Zone relay buzzing and stuck on so heat will not stop

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RedRancher

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Hi HVAC family,

I have a 4 zoned system, each zone with its own circulator pump, since they are isolated there are no zone valves. My furnace is a propane fired cold start boiler. 3 of the 4 zones are for forced hotwater/baseboard heat. The remaining zone is for the indirect hotwater tank. The 4 zones are connected to an old (1999) Taco SR504 Switching Relay which also has 4 leds on the side of the panel indicating what zone(s) are active. Recently I noticed that zone 2 tends to stay on past call for heat, upon inspection the relay for zone 2 hums and buzzes and the related led flutcuates between lowly lite and off. So the relay is not fully disengaging. At first I thought it was the thermostat so I just removed it all together and it did not change. Finally to get the zone to shut off I pulled the hot wire from the circulator pump. Even after pulling the thermostat and hot wire from the circulator pump the relay is still freaking out. My thoughts are 1) the relay is on its way out 2) my 3 thermostats (battery operated with no c-wire) are pulling too much power such that the relay doesn't have enough power to fully close 3) or something is shorted? Something to note is that the SR504 Switching Relay has its own transformer but not sure if that transformer has anything to do with powering the relay. If interested the SR504 instruction are here they were hard to find (archived) https://www.tacocomfort.com/documents/Archives/switchingrelays_iom.zip. Finally my 3 thermostats are ST55, Sensi Smart Thermostats found here https://sensi.copeland.com/en-us/shop/sensi/sensi-sku-st55
 

Fitter30

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At the Taco sr504 did you disconnect the W wire at zone 2 thermostat terminal? Noise coming from taco control? Or put a meter on R and W thermostat connection. 0 vac pull W wire at thermostat see if the noise stops if it doesn't go to the taco control disconnect it.If R to W reads 24vac?
 
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RedRancher

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At the Taco sr504 did you disconnect the W wire at zone 2 thermostat terminal? Noise coming from taco control? Or put a meter on R and W thermostat connection. 0 vac pull W wire at thermostat see if the noise stops if it doesn't go to the taco control disconnect it.If R to W reads 24vac?
I did not but will try that now.
 
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