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How to Replace a zone valve

A common problem with hydronic heating systems is a zone valve that is not opening or an end switch that is not working properly. This article will show you how to diagnose and replace a zone valve.

There are two functions of a zone valve. The first is to open when there is a call for heat and let hot water circulate through that zone. The second is for the end switch to send a signal to the boiler and circulating pump to start.

If the motor is not working, it's pretty straightforward; the valve is not opening to let the water circulate. This will not allow that zone to heat up.

The other problem is an end switch not working and in turn will not signal the boiler to fire. This is harder to diagnose because the zone will heat sporadically. I've talked to homeowners who have had frustrating issues for years with a zone not heating properly. The reason for this is, the valve is opening but the boiler and circulating pump are not coming on. This zone will heat but only if another zone calls for heat and fires the boiler.

  1. Step 1
    Step 1
  2. Step 2
    Step 2
  3. Step 3a
    Step 3a
  4. Step 3b
    Step 3b
  5. Step 4
    Step 4
  6. Step 5
    Step 5
  7. Step 6a
    Step 6a
  8. Step 6b
    Step 6b
  9. Step 6c
    Step 6c
  10. Step 6d
    Step 6d
  11. Step 7
    Step 7
  12. Step 8
    Step 8
  13. Step 9
    Step 9
  14. Step 10
    Step 10
  15. Step 11
    Step 11


  1. First, turn off the main power to the boiler and heating controls. Most hydronic heating systems will have a switch close to the system. If a main switch is not available, the power will need to be turned off at the breaker box. Systems will vary, some will have one switch to control both the boiler and heating controls, others will have more than one.
  2. Next, the main water supply will need to be shut off. Look for the boiler regulator/fill valve. This will be where the heating system ties into your domestic water supply. If there is a valve located in this area turn the water off there. If no valve is available, the supply might need to be turned off to the entire house. This can be a problem in older homes when the valves will no longer operate.
  3. (Caution: The boiler water is very hot! Be sure that the boiler has cooled off before draining.)

    Drain the system before changing the valve. If possible, close any valves above the zone valve being replaced. The example shown is a house with radiant floor heating and the valves were closed at the manifold. If your house has baseboard heating, look for any valves that will isolate the system and prevent draining down the entire system. This is not necessary or even possible on every system; doing this will help from having to purge the system when you restart it.

    After the system is isolated, check the Temperature/Pressure gauge to see the temperature is at a safe level. The Temperature/Pressure gauge is usually located on the main-supply piping coming off the boiler. Once you have determined the temperature is at a safe level, you can attach a hose to the main boiler drain and drain the boiler and associated piping. Leave the hose attached and the drain open. After cutting into the line more water will drain out. This will help when you go to solder in the new valve.
  4. Before cutting out the old valve, remove the wires from the zone valve powerhead. I will cover the rewiring your Caleffi zone valve in Step 8.
  5. Cut out the old valve using a tubing cutter or hacksaw. Make sure to leave enough pipe to solder the new valve or coupling to.
  6. On this installation, I soldered the one side of the valve to the pipe and used a copper repair coupling on the other. Depending on the installation it might be better to use two repair couplings. Measure and cut copper pipe to fit. I like to use repair or slip couplings to ease the repair. The repair coupling gives you the ability to slide the coupling on the pipe and position the new piece of copper, and then slide the coupling into place. I soldered the valve in but there is no reason you couldn't use compression or Tectite fittings. Before doing any soldering, make sure the pipe and fittings are cleaned with sand cloth and fitting brushes. Apply flux to both the pipe and fittings. Open the valve by hand before soldering to ensure there will not be any damage to the flapper. Also, take note of the flow direction on the valve body marked with an arrow. (Hint- On most systems the zone valves are located on the return piping.)

    Before soldering, disable any smoke or CO detectors. It's also a good idea to have a fire extinguisher on hand. Go ahead and solder all connections. Don't forget to rearm the smoke and CO detectors.
  7. After everything is soldered, turn the water supply on and check for leaks. You should hear the water rushing into the piping. Check the Temperature/Pressure gauge to make sure that the boiler regulator/fill valve is operating. The pressure should read between 12-15 psi. Most newer boiler systems require a check valve between the heating system and your domestic water supply. It's not uncommon for this check valve to stick and not let the system fill up. If this is the case, remove the check valve and break the plunger loose with a screwdriver. If there are no leaks and there is pressure in the system, open up the main supply and return valves.
  8. Reinstall the wires on the powerhead. First, I will cover "4 wire" zone valves, this will be pretty straightforward. You will connect one wire from the transformer and one from the wall thermostat to the motor terminals on the new Caleffi valve. On most 4-wire zone valves the thermostat terminals are labeled TH and TR. The next two wires are the switch wires. These are the end-switch wires and go to the TT terminals on the boiler or to a pump relay or other low voltage control. On most 4-wire zone valves these terminals are labeled ES and ES.

    Next, I'll cover Taco "3-wire" valves. The Taco terminals are labeled 1, 2, and 3. These zone valves share the #2 terminal between the motor and end switch. Just like the "4-wire" valve you will take the transformer wire off of terminal #2 and attach it to one of the motor terminals. The wire from the #1 terminal that comes from the wall thermostat attaches to the other motor terminal. There will be a second wire coming off of the #2 terminal and will be routed to one of the TT terminals on the boiler. This wire will go to one of the switch terminals on your new valve. Lastly is the #3 terminal, which will be attached to the other TT terminal on your boiler. This wire will be attached to the remaining switch terminal.
  9. At this point install the powerhead. Using your fingers, close the valve. Push the button in on the powerhead while pushing it onto the valve until it clicks.
  10. Open all valves and switch the main power back on. Turn the corresponding thermostat up and all other thermostats down. This will make sure the zone valve is opening and the boiler is firing. Next, you need to make sure the zone is circulating and not air-locked. I recommend using an infrared thermometer to check the supply and return temperatures. If the zone is air-locked it will not heat up and you will need to bleed that zone until it starts to heat up.
  11. Replacement complete

The bleeding of the system will be covered in another article.

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