HVAC Glossary

SEER Seasonal Energy Efficiency Ratio

SEER measures how efficiently a unit cools over an entire season, not just at peak load. It accounts for varying outdoor temperatures and partial-load operation throughout a typical European cooling season. A higher SEER means lower electricity bills for the same amount of cooling.

Technically, SEER is the ratio of total cooling output (kWh) to total electrical input (kWh) over a standardized season. EU energy labels map SEER values to letter classes.

SCOP Seasonal Coefficient of Performance

SCOP is the heating equivalent of SEER. It measures how efficiently a heat pump heats over a full heating season, weighted across different outdoor temperatures and load conditions. A SCOP of 4.0 means the unit delivers 4 kWh of heat for every 1 kWh of electricity consumed, on average over the season.

The default SCOP value refers to the "Average" European climate. Separate values exist for Colder and Warmer climate zones.

EER Energy Efficiency Ratio

EER is the cooling efficiency measured at a single set of test conditions: 35°C outdoor, 27°C indoor, 100% load. Think of it as a snapshot of efficiency at peak summer conditions. While SEER shows the full movie of a cooling season, EER shows one frame at full load.

COP Coefficient of Performance

COP is the heating efficiency measured at standard test conditions: +7°C outdoor temperature at full load. Like EER for cooling, COP is a single-point measurement rather than a seasonal average. A COP of 4.5 means 4.5 kW of heat output for every 1 kW of electricity consumed at those specific conditions.

ηs Seasonal Space Efficiency

Eta-s (ηs) expresses SEER or SCOP as a percentage, as defined by EU Ecodesign regulations. It converts the ratio into a percentage that accounts for electricity generation losses (using a conversion coefficient of 2.5). This is the value used to determine EU energy label classes.

Pdesign Design Capacity

The rated output capacity in kW at design conditions — the maximum output you can expect when you need it most. For cooling (Pdesignc), this is measured at 35°C outdoor temperature. For heating (Pdesignh), it's measured at −10°C outdoor for the Average climate zone.

Use this to size a unit for your space. A larger room or poorly insulated building needs a higher Pdesign value.

Partial Load Conditions

Heat pumps rarely run at full blast. They're tested at standard load points that simulate real-world conditions across a season. Cooling and heating use different test conditions:

Cooling (A–D)

• A — 100% load at 35°C outdoor (peak cooling)
• B — 74% load at 30°C outdoor
• C — 47% load at 25°C outdoor
• D — 21% load at 20°C outdoor (mild day)

Heating (A–F)

• A — 88% load at −7°C outdoor (cold winter day)
• B — 54% load at +2°C outdoor
• C — 35% load at +7°C outdoor
• D — 15% load at +12°C outdoor (mild day)
• E — at TOL (lowest operating temperature)
• F — at Tbiv (bivalent temperature)

Units with variable capacity control (inverter) tend to perform much better at partial loads, where most operating hours actually occur.

Capacity Control Fixed vs Variable

Fixed (single-speed) units run the compressor at full power and cycle on/off to maintain temperature. This wastes energy during start-up cycles and causes temperature swings.

Variable (inverter-driven) units modulate the compressor speed to match the actual demand. They run continuously at lower power rather than cycling, delivering better comfort, lower noise at partial load, and significantly higher seasonal efficiency.

Minimum Modulation LRcontmin

Variable-speed (inverter) units can reduce their output below rated capacity. The minimum modulation ratio shows the lowest continuous output as a fraction of full capacity. For example, a value of 0.30 on a 5 kW unit means it can run as low as 1.5 kW without cycling on and off.

Lower minimum modulation is better — it means the unit can match small heating or cooling demands without overshooting and cycling, which improves comfort and efficiency. Fixed-speed units cannot modulate at all and always run at 100%.

Qce / Qhe Annual Electricity Consumption

Estimated annual electricity consumption in kWh/year — Qce for cooling, Qhe for heating. These are calculated from the design capacity and seasonal efficiency using standardized usage hours for the Average European climate.

Multiply by your electricity price to estimate annual running costs. Your actual consumption will vary based on your climate, building insulation, and usage patterns.

TOL Temperature Operating Limit

The lowest outdoor temperature at which the manufacturer declares the heat pump can operate. In practice, this value is often conservative — many units continue to run below their declared TOL, just at reduced capacity and efficiency. Some units even have certified colder-climate performance data (G point) at −15°C despite a higher declared TOL. A lower (more negative) TOL is better, especially in cold climates.

Many modern units operate down to −15°C or lower. If you live in a region with harsh winters, check that the TOL is well below your typical coldest nights. When comparing units, also look at the colder-climate partial load data for a more complete picture of low-temperature performance.

Tbiv Bivalent Temperature

The outdoor temperature below which the heat pump can no longer cover its rated heating output on its own. Below this point, the unit still runs but supplemental heating may be needed to make up the shortfall. The heat pump doesn't stop at Tbiv — it just can't deliver its full rated capacity anymore.

A lower Tbiv means the heat pump can maintain its rated output at colder temperatures before needing supplemental heating.

Climate Zones Average / Colder / Warmer

EU regulations define three reference climates for calculating SCOP. Each uses different outdoor temperature distributions and design temperatures:

• Average — Typical central European climate (reference city: Strasbourg). Design temperature: −10°C. This is the default SCOP shown on energy labels.
• Colder — Northern/Scandinavian climate (reference city: Helsinki). Design temperature: −22°C.
• Warmer — Mediterranean climate (reference city: Athens). Design temperature: +2°C.

GWP Global Warming Potential

GWP measures how much greenhouse effect a refrigerant causes if released into the atmosphere, relative to CO₂ (which has a GWP of 1). A refrigerant with GWP 675 would cause 675 times more warming than the same weight of CO₂ over 100 years.

The EU F-gas Regulation is progressively restricting high-GWP refrigerants. Units with lower GWP refrigerants are more future-proof and environmentally friendly.

Common Refrigerant Types

Refrigerants are the working fluid inside a heat pump that absorbs and releases heat. Different refrigerants have varying efficiency, environmental impact, and regulatory status:

Sound Power vs Sound Pressure dB(A)

Spec sheets show sound power levels (Lw), which measure the total acoustic energy a unit emits. What you actually hear is sound pressure (Lp), which depends on distance and surroundings. Sound pressure at 1 metre distance is typically 10–15 dB lower than the sound power value.

The decibel scale is logarithmic: every 10 dB increase sounds roughly twice as loud. For reference: 40 dB(A) is a quiet library, 50 dB(A) is light rain, 60 dB(A) is a normal conversation.

Cd Degradation Coefficient

When a compressor cycles on and off (rather than running continuously), there's an efficiency penalty during each start-up. The degradation coefficient Cd quantifies this loss — it's applied proportionally to how far below full capacity the unit operates. A Cd of 0.25 means up to 25% efficiency penalty at the worst case (near-zero load), but much less at typical partial loads. Lower values indicate less penalty.

This mainly matters for fixed-speed units that cycle frequently. Inverter units modulate continuously and are less affected.

Standby Power Psb / Poff / Pto / Pck

Heat pumps draw small amounts of power even when not actively heating or cooling. These low-power states add up over a year (8,760 hours):

• Psb (Standby) — unit is ready to respond to a heating/cooling demand
• Poff (Off mode) — unit is plugged in but fully off
• Pto (Thermostat off) — temperature is reached, compressor paused
• Pck (Crankcase heater) — keeps compressor oil warm to prevent damage on start-up, mainly in cold weather

Mounting Types

How the unit is physically installed:

• Split — separate indoor and outdoor units connected by refrigerant lines. Most common for residential use. Requires professional installation with pipe routing between units.
• Multi-split — one outdoor unit connected to multiple indoor units. Good for heating/cooling several rooms independently.
• Monoblock — everything in one outdoor unit, connected to indoors via water pipes only. Simpler installation, no refrigerant handling needed indoors.