- Air-to-air
- A heat pump that heats room air through indoor units; it makes no hot water. Explained
- Air-to-water
- A heat pump that takes heat from outside air and heats water for radiators and a cylinder. Explained
- Backup heater
- An electric element that adds heat when the heat pump alone cannot, at one unit of heat per unit of electricity. Explained
- Bivalent point
- In a hybrid, the outdoor temperature below which the boiler takes over or helps. Explained
- Compressor
- The electrically driven part that squeezes refrigerant vapour, raising its temperature. Explained
- Condenser
- Where hot refrigerant gives its heat to the heating water and turns back to liquid. Explained
- COP
- Coefficient of performance: heat out divided by electricity in, at one set of test conditions. Explained
- Cylinder
- An insulated tank storing hot water for taps and showers. Explained
- Defrost cycle
- A brief reversal of the cycle to melt ice on an air source unit's outdoor coil. Explained
- Evaporator
- Where cold refrigerant takes in heat from the air, ground or water and boils. Explained
- Expansion valve
- Drops the refrigerant's pressure so it turns cold again for the next loop. Explained
- Flow temperature
- The temperature of the water leaving the heat pump for the radiators or floor. Explained
- Ground loop
- Buried pipe, in trenches or boreholes, through which a ground source heat pump collects heat. Explained
- Heating curve
- The setting that links outdoor temperature to flow temperature under weather compensation. Explained
- Hybrid
- A heat pump and a boiler sharing the heating of one home. Explained
- Refrigerant
- The fluid that boils at low temperature and carries heat round the cycle. Explained
- SCOP
- Seasonal coefficient of performance: a standard-season average from tests, shown on labels. Explained
- Sound power
- The total sound a unit emits; sound pressure is what reaches a particular place. Explained
- SPF
- Seasonal performance factor: what a real system achieved over a real season. Explained
- Standing charge
- A fixed daily charge for each fuel supply, paid whatever you use. Explained
- Weather compensation
- A control that sets the flow temperature from the outdoor temperature. Explained
Words that sound alike
"Heat pump" and "air conditioner" describe machines built on the same refrigerant cycle, run for different purposes. "Air source" and "air-to-air" both take heat from outside air, but the first usually means an air-to-water system that heats radiators. And "efficiency" can mean a laboratory COP, a calculated SCOP or a measured SPF; when someone quotes a figure, it is worth asking which.