A heat only electric boiler — sometimes called a regular boiler, conventional boiler, or electric flow boiler — is designed to work with open-vented heating systems that use a feed and expansion (F&E) tank, typically located in the loft. It provides central heating only, circulating hot water around your radiators or underfloor heating circuit. Like a system boiler, it does not produce domestic hot water directly — hot water is handled separately by a cylinder or immersion heater.
This type of boiler is the right choice when you are replacing an existing regular gas or oil boiler on an older open-vented system and do not wish to convert the pipework to a sealed pressurised system.
Heat Only vs System Boiler — What's the Difference?
This is one of the most commonly confused distinctions in heating, and getting it wrong can cause real installation problems.
A heat only (regular) boiler works with an open-vented system — one that is not pressurised and relies on a small cold water feed and expansion tank in the loft to maintain the water level and accommodate expansion. The system operates at low, gravity-fed pressure and does not require a pressurisation unit or sealed expansion vessel on the boiler itself.
A system boiler works with a sealed, pressurised system — it typically includes an internal expansion vessel and circulating pump, and the system is filled to a set pressure (usually 1–1.5 bar) directly from the mains. System boilers are designed to work with unvented cylinders at mains pressure.
The simplest way to identify which type of system you have is to check the loft. If there are one or two small cold water tanks feeding your heating circuit, you have an open-vented system suited to a heat only boiler. If there are no tanks and your heating system has a pressure gauge and filling loop, it is already a sealed system suited to a system boiler.
Can You Convert from Open-Vented to Sealed?
Yes — it is possible to convert an open-vented system to a sealed pressurised one at the same time as replacing the boiler, removing the loft tanks in the process. This is sometimes done to allow the installation of a system boiler and an unvented cylinder. However, the conversion requires additional components, system flushing, and a pressure test, and adds cost and complexity to the installation. For a straightforward like-for-like boiler replacement on an existing open-vented system, fitting a heat only electric boiler is considerably simpler and more cost-effective.
No Flue, No Gas, No Annual Service
As with all electric boilers, heat only models require no flue, no gas supply, and no annual gas safety inspection. They are 100% efficient at point of use, completely silent in operation, and compatible with solar PV systems for reduced running costs. Maintenance requirements are minimal compared to gas equivalents — there are no burners, heat exchangers, or combustion components to service.
⚠️ Check Your Electrical Supply
The same electrical supply considerations that apply to electric combi and system boilers apply here. At the lower end of the power range — 4kW to 7kW — a standard domestic circuit is adequate. At 14kW or 14.4kW single phase, you are drawing approximately 63 amps and will need a dedicated 10mm² circuit with an appropriately rated MCB. Higher outputs require a three-phase supply. Always have your installer confirm your consumer unit has sufficient capacity before ordering.
Do You Also Need a Hot Water Cylinder?
A heat only boiler provides central heating only — it has no domestic hot water function whatsoever. If you are replacing a gas regular boiler that currently heats a hot water cylinder via a coil, your existing indirect cylinder will continue to do that job once paired with the new electric boiler, provided the cylinder's coil connections are compatible. If your existing cylinder is old or vented (an old copper cylinder with no pressure rating), it may be worth replacing it with a modern indirect unvented cylinder at the same time to improve hot water pressure and performance throughout the property.
Browse our full range of heat only electric boilers below, or contact our team for advice on the right output for your system size and property.
