Taco Zone Valve - Challenge

PVF1799

I have a Burnham Gas Boiler with Field Control Unit to turn on the stack blower. This runs for 1 minute before the furnace lights. It is controlled by the zone valve. Now - the blower has not shut-off in weeks. The last time this happened it was a zone valve - but I can't recall how the guy tested each one of the five (zone valves). I think he twisted two together - perhaps 2 and 3? This allowed the furnace blower motor to shut-off, when he found the right one.

I have voltage on 1 and 2 to all ZV and continuity between 2 and 3 on all ZV. Wondering if the relay on the side of the blower control has failed.

Your help is much appreciated.

EBEBRATT-Ed

24v power from your transformer one side should go to 2 on the zone valve. The other 24v wire goes to the thermostat then powers terminal 1 on the zone valve. So when the stat is calling you should have 24 volts on 1 & 2. #2 makes internally to terminal #3 that's your end switch. so your load, whatever the end switch starts would be connected to zone valve terminals #1 &#3. If you have multiple zone valves mark and trace the wires, disconnect them and troubleshoot them individually

PVF1799

Ed - Thanks for your comment. If I understand correctly - Disconnecting 1 and 3 from 1 ZV at a time should isolate the ZV that is NOT allowing the fan to shut-off. Alternatively, as #3 is the load as you mentioned. Disconnecting #3 from one ZV at a time should isolate the ZV with the issue.

I should note this system has been stable since 1995, except for an occasional ZV change - the valve not opening kind of failure.

Thanks again for your help.

EBEBRATT-Ed

I would disconnect #3 from the valves to isolate the problem to see if the fan shuts down. After that 24 volts back on 1 & 2 should open the valve. 24 v from 1-3 shows zone valve end sw is made (with an open valve). Then remove power from 1 valve should close. leave wire off of 1 and check for power between #3 and #1wire (not the terminal) you should loose power when the end sw opens

PVF1799

Ed - The answer is I started pulling #3 off of the ZV's. Did 5 with no result - Wouldn't you know it was #6. Popped off #3 wire and the fan shut-off. Left it off for now because it's the garage heat and don't need it now:).

Thanks Ed - the key was knowing that #3 is "Load" - Knowing that saved the day. Thanks a bunch.

EBEBRATT-Ed

Glad it worked out!!