Larger buildings often have a centralised gas hot water system, sized to meet the needs of all apartments.
The heated water is normally pumped continually around a ‘recirculating loop’ so that hot water is never too far away when it is needed. The loop (always full of hot water) extends throughout the building, with individual feeds radiating to each apartment.
For most centralised gas hot water systems:
- the gas needed to provide hot water is centrally metered at the hot water system
- each apartment is fitted with a water meter at the apartment’s hot water entry point which registers the volume of (hot) water flowing into the apartment
- a water meter on the centralised hot water system also records the volume of cold water entering at the inlet
- the gas distributor calculates the amount of gas used to heat the water, and charges each customer in the complex based on how much hot water they have used
- the water usage is multiplied by a ‘common factor’, which varies from quarter to quarter, depending on how much gas has been used to heat the total volume of water for the building
- the common factor applied to the billing quarter will be listed on the bill, usually below or beside the meter readings
Some older buildings don't meter individual apartment consumption and apportion gas costs according to unit entitlements.