Seiya S4

The Seiya S4 is Toshiba's entry-level wall split and the cheapest way into the brand's UK range, and it is a better unit than its price suggests. It comes in seven sizes from an unusually small 1.5 kW up to 6.5 kW, holds an A++ label in both cooling and heating on most of them, and drops to the same 19 dB(A) quiet mode as Toshiba's flagship Haori on the small bedroom sizes. It has been on sale since 2019, so it is a proven design with a long track record behind it, not a budget experiment.

What you give up is the trimmings. Wi-Fi needs an optional adapter rather than coming built in, filtration is a basic dust filter with a self-cleaning coil rather than a proper air-purification pack, the styling is a plain white box, and the two largest sizes run louder and slip to A+ in heating. For cooling a single room on a sensible budget those trades barely matter, and the Seiya is the value pick of the Toshiba range. If you will heat with it heavily, or want Wi-Fi and better filtration out of the box, the step up to the Shorai Edge is worth the extra.

The Seiya is a wall-mounted R32 split system: an indoor unit on the wall paired with an outdoor condenser, heating as well as cooling because it is a reverse-cycle air-to-air heat pump. R32 is the lower-impact refrigerant now standard on quality splits, and by law any work on the sealed circuit needs an F-Gas certified engineer, so this is a fitted product priced by quote, not a box you order and plug in. The current UK generation is the S4 series, sold since mid-2025; the earlier E2 series is officially archived but still sold as run-out stock, so check the model codes on any quote. The S4 has the same core performance, but you should know which generation you are buying.

It sits at the bottom of Toshiba's residential ladder, below the mainstream Shorai Edge and the designer Haori, and it is built to a clear brief: dependable cooling and heating at the lowest price Toshiba will put its name to. The essentials are stronger than the entry billing suggests. Seasonal efficiency, SEER for cooling and SCOP for heating, reaches 7.0 and 4.60 with an A++ label both ways on the 1.5 to 4.2 kW sizes, and quiet mode drops to 19 dB(A) indoors on the small sizes, the same headline figure as the Haori at twice the price. The savings show elsewhere: an infrared remote with Wi-Fi as an optional adapter, a basic dust filter with a self-cleaning coil instead of an air-purification pack, plain styling and simpler 2D airflow.

Seven sizes run from 1.5 to 6.5 kW, and the 1.5 kW is a genuine point of difference: most ranges start at 2.0 or 2.5 kW, which is oversized for a box room or small bedroom. It suits budget-conscious buyers cooling one room who want a big-brand split with a proper installer network behind it rather than an unknown badge at a similar price. The two largest sizes are the weakest picks: minimum indoor noise rises to 26 and 29 dB(A) and heating slips to A+, so for a large room it is worth comparing the step-up ranges before deciding.

Most Seiya buyers are weighing it against Toshiba's own step-up models, so here is the honest steer. The Shorai Edge is the mainstream mid-range: A+++ both ways with SEER 8.6 and SCOP 5.1 on the 3.5 kW, built-in Wi-Fi, better filtration and a choice of white or black. The Haori is the designer flagship: similar top-tier efficiency with changeable fabric covers and the full air-quality kit, from around £1,995 fitted. The Seiya matches them on the figure that matters most in a bedroom, the 19 dB(A) quiet mode, and undercuts both by several hundred pounds.

The efficiency gap matters less than the labels imply if you mainly cool. Cooling a single UK room costs tens of pounds a year, so a higher SEER saves pennies. Heating is where the step up pays: at SCOP 5.1 against the Seiya's 4.60, a Shorai Edge delivers the same warmth on roughly 10% less electricity, which adds up if the unit becomes your main heater. So: buy the Seiya for cooling-led, single-room use on a budget; pay for the Shorai Edge if you will heat with it a lot or want Wi-Fi and better filtration built in; pay for the Haori only if the look of the unit is the point.

Unit-only, a Seiya S4 set runs from roughly £600 for the smallest sizes to about £1,500 for the 6.5 kW, but it must be professionally fitted, so the number that matters is supplied and fitted. Through independent installers budget roughly £1,300 to £2,000 for a single-room install depending on size and how hard the pipe run is. Currys sells fixed-price packages installed by British Gas trusted partners at £2,492 for the 2.6 kW and £2,539 for the 3.3 kW, including up to 15 m of pipework, waste disposal and commissioning, with lead times around 45 days; you pay a premium over a good local quote for the national-brand convenience. Whichever route you take, get two or three quotes, because installers price the same job differently.

Running costs are low. At the July 2026 Ofgem price cap unit rate of 26.11p per kWh, the standardised annual figures for the 3.3 kW size work out at about £43 a year in cooling and about £215 a year if you also use it for regular heating, with real costs depending on your room, hours and tariff. As a reverse-cycle unit it can in principle qualify for the £2,500 Boiler Upgrade Scheme grant for air-to-air heat pumps in England and Wales, but only if it replaces a fossil-fuel heating system and is fitted by an MCS-certified installer; comfort cooling added alongside a gas boiler does not qualify, and it is separate from the £7,500 air-to-water grant. Prices and rates correct as of August 2026; the energy price cap changes quarterly, so check current rates before budgeting.