Combination boilers or "Combis" are a particularly attractive form of heating because they dispense with the need for the hot water cylinder in the airing cupboard, and also the large cold water cistern ("tank") plus small header cistern in the loft which are otherwise needed in conventional vented heating systems. All the major components are housed in one unit which makes them very convenient to the installer. They are especially suitable for smaller households with inconsiderable heating and hot water demands. In the past combination boilers have been less suited to larger households, however Ideal's latest models can cater for large households hot water needs. For example the Ideal isar boiler range has a 35kW which output model will cater quite easily for a homeowners heating and hot water needs in a 3 or 4 bedroom house. For anything larger Ideal recommend their istor boiler (see below). Unlike heat-only or system boilers, the casing normally houses the pump and DHW heat exchanger and motorized valve - the system is all in 1 box. Because it is a sealed system, Central Heating (CH) water expansion has to be accommodated in a pressure vessel which is also within the casing. Instead of an open safety-vent pipe looping over the expansion cistern in the loft there will be a pressure-relief valve. A gauge is usually provided to indicate that the CH system is under pressure. It will need occasional re-pressurising from the mains water supply so a facility for a filling loop (flexible braided pipe) is provided. To comply with the water regulations the loop must be physically removed upon completion.