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How do you check the output from a three wire taco valve

bink Member Posts: 97
How do you check the electrical function of the number 3 terminal to the t-t in the control. Can this be checked with it wired up to see if it has closed. Can this be checked with a voltmeter when the valve is complete on turning the boiler and circulator on. I tried this but was confused by the readings in the ac milli volt range. The tech I talked to at Taco said to check with ohm meter for continuity when closed. Assume you would disconnect the wire to check the resistance.
Just want to see how this is checked.


  • lchmb
    lchmb Member Posts: 2,997
    disconnect the 3rd wire...set your meter to check resistance and after the valve is fully open check for continuity between the middle lug and the 3rd lug...
  • bink
    bink Member Posts: 97
    Thanks lchmb. That helps.
    I wondering if anyone tried measuring current amp draw though that wire. Might be difficult but I am trying To think of a test to determine if my zone valves are causing a problem I have with random starts of the burner circulator a few seconds then shutoff. I call it a ghost run. This happens 10 or more times a night in between normal runs for the zones. Literally driving me crazy.
    Anyway thanks for the response.
  • lchmb
    lchmb Member Posts: 2,997
    do you have power stealing t-stats on your system? One other thing I would try, disconnect each end switch (3rd wire) one at a time til you find the trouble zone. Then replace the zone head..see if it's the issue..
  • unclejohn
    unclejohn Member Posts: 1,830
    The # 1 and #2 terminals of the valve are power for the heater. # 3 terminal and # 2 terminal will close when valve strokes open. To check if 2 & 3 are closed use your volt meter and check for 24v between 1 and 3. When valve is powered you will always have 24v between 1 and 2.
  • bink
    bink Member Posts: 97
    Thanks for the suggestions. I have 2 round old Mercury Honeywell thermostats and one new digital Honeywell.
    I appreciate the help. If zone valve is open does the voltage between 1 & 3 go 0?
  • Paul48
    Paul48 Member Posts: 4,469
    If those thermostats have anticipators, that's probably your problem. As the room temp hovers around the t-stat setting, the boiler will do that with the wrong anticipator setting.
  • Joe Mattiello
    Joe Mattiello Member Posts: 663
    The boiler short cycling could be for few reasons; a couple was mentioned in the tread. Power robbing thermostats is one reason. If thermostat requires 24 volts to illuminate, retain memory, in some cases close the switch to power the valve, and energize the boiler. Placing a 1k, .5 Watt resistor across terminals 1, and 2 should mitigate the thermostat acting erratically. This would happen because of the valve inherently cycling on, and off every 30 seconds not to overheat the valve.
    Another potential problem is the anticipator setting which should be set to min .9. Some people turn it up to max 1.2. if the anticipator setting is not equal to the valve amp load, it will cause the boiler to short cycle.
    The boiler could be oversized too. assuming the boiler is sized for total load, and only 1 or 2 zones are calling for heat, the boiler might not have enough time to dissipate the heat, and shut off too quickly. sometimes using an outdoor reset control will mitigate. If you continue to have problems, try giving Taco tech support another call.
    For your reference, I attached a technical document which explains the operation of the valve. Thanks
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
  • bink
    bink Member Posts: 97
    Thanks again for the comments. We checked the two Honeywell round thermostats they are both at .9 anticipator setting. The other thermostat is a new Honeywell digital model 3000. It has batteries for the light so I am not sure if this the problem. I will keep observing the zone panel to see if I can see if one of the zones is the culprit.
    Joe, I can only see the 1st page of your tech document. Is there another way for me to see this.
    The guys on this forum have been very helpful and I really do appreciate your help.