Zones in a residentuial boiler system refers to the ability to heat only certain parts of the home at a time without incuring the expense of heating the whole home. For example, if no one is going to be in the west side of the home all day, you could just simply turn down the temperature in the zone for the west side of the home. This would have no effect on the temperature of other parts of the home. Zoning gives you option to adjust the heat higher or lower in seperate areas of the home.

There is a greater up front cost when going with a zoned boiler system. There are multiple of ways to pipe a zoned system. You can have each zone on its own pump. When that zone calles for heat, it would just pump water to that area.  The initial up front cost of this type of system includes the cost of the the additional piping to each zone, as well as the cost of all the controlls that are needed for each pump.

There is a system that has only one pump, and each zone would be controlled be a zone valve. The valve would open, allowing water flow to the required zone, only when a call for heat was needed in that zone. Most zone valves work by a call for heat from the thermostat, then the valve will start to open. Once the zone valve has fully opened, it would start the pump. If only one zone valve turned on the pump, water would only flow down that zone. If there was a second zone valve open when the call for heat came, the water would just start to flow down that zone also, due to the pump already being on.