Troubleshooting a zone valve that won't close is important to ensure the proper operation of your heating unit. If a zone valve is not working correctly it can increase your heating cost, decrease the life of your heating unit, and not heat the space properly. Most commonly when you have a zone valve that won't close it could be one of three things; the powerhead, the valve assembly, or the manual valve lever.
Below are some tips on troubleshooting these three components to identify your problem. Before starting troubleshooting cut off water to the boiler to avoid injury and burns from hot water or steam. Caution must be taken when working with boilers because of the pressurized what that it takes to make the boiler work.
Checking the Power Head
If the motor to the power head has gone bad, the valve can be stuck in an open position. Using a small hammer tap the valve lightly. If the cover closes then that confirms that you have a bad power head. If the valve stays open you may want to continue to troubleshoot for blockages or damage to the valve head. Replacing a power head is fairly easy and doesn't take many steps. You will replace the head just as you remove it making sure to label all wiring as you go along to ensure proper connections.
Checking the Manual Lever
If the motor appears to be working properly on the power head you may need to re-engage the manual lever. If the lever moves freely that can indicate a problem and you can try to following to correct it. Behind the valve flange you should see a small plate with a metal spring attached to it. Reposition the metal plate until they snap and the valve closes. If the valve opens and closes easily this may correct your problem.
Replacing the Valve Assembly
If checking the power head and the manual lever doesn't correct the problems with the zone valve you may need to replace the entire valve assembly. This is the easiest way to replace any internal parts that may be damaged or worn. Replacing the assembly will require you to solder a new valve into place. Adapter kits can be purchased to avoid this, but check with the manufacturers directions to ensure proper installation. When replacing the assembly always clearly mark all the parts as you go along to make for easier installation of the new assembly.
If trying all of the above steps don't work, then you may want to consult a heating and air professional. Problems with the zone valves can indicate a larger problem within the boiler system which a professional can confirm. Also if you are not sure what you are looking for you can cause more damage and make repairs higher in cost.