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Honeywell zone valve end switches damaged but not destroyed by surge - difficult to troubleshoot
I work a lot with Honeywell zone valves and I ran across an unusual problem yesterday. I thought I'd explain what happened for the benefit of others. The system I'm discussing has utilizes a total of four honeywell zone valves: three heating zones and a water heater, wired through a switching relay to the circulation pump. Our foreman was having a heck of a time figuring out an intermittent problem; the zone valves appeared to be working, but the switching relay wasn't always turning on. He had a volt / ohm / continuity meter but wasn't well acquainted with how to use it. I skipping the gory details, here is what I found.
The zone valves motors were working fine. They were pressing the end switch button inside the zone valve as designed. When the switch was closed (on three of the four zone valves), instead of going to the near-zero resistance expected from a closed contact I found a positive resistance of about 4 kilaohms (I dont' recall the exact reading but it was above 1k). In other words I found the contacts on the end switches had somehow been damaged but not completely destroyed.
This was in a Native village with running a microgrid. I asked around and found out the village had recently experienced a power surge. The three zone valves with partially-fried contacts had been actuated at the time of the surge; they all had the same symptom; high resistance at the end switch, but not a complete failure. The water heater zone was turned off at the time and experienced no damage.
The circuit I was troubleshooting was the equivilent to one, two, or three > 1k resistors in parallel. Overall resistance declines when you add more of the same value in parallel. When all three zone valves were actuated enough current flowed to actuate the switching relay, but when one or two closed the resistance was too high (that's why it was intermittent). I robbed a working switch from an old zone valve I had handy and got one of the heating zones working; I put the other two on manual, explained to the homeowner that only one thermostat would "work" until the other two zone valves were fixed, and returned to home base. I'll be sending out a couple of new power-heads to our foreman today.
It's interesting to me that the zone valve end switches were the only components in the system damaged by the voltage surge. The zone valve motors were ok. The transformer was ok. The switching relay was ok. The thermostats were ok. If anybody else out there has run into this issue it would be interesting to hear.