What a 55 degree flow temperature costs, in published SCOP
Three United Kingdom heat pump ranges publish SCOP at both 35 and 55 degrees. The gap between them is a quarter of the seasonal efficiency, and it is the emitter decision priced.
What this means
The same heat pump is rated twice, at a 35 degree Celsius flow temperature and at 55. Across the Grant Aerona 290 range the SCOP falls from between 4.56 and 5.08 to between 3.33 and 3.74. Across the Worcester Bosch Compress 5800i AW range it falls from between 4.58 and 4.65 to between 3.32 and 3.52. That is between a fifth and three tenths of the seasonal efficiency, and radiator size is what decides which figure a house gets.
Equipment and model context
Cross-brand explanation; product figures come from the model documentation
- Vaillant aroTHERM plus VWL 35/6 to VWL 125/6
- Worcester Bosch Compress 5800i AW 4, 5 and 7 kilowatt
- Grant Aerona 290 4, 6.5, 9, 12 and 15.5 kilowatt
United Kingdom and European air-to-water heat pumps rated under EN 14825. The figures quoted are manufacturer-published seasonal ratings for a climate profile, not a prediction for one building, and the standard has more than one edition so figures from different sheets are not always on the same basis.
What this guide covers
- A survey recommends replacing radiators and the reason given is efficiency.
- Two quotations propose the same heat pump at different design flow temperatures.
- Running costs are higher than the SCOP on the quotation implied.
What changes the answer
- Seasonal efficiency is rated at two flow temperatures and only one of them describes a given house.
- Emitter surface area is what sets the flow temperature a house can be heated at.
The two ratings on every data sheet
EN 14825 asks for a seasonal coefficient of performance at a low-temperature application and at a medium-temperature one, which for these ranges appears as a pair of numbers at 35 and 55 degrees Celsius. Manufacturers print both. Marketing quotes the first.
Grant publishes the pair for all five Aerona 290 sizes. The 6.5 kilowatt unit is 5.08 at 35 degrees and 3.62 at 55. The 15.5 kilowatt is 4.56 and 3.33. Worcester Bosch publishes the same pairing on the Compress 5800i AW: 4.65 and 3.50 on the 5 kilowatt, 4.58 and 3.52 on the 7.
Held against a gas boiler the difference is stark in both cases. Held against each other, one of those numbers is worth roughly a third more heat per unit of electricity than the other, from identical hardware.
Why the flow temperature is a radiator question
A radiator gives out heat in proportion to how much hotter it is than the room. Drop the water going into it from 55 degrees to 35 and the temperature difference driving that transfer falls by more than half, so the same radiator delivers a fraction of what it did on a boiler.
The output has to come back from somewhere, and the two routes are area and flow. A larger radiator, or a second one, restores the surface. Underfloor heating restores it with an entire floor. Fan coils restore it by moving air across a smaller surface instead.
This is why a heat pump survey measures rooms and lists emitters rather than simply sizing an outdoor unit. The outdoor unit choice sets the capacity; the emitter schedule sets which of the two published SCOP figures the house will operate near.
Flow temperature also moves the sound figure
Vaillant publishes sound power for the aroTHERM plus at three conditions rather than one, and the figures rise with the water temperature. The 3.5 and 5 kilowatt units are 51 dB(A) at A7/W35, 53 at A7/W45, and 54 at A7/W55. The 7 kilowatt runs 53, 53, and 55.
So the quiet figure quoted in a brochure belongs to the low-temperature condition. A system designed to run at 55 degrees is operating at the top of that scale for most of the heating season rather than the bottom of it.
For a terraced or semi-detached property where the outdoor unit sits near a boundary, that is a planning and neighbour question as much as a comfort one, and it is decided by the same emitter schedule that decides the efficiency.
What the published figure does not promise
SCOP is a seasonal figure computed for a standard climate profile under a test standard. Grant states that its Aerona 290 outputs are rated at minus 5 degrees air and 55 degrees water, which is a different condition again, and NIBE publishes SCOP for average and cold climates separately.
The standard also has editions. NIBE's S2125 sheet cites EN 14825:2018 and its F2120 page cites EN 14825:2013, so two NIBE figures printed in the same font are not on the same basis. Comparing across manufacturers carries the same risk in a quieter form.
Treat the pair of numbers as what they are: a like-for-like statement of how much a given machine loses when it is asked for hotter water. That comparison holds inside one data sheet, which is the comparison an emitter decision needs.
| Model | SCOP at 35 | SCOP at 55 |
|---|---|---|
| Grant Aerona 290, 4 kW | 5.00 | 3.66 |
| Grant Aerona 290, 6.5 kW | 5.08 | 3.62 |
| Grant Aerona 290, 9 kW | 4.74 | 3.69 |
| Grant Aerona 290, 12 kW | 4.74 | 3.74 |
| Grant Aerona 290, 15.5 kW | 4.56 | 3.33 |
| Compress 5800i AW, 4 kW | 4.58 | 3.32 |
| Compress 5800i AW, 5 kW | 4.65 | 3.50 |
| Compress 5800i AW, 7 kW | 4.58 | 3.52 |
| What it touches | Moving from 55 to 35 degrees |
|---|---|
| Seasonal efficiency | Rises by roughly a third on the ranges above |
| Radiator sizes | Larger emitters or underfloor to recover output |
| Sound power | Falls by up to 3 dB(A) on Vaillant's published figures |
| Installation cost | Rises with the emitter work |
| Running cost | Falls with the efficiency |
| Hot water | Unchanged. The cylinder charge runs hotter regardless |
Each link constrains the next, and the direction of travel runs from the building outward. Choosing the outdoor unit first inverts it, which is how a system ends up rated at one figure and running at the other.
- Heat loss per roomCalculated for the fabric, at the design outdoor temperature
- Emitter surfaceRadiator size, underfloor area, or fan coil
- Flow temperatureThe lowest that still delivers the room output
- SCOP the house lands nearWhich of the two published figures applies
Questions people ask about this
How much efficiency does a higher flow temperature cost?
On the ranges quoted here, between a fifth and three tenths of the seasonal figure. Grant's 6.5 kilowatt Aerona 290 is 5.08 at 35 degrees and 3.62 at 55. Worcester Bosch's 4 kilowatt Compress 5800i AW is 4.58 and 3.32.
Do I have to change my radiators for a heat pump?
It depends on whether the existing ones can deliver each room's heat loss at the flow temperature being designed for. A radiator fed with 35 degree water has less than half the temperature difference driving it that it had at 55, so the required output has to be recovered through area or through a different emitter type.
Can a heat pump run at 55 degrees or higher?
All three ranges here publish a flow temperature of 75 degrees Celsius, so the capability exists. What changes is the seasonal efficiency, and on Vaillant's published sound figures the noise as well, since sound power rises from A7/W35 to A7/W55 on every aroTHERM plus size.
Why do two heat pumps with the same SCOP cost different amounts to run?
SCOP is calculated for a standard climate profile at a stated flow temperature and under a stated edition of EN 14825. A house with a different heat loss curve, a different design flow temperature, or a different hot water pattern will not reproduce the test conditions the figure came from.
Is a bigger heat pump more efficient?
Not on Grant's published table. The 6.5 kilowatt Aerona 290 has the highest figure at 35 degrees, at 5.08, and the 15.5 kilowatt the lowest, at 4.56. At 55 degrees the order changes again, with the 12 kilowatt leading at 3.74.
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: Grant UK, Worcester Bosch, Vaillant and NIBE 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
- Confirm against the exact model manual
- Last checked
- 2026-09-07