Long time lurker, 1st time posting...
I have an older style TACO zone valve (black case) controlling the hot water heater (ESP 100c2-1 dated 01-03) - gas furnace <10y/o. Worked fine until about a month ago when it wouldn't shut off. Since then, its either been running continuously or I have to manually turn it off and on (rotating button on powerhead). I checked the voltage from the transformer - ~24v AC, checked the thermostat on the waterheater for both the hot and cold conditions - ok. I know everything operates fine because when its on, there is hot water and when its off for an extended period of time, there isn't. The problem its that it doesn't operate in auto mode.

Spoke with the plumbing supply counter guy who informed me the older style TACO zone valve heads had a very high failure rate and were redesigned. The new powerheads are green, so I bought a replacement head (ZA024Q4A1) - not the valve. Checked the wiring (swapped wires for the w/y and c connector as they were opposite that of the older style), installed and turned on power. The new valve will not power up (green LED is off). Tried to manually turn the valve with the button, it rotates 90 degrees but the furnace does not come on. Reinstalled the old TACO head, works fine in manual mode. I noticed that the old and new heads both have the same size valve sprocket on the back so the valve itself is turning.

My questions: Is the new designed TACO head not compatible with the old valve even though it fits? Is the new head faulty to begin with? Is there a better replacement for the old style TACO powerhead?

I have pics of the heads if needed. Any help would be appreciated. thanks - Paul

EDIT: Here is a pic of the old valve:
http://smg.photobucket.com/albums/v626/ppap/?action=view&current=valve.jpg

On the new valve, the green light never goes on.

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The green led won't come on unless the thermostat is calling for heat.

Were the connection terminals the same on both valves?

There shouldn't have been any reason to switch wiring.

A pic of old valve might help.

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There could be something in the circuit that is marginal to let it work. You can get 24 volts; but a bad wire or contact could not pass enough amperage to operate the head.

Try applying the transformer 24 volts directly to the c and w/y terminals, to bypass and disconnect the thermostat, so the c and w/y terminals get power directly, to check if there is a problem in the wiring, thermostat, circuits. When you get power to the terminals, it should turn within a minute if the power head is good.

If there's a short in the circuit, the power head will remain in the open position, looking like it's not operating; but the LED would be on.

Quote:

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1. Upon initial field installation the capacitor
requires a full charge, up to 20 seconds, before the
valve starts to turn. Charging time will vary (typically
less) during normal operation.
2. When the capacitor is charging the green LED
light will FLASH.
3. Once the capacitor is charged, the green LED will
stop flashing but remain ON. At this point the
valve’s operator will rotate the ball valve. The
green LED will remain ON as long as the thermostat
is calling (the unit is powered).
4. Once the thermostat is satisfied the green LED will
turn OFF and the valve will rotate 90° into its normal
position or non-powered position. For example:
If the actuator is an NC (normally closed) version,
the operator would open the valve when the
thermostat calls. Once the thermostat is satisfied
the operator would then rotate the valve 90° to its
normally closed position.
.
WARNING: Do not use zone valves on indirect
water heaters without a tempering device.
http://s3.pexsupply.com/manuals/1303748424350/54362_PROD_FILE.pdf

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Based on your last post, I decided to test the new valve using the source power supply. Sure enough, the new valve operates - LED flashes, goes solid, turns the valve. This means I have a voltage drop issue somewhere at the end of the circuit. I will probably rewire and check the thermostat for proper current. The thermostat is operating correctly, in theory, as its in closed circuit mode when cold and open circuit when hot. Possibly under load it has resistance...

The old TACO ESP valve s definitely junk as its operation is sporadic. Tested directly at the power source.

My question is can I do a direct replacement of the old TACO ESP power head without replacing the valve itself --> http://smg.photobucket.com/albums/v626/ppap/?action=view&current=valve.jpg

new TACO Zone Sentry --> http://smg.photobucket.com/albums/v626/ppap/?action=view&current=valve2.png

Also, the old TACO ESP had a switch for NO/NC (normally open/closed). The new TACO Zone Sentry is fixed at NC. What does this refer to? Is it the state of the thermostat when cold?

thanks Paul

As the instruction sheet in the above URL says that the head slips on and off by squeezing the retaining clips, either the head slips over the D valve shaft and clips in place, or won't, which answers the question for you.

Be aware there is a note about 'power robbing' thermostats in the instructions that draw current to charge so they operate. Sometimes they need to operate a relay to close or open contacts, called an 'isolation relay,' so the transformer power doesn't only go through the thermostat circuit and the thermostat can draw power to charge no matter what.

NC normally closed would open their contacts when the valve operates. Most have NC contacts. They would be normally closed even when the valve head is brand new in the box.

NO normally open contacts would close when the valve operates.

Most thermostat circuits control relays by closing contacts for burners and circulators; but there are others.

Problem solved. I found a wire that appeared to be broken with some continuity. The ohm meter passed it but I noticed varying current when I moved it. The valve head fits fine and operates correctly (in auto mode). I monitored it for a complete cycle. It still does not operate in manual mode but at this point, it doesn't matter. Thank you for the information.

Thanks for the update. Others might need to know, so telling them that the ESP head interchanges new to old valve could be useful and a relief.