No measurable voltage at TT connections when thermostats are calling for heat

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Bunny Laroche

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Hello, we have an outdoor reset module installed with its wired connected to our Westinghouse 80k BTUh boiler's O/S connections on its terminal block. A measurable voltage can be observed on these contact points. We assume this voltage equates to a temperature at which the boiler would operate if it was running with gas turned on. However, we also have a TT connection on the Taco Zone Controller connected to the boiler's terminal block at the appropriate TT connections. When my girlfriend puts the two zone's thermostat's into their test mode we observed that the Zone Controller opens the appropriate Zone Valves as well as the two circulators. However, there does not appear to be any measurable voltage observed on the TT connections. Is this because the boiler's outdoor sensor is present and connected? But my girlfriend said she still expected to measure a voltage on the Zone Controller's TT connections and is baffled why no voltage is present on these connection points at the Zone Controller when the Thermostats are calling for heat and the Controller does indeed open the Zone Vales and start the circulators.
 

NY_Rob

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TT contacts on the boiler are just looking for closure from the zone valve end switch not voltage to activate.

You have the Westinghouse WBRUNG080W mod-con boiler?
 
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Bunny Laroche

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Hi again NY_Rob, I am lavishing all of my appreciation on you sofar for your prompt and helpful replies.

TT contacts on the boiler are just looking for closure from the zone valve end switch not voltage to activate.
Ok that makes seems to make sense to my girlfriend. Apparently, she was thinking there should be a voltage to measure on the TT since it is voltage from the Outdoor Sensor that tells the boiler to start. So she said she will check for continuity on the TT connection with the thermostats calling for heat when she comes over tonight.
 

NY_Rob

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Yes, you should see continuity across the TT contacts when the zone valves fully open and their end switches close.

The ODR sensor is just an analog input for the boiler microprocessor used to determine the water temperature setpoint for space heating , it doesn't affect boiler startup, etc..
 

Jadnashua

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When the switch closes, the two contacts of the TT terminals are just connected together, making it functionally like two locations along a single wire...you are not measuring across a load...it is like putting the multimeter probes on the same point. You might measure a voltage between them when the switch is open, it depends on the rest of the circuit, but should not when they are closed. If you do, that means the switch is not great, and has some internal resistance where it should be essentially zero (a straight wire).

An outdoor reset is typically a thermistor...a device that changes resistance with temperature changes. The system applies voltage on one end, and through the circuit, as the temperature changes, it will increase or decrease the resistance through it, and that voltage change is interpreted as the temperature, which the system uses to decide various things of how to operate efficiently.
 
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