✔ Range-Rated Flex — One Boiler, Any Property
The Primus Flex's defining feature: both heating and hot water power outputs are adjustable at installation from 2kW up to the model maximum. The installer sets the boiler output to match the heat loss calculation, rather than selecting a fixed-output model that may be over- or under-sized. For the DHW circuit, EHS recommend not reducing below 12kW if the property has a shower, to ensure adequate temperature rise at the required flow rate. This range-rated approach means a single boiler suits a wide variety of flat and small home types.
✔ No Flue — No Gas — No Annual Certificate
There are no combustion gases, no flue penetration, and no requirement for a Gas Safe annual safety certificate. The boiler can be installed in any interior room — kitchen, hallway, cupboard — without the constraint of a flue route to an external wall. For leasehold flats where gas boiler flue positions are restricted, and for properties in listed buildings or conservation areas, this removes a frequently insurmountable installation barrier. There is no risk of carbon monoxide emission.
✔ Continuous Hot Water — 6.5–7 L/min
At full 14kW output, the boiler raises incoming cold water temperature by 30°C at 6.5 litres per minute — sufficient for a standard shower (39°C is a typical shower temperature). The included Caleffi 5 L/min flow restrictor limits the mains flow to ensure the heat exchanger delivers adequate temperature regardless of incoming mains pressure variation. EHS specify this as a shower-capable boiler for single-bathroom properties; for baths or multi-bathroom homes, a system boiler with a cylinder is recommended.
✔ COOLFIT Overheat Protection
COOLFIT is EHS's active overheat protection system. Rather than relying solely on a passive thermal cut-out, COOLFIT monitors operating temperatures and manages the boiler's power delivery to prevent overheating events before they trigger a safety shutdown. Combined with the standard high-temperature sensor and thermal reset switch within the Flex boiler, this provides layered protection that prolongs component life and reduces nuisance fault conditions.
✔ Touch Screen — Built-in Timer — Whisper Quiet
The colour touch screen display provides intuitive access to operating mode selection (heating / hot water / both), temperature settings, timer programming, frost protection, and pump purge functions without requiring any external controller for basic operation. A built-in 24-hour timer allows heating periods to be set directly on the boiler. Near-silent operation — with minimal moving parts (pump and a 3 bar safety valve) — makes it suitable for installation adjacent to living areas and bedrooms in flats.
✔ Solar PV & Economy Tariff Compatible
The Primus Flex can be integrated with solar PV systems to absorb surplus generated electricity for heating — reducing grid import during generation periods. The built-in timer makes it straightforward to schedule heating to run during cheap-rate periods (Economy 7, Octopus Go, Agile), and the range-rated output allows the boiler to run at a lower output level during self-consumption windows. Compatible with underfloor heating systems at the lower flow temperatures the UFH requires.
✔ Installer Kit — All Components, Extended Warranty
The bundled installer kit provides every major plumbing component required for a compliant installation: Sentinel magnetic filter, cleaner and inhibitor (system treatment per BS 7593); EPH Combi Pack 4 (Boiler Plus compliant wireless thermostat with OpenTherm); WRAS filling loop; automatic bypass valve (mandatory per EHS specification); isolation valves; and Caleffi 5 L/min flow restrictor. Purchasing the boiler with the installer kit extends the standard 5-year warranty by a further 2 years — to 7 years total — when the boiler is registered on the EHS website.
✔ Direct Replacement for Gas Combi — No Pipework Changes
The Primus Flex connects to existing ¾" heating flow/return and ½" cold/hot water pipework — the same connection sizes as most domestic gas combi boilers. Existing radiators and pipework do not need to be replaced or modified. The only substantive work is the electrical supply upgrade (63A dedicated circuit) and the removal of the old gas boiler and flue. This makes the Primus Flex one of the most straightforward electric conversions available for flats currently served by a small gas combi.
Is 14kW enough to heat my property?
This depends entirely on the heat loss of the property — the size, insulation standard, and number of radiators. EHS recommend completing a heat loss calculation (to BS EN 12831) before ordering. As a rough guide, a well-insulated one or two-bedroom flat up to approximately 70m² is typically suitable for a 14kW electric combi. Because the Flex is range-rated, the installer can set the heating output lower than 14kW if the heat loss calculation indicates this — the boiler will then modulate between 2kW and the set maximum, which can improve efficiency on mild days. EHS offer assistance with heat loss calculations — contact info@ehs-heating.com.
Why is a 5 or 6 L/min flow restrictor needed for the shower?
The heat exchanger in a 14kW electric combi can only add a fixed amount of heat per litre of water that passes through it. At 14kW and 6.5 litres per minute, the temperature rise is approximately 30°C — so if cold water enters at 10°C, hot water leaves at 40°C, which is comfortable for a shower. If mains pressure is high and the flow rate increases to 8 or 9 L/min without restriction, the same 14kW heats each litre by less — water might only reach 30°C, which is not comfortable for bathing. The Caleffi 5 L/min flow restrictor in the installer kit caps the flow at a rate the boiler can always satisfy regardless of mains pressure, ensuring consistently hot water every time the shower is run.
Why is the automatic bypass valve mandatory?
When thermostatic radiator valves (TRVs) close down as rooms reach temperature, the heating circuit becomes increasingly restricted. Without a bypass, the circulation pump forces water against a closed head — pressure rises sharply and flow drops. The EHS Flex boiler has a paddle flow switch that detects inadequate flow and triggers an F6 error code or shuts the boiler down to protect the heat exchanger. The automatic bypass valve prevents this by opening progressively as pressure rises, maintaining minimum flow through the boiler circuit regardless of how many TRVs are closed. This is why EHS specify it as mandatory, not optional. The bypass valve is included in the installer kit.
