Bosch Compress heat pump review UK: 5800i vs 7400i

Quick answer: The Compress 5800i AW is the Bosch heat pump to buy: R290, 75°C flow, SCOP up to 4.65 and a 41.5 dB(A) sound power figure, the lowest we’ve seen published. A 7kW outdoor unit was £3,693.80 at a UK merchant in July 2026. The older 7400i AW only makes sense at a discount.

We haven’t tested these units. This review uses Bosch’s published specifications, the official ErP fiche for the 7400i AW, and UK merchant prices, all checked July 2026.

One heat pump, two badges

In the UK these units sell under two names. Builders’ merchants list “Bosch” Compress units; the heating trade and the consumer site sell the same machines as Worcester Bosch. Same German parent, same hardware, different sticker. If your quote says Worcester Bosch Compress 5800i AW and the merchant invoice says Bosch, nothing is wrong.

Compress 5800i AW: the current platform

The 5800i AW is the one generating the noise coverage, ironically by not generating noise. Worcester Bosch publishes a 41.5 dB(A) sound power figure and calls it its quietest outdoor unit ever, with Quiet Mark certification (Worcester Bosch, checked July 2026). For comparison, Mitsubishi’s Ecodan R290 publishes 56 to 58 dB(A). Decibels are logarithmic, so that gap is large, and it can decide whether a unit passes the MCS noise assessment beside a boundary fence.

The rest of the sheet: R290 refrigerant, flow temperatures up to 75°C, SCOP up to 4.65, ErP A+++ for heating at 35°C. UK merchants list 5kW and 7kW outdoor units, with wall-hung or pre-plumbed cylinder indoor units to pair (City Plumbing; Mr Central Heating, July 2026). The 7kW outdoor unit weighs 182kg (MKM listing), so plan the base properly.

Compress 7400i AW: the outgoing platform

The 7400i AW (model code CS7400iAW) is the previous generation, and the official ErP fiche tells its story plainly: R410A refrigerant, seasonal space heating efficiency of 196% at low temperature and 133% in the average application, a heating water limit of 60°C, and 50 dB(A) outdoor sound power (Bosch ErP fiche 6721842465). The fiche was printed in September 2021. This is a mature design, and we’ve covered it separately in our full CS7400iAW review.

Notice the odd result: at 35°C flow the older unit’s ErP numbers convert to a SCOP around 5.0, above the 5800i’s 4.65. R410A performs well at low flow temperatures, so that’s plausible. But the 5800i wins everywhere else: 15°C more flow headroom for radiators and hot water, a refrigerant with a future, and a night-and-day noise figure. Our guide to SCOP and running costs explains why behaviour at 55°C matters more for retrofits than the 35°C headline.

So what does SCOP 4.65 mean in cash? At the current 26.32p/kWh electricity cap, heat could come out around 5.7p per kWh if the installation actually achieves that figure. A 90% efficient gas boiler at the 7.97p gas cap works out at roughly 8.9p/kWh of heat once flue losses are counted. Real installations usually land below lab SCOPs, so treat that as the optimistic end of the range.

Prices and grants

Hardware is one line in the quote. A full installation, with cylinder, controls and system work, comes out around £11,014 on our modelled mid-range estimate; see how we calculate costs and test your own home in the calculator. The Boiler Upgrade Scheme takes £7,500 off MCS-certified installations in England and Wales. The £9,000 figure you may have seen applies only to homes currently heated by oil or LPG, from 21 July 2026 to 31 March 2027. VAT is 0% until 31 March 2027.

Warranty and support

Merchant listings show a 2-year base warranty on the hardware (MKM, July 2026). Worcester Bosch extends guarantees through its accredited installer network, and the length depends on who fits it. So pin the number down in writing. The brand’s service machine is the real asset here: a 4.6 Trustpilot score from over 50,000 reviews (checked July 2026), built overwhelmingly on boilers, but the engineer network behind it answers heat pump calls too. Our installer guide covers what accreditation to look for.

Which Compress to buy

The 5800i AW, and it isn’t close. Buy the 7400i AW only if you’re offered a genuine discount, your system is designed for 55°C flow or less, and you accept an R410A service future. For how the 5800i stacks against the rest of the market, including Vaillant’s Arotherm Plus and Mitsubishi’s Ecodan, our brand comparison has the full table.

FAQs

How quiet is 41.5 dB(A) in practice?

Worcester Bosch compares it to a kitchen fridge, which is marketing but not wildly off. What you hear also depends on distance and reflective surfaces, so the MCS noise calculation is the test that counts.

Is R410A in the 7400i AW a problem?

Not day to day: the circuit is sealed and no refrigerant work happens at installation. The risk is long-term. R410A is being phased down under F-Gas rules, so service refrigerant will get pricier over the unit’s life.

Do both models get the £7,500 grant?

Yes. Any MCS-certified air source heat pump installation in England or Wales qualifies for the £7,500 Boiler Upgrade Scheme grant. The £9,000 rate is only for oil and LPG-heated homes, 21 July 2026 to 31 March 2027.