Compress 5800i AW: three efficiency figures, three winners
The 4, 5, and 7 kilowatt Compress 5800i AW rank differently on SCOP at 35, SCOP at 55, and COP at A7/W35. Which published number answers which question.
What this means
Worcester Bosch publishes SCOP at 35 degrees of 4.58, 4.65, and 4.58 for the 4, 5, and 7 kilowatt units, SCOP at 55 degrees of 3.32, 3.50, and 3.52, and COP at plus 7 air and W35 of 3.59, 3.16, and 3.07. No single unit leads all three, so the figure that matches the design decides which model reads best.
Equipment and model context
Worcester Bosch Compress 5800i AW air source heat pump
- Compress 5800i AW 4 kilowatt
- Compress 5800i AW 5 kilowatt
- Compress 5800i AW 7 kilowatt
The three-size Compress 5800i AW range as Worcester Bosch publishes it for the United Kingdom. Figures are manufacturer-declared laboratory ratings under their own test conditions and describe the outdoor unit in a rated configuration, not a building.
What this guide covers
- Two sizes are within capacity range for the house and the efficiency figures disagree.
- A quotation cites a COP that is lower than the SCOP on the same data sheet.
- A tight installation where cabinet size and sound both matter.
What changes the answer
- SCOP and COP are measured under different standards at different load points.
- The efficiency ordering across three sizes is not the same at every rated condition.
| Figure | 4 kW | 5 kW | 7 kW |
|---|---|---|---|
| SCOP at 35 degrees | 4.58 | 4.65 | 4.58 |
| SCOP at 55 degrees | 3.32 | 3.50 | 3.52 |
| COP at plus 7, W35 | 3.59 | 3.16 | 3.07 |
| Sound pressure at 1 m | 32 dB(A) | 34 dB(A) | 34 dB(A) |
| Sound power | 40 dBA | 42 dBA | 42 dBA |
| Energy class, 35 and 55 | A+++ and A++ | A+++ and A++ | A+++ and A++ |
Why the seasonal figure sits above the single-point one
On the 4 kilowatt unit the SCOP at 35 degrees is 4.58 and the COP at plus 7 air with 35 degree water is 3.59. The larger number describes a whole season and the smaller one describes a single test point, which sounds backwards until the load is taken into account.
The single-point figure is measured with the machine delivering its rated capacity. The seasonal figure is assembled from a set of load conditions across a climate profile, and an inverter-driven compressor spends most of those conditions running below rated capacity, where a heat exchanger sized for full output is being asked for less.
So the two numbers are not competing claims about the same thing. One says what the machine does when it is working hard, and the other estimates what it does across a year in which the load is below rated capacity for a large share of the hours.
Three sizes, three orders
At 35 degrees the 5 kilowatt unit leads on 4.65, with the 4 and 7 kilowatt units level at 4.58. At 55 degrees the order changes: the 7 kilowatt leads on 3.52, then the 5 on 3.50, then the 4 on 3.32. On COP at A7/W35 the order reverses completely, with the 4 kilowatt at 3.59, the 5 at 3.16, and the 7 at 3.07.
The consequence for a specification is direct. A well-insulated house designed around underfloor heating at 35 degrees is being sold on the first column. A retrofit designed at 55 degrees is being sold on the second. Reading the wrong column picks the wrong unit within one range.
The spread at 55 degrees is worth noticing on its own. Between 3.32 and 3.52 there is six percent of running cost separating the smallest unit from the largest, on a design condition where a retrofit spends much of the winter.
What the range is designed to do
All three sizes reach a 75 degree Celsius flow temperature from the heat pump alone, which is what makes the range a candidate for a house keeping its existing radiators. R290 is why, and Worcester Bosch publishes the charge as 0.95 kilograms with a stated global warming potential of 3.
The energy labelling reflects the same split as the SCOP columns: A+++ at 35 degrees and A++ at 55 across all three sizes. The label does not separate them; the SCOP figures do.
Sizes are 4, 5, and 7 kilowatts, which places the range at one to three bedroom houses rather than at large properties. A house needing more than 7 kilowatts at design condition is outside the range rather than at the top of it.
Sound and the indoor side
Sound pressure at one metre is 32 dB(A) on the 4 kilowatt unit and 34 on the 5 and 7, with sound power of 40 dBA and 42 dBA respectively. Two decibels separates the smallest unit from the other two, and all three share the same outdoor cabinet at 800 millimetres high by 1100 wide by 540 deep.
The indoor unit is 700 by 400 by 290 millimetres, which is a wall-mounted box rather than a floor-standing tower. Cylinder capacity is a separate choice, with a 180 litre slimline option alongside 210 and 250 litre standard cylinders.
That split matters for a retrofit where the old boiler cupboard is the only space available. A wall-mounted hydraulic unit and a separately sited cylinder can be placed in two rooms, which an integrated tower cannot.
| The design | The figure that describes it |
|---|---|
| Underfloor heating at 35 degrees | SCOP at 35, where the 5 kilowatt leads |
| Existing radiators at 55 degrees | SCOP at 55, where the 7 kilowatt leads |
| A house at design condition much of the winter | COP at A7/W35, where the 4 kilowatt leads |
| A boundary close to the unit | Sound pressure, where the 4 kilowatt is 2 dB lower |
| A design load above 7 kilowatts | Outside this range rather than at the top of it |
| No room for a cylinder in the plant space | The wall-mounted indoor unit with a remote cylinder |
Both numbers appear on the same data sheet in the same units and answer different questions. Reading them as a ranking of one machine against another is what produces the impression that the sheet contradicts itself.
- COPOne air and water temperature pair, at rated capacity
- SCOPA climate profile of load conditions, mostly part load
- The 35 columnA low-temperature emitter design
- The 55 columnA medium-temperature design on existing radiators
Questions people ask about this
Which Compress 5800i AW is the most efficient?
It depends on the column. The 5 kilowatt unit leads on SCOP at 35 degrees with 4.65, the 7 kilowatt leads on SCOP at 55 with 3.52, and the 4 kilowatt leads on COP at plus 7 air and W35 with 3.59.
Why is the COP lower than the SCOP on the same unit?
They are measured differently. COP is taken at one air and water temperature pair with the machine at its rated capacity, while SCOP is assembled across a climate profile in which an inverter-driven compressor runs below rated capacity for much of the time.
How loud is the Compress 5800i AW?
Worcester Bosch publishes sound pressure at one metre of 32 dB(A) on the 4 kilowatt unit and 34 dB(A) on the 5 and 7 kilowatt units, with corresponding sound power of 40 and 42 dBA.
Can the Compress 5800i AW keep my existing radiators?
It reaches a 75 degree Celsius flow temperature from the heat pump alone, which is what makes that possible. The published efficiency at 55 degrees rather than 35 is what the choice costs, and on this range that is a drop from around 4.6 to between 3.3 and 3.5.
What refrigerant does the Compress 5800i AW use?
The published charge is 0.95 kilograms of R290, and Worcester Bosch states its global warming potential as 3. The same property that delivers the high flow temperature is what brings the clearance rules a hydrocarbon refrigerant carries.
Evidence recordHow this page was checked
official manufacturer support article · checked 2026-09-07
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Worcester Bosch and Vaillant technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article
- Scope of the definition
- Worcester Bosch Compress 5800i AW air source heat pump
- Last checked
- 2026-09-07