Combi Boiler
/ ˈkɒmbi ˈbɔɪlə /
Also known as: combination boiler, combi, combination heating unit
Definition
A combi boiler (combination boiler) is a wall-mounted gas or oil-fired boiler that provides both central heating and instantaneous domestic hot water in a single unit. It heats water directly from the mains supply on demand as a tap is opened, with no need for a separate hot water cylinder, cold water storage tank, or header tank. Combi boilers are the most common boiler type installed in UK homes.
In practice
When a hot tap is opened, the boiler detects flow and diverts from heating mode to hot water mode, firing the burner to heat the mains cold water as it passes through an internal heat exchanger. Hot water is available almost instantly (after a brief purge of cold water from the pipe), and continues for as long as the tap remains open. The maximum hot water flow rate is limited by the boiler's output - a 28kW boiler delivers approximately 12-13 litres per minute at a 35°C temperature rise.
All modern combi boilers are condensing boilers - they recover heat from the flue gases by cooling them to below dew point, extracting latent heat and producing a condensate discharge that must be piped to an internal or external drain. Efficiency ratings (ErP) are typically 89-93% seasonal efficiency. Combis have an integral expansion vessel, pressure gauge, and safety relief valve - the filling loop allows the sealed system to be topped up when pressure drops.
Building Regulations
Approved Document J covers combustion appliances including gas boilers. All gas work must be carried out by a Gas Safe Registered engineer. Approved Document L has required condensing boilers in new installations since 2005. The flue terminal must be positioned to meet the clearance requirements in AD J Diagram 11, and the condensate drain must discharge safely to avoid freezing in exposed locations.
Full Building Regulations guidanceSee also