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Current draw through a thermostat

You guys helped me design my in-floor heating system 10 years ago.  I sure appreciated the help then - it has been a great system!

I've got a new hobby  - playing with the Arduino microprocessors.  My first project is a data logger that logs to a SD card when the boiler goes on and which zones are calling for heat and what water temperature is going to the floor.  It turns out the zone status is easy since I have Honeywell zone valves that have an extra set of contacts that close when the valve is open - that's easy for the Arduino to sense.  But to sense that the boiler is on is a different matter.  I have a Tekmar 371 controller that tells the Weil-Mclain Gold boiler to come on.  I believe there is just a relay in the 371 that shorts the two wires going to the thermostat wires of the boiler.  I want to sense the voltage on those two wires and record when the voltage changes.  When the boiler is off, there is about 24VAC between the two wires, and when it's on there is, of course no voltage.  I can take that signal and put it through a rectifier and eventually get it down to 5VDC which the Arduino can see.  My question is:  how much current can my electronics draw through those two thermostat wires coming from the boiler without getting in trouble?  I don't expect a lot of current, but doesn't a normal thermostat draw on the order of 1 amp (because of the anticipator circuit).   So as long as I'm below that I'm OK??  Am I thinking correctly??

Thanks for the help!


  • Tim McElwain
    Tim McElwain Member Posts: 4,554
    Thermostats do not draw amperage

    of any concern as they are just a switch. The anticipator in the thermostat however has to be matched to the amperage draw of the primary control (the control the thermostat is directly wired to) put an amp meter in series with the control you are going to wire the thermostat to and it will tell you how much amperage the primary control is drawing and that is what you set the anticipator to in the thermostat. To be safe while you are checking this set the anticipator in the thermostat to 1.2. Then once you have the exact amperage draw you can reset it.

    I would check with each of the control manufacturers to see if all the controls are compatible with one another.
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