On a hot water heating system. The heating system is split up into separate zones, heating different parts of your home, basically for every thermostat you have you will have a zone valve.
(if your system uses zone valves some systems have a separate circulator pump for each zone)  So if you have three thermostats in you home you should have three zone valves on you boiler.

If you have a garage unit heater that runs off your boiler then you will have four (in some areas the unit heater zone is not controlled with a zone valve and the water is allowed to flow freely) and if you have an indirect water heater for your domestic hot water you will have 5. 

Most zone valves run on 24 volts. There is a 24 volt transformer some place near or on your boiler  that powers the zone valves.

Danger Changing a zone valve can be dangerous, you can be scalded, or electrocuted (they are not all only 24 volts) It is best to have a professional do it for you.

There are four major zone valve manufactures. Taco, Honeywell, White Rogers, And Erie. There are others out there, but I can't remember the last time I saw one that was not manufactured by these companies.
This is what happens, you need heat so you turn on the thermostat this sends power to the zone valve, the zone valve opens, inside the zone valve is an end switch when the valve is opened this switch tells the aquastat relay  that you are calling for heat it send power to the pump and the pump sends the hot water through the pipes to your base board or radiators. and you have heat.    

The aquastat also regulates the temperature of the boiler water.

This is just a simple bmp diagram of how they are wired

The Honeywell zone valve These valves have been around for a long time, and have changed over the years. But you can still (in most cases) replace the Head and the ball with out having to replace the body.

On the newer ones there are only 2 screws 1 top left and the other bottom right. you don't have to turn the water off or drain anything to do this.

	Click on picture to see larger picture this is a newer one see the brass screw top left. To replace this head. just loosen the two screws and pull the old head off put the new head on tighten the screws then connect each wire just like the old one was. (make sure you turn the power off first) and you are done. Buy a new Honeywell zone valve by clicking here
	This is an older one notice that it has two screws on the same side. (it has 2 others on the bottom) this is the older style. To replace the head on this you will have to turn the water off and drain this pipe. But you will only have to do this once because the new zone valve parts mount to this body. So you don't have to look for special parts for an older Honeywell zone valve, just buy a new 3/4" sweat zone valve. And I will show you how to make it work.
	This is what the old head looks like when it is removed notice how it is all one piece the ball and flange are connected to the head.
	This is the back of a new head
	This is the new body with out the head on it, notice it has 4 nuts on it, 2 of them are tapped and 2 have high points on them these have to go back the same way they came off. Buy a new Honeywell zone valve by clicking here