The?Firebird UK?Envirogreen Slimline Combipac is the smallest external Combi on the market. The Combi (combination) boiler is an instant domestic water heater and a central heating boiler combined. Water is heated directly via a thermal store when a tap is turned on, saving not only on cost, but also on the space required for a hot water storage tank. Providing instant access to hot water, this type of boiler is particularly suited to smaller households with one bathroom and shower/ensuite. The Envirogreen Slimline Combipac is similar in specification to the internal Envirogreen Slimline Combi, with the addition of a built-in frost thermostat.