more water than boilers at a smaller temperature drop, so their pipes are often a size bigger: 28 mm where a boiler might use 22 mm.

A motorised valve that sends the warm water either to the heating or to the coil in the hot water cylinder. The heat pump does one job at a time.

While the cylinder reheats, the rooms coast on the heat already in the radiators, floor and walls.

The heat pump’s own controls and the room thermostat. With weather compensation on, it sets the flow temperature from the outdoor temperature: cooler water on mild days, warmer on cold ones.

Approved Document L asks for new heating systems designed to run at 55°C flow or lower.

A radiator’s output falls quickly as the water cools: at a mean of 40°C in a 21°C room it gives under 30% of its rated output. Bigger or double-panel radiators give the same heat from cooler water.

Output = rated output × (ΔT ÷ 50)^1.3, where ΔT is the mean water temperature minus the room temperature.

Pipes in the screed turn the whole floor into a large, gentle radiator. Because the area is so big, water at 30–35°C is enough to heat a well-insulated room.

A floor slab warms and cools slowly, so set it and leave it rather than switching it on and off.

A well-insulated tank with a large coil inside. A heat pump heats water more gently than a boiler, so the coil is bigger and the cylinder stores enough for the day. Hot water rises, so the tank fills with heat from the top down.

Heating water to 50°C needs a flow of 55°C or so, which is why hot water costs more per kWh than heating. Many systems also lift the cylinder to 60°C once a week to control legionella, which costs a little more.

Insulation sets how much heat the house needs. This one loses 130 W for every degree between inside and out, so on a −3°C day at 21°C indoors it loses 3.1 kW. Free heat from people, cooking and appliances covers some of that, but the heating is sized for all of it.

Halve the heat loss and the heat pump can run cooler water through smaller emitters.