I want to ensure I'm safe.

Is it normal to find fluctuating 12 volts when the thermostat (2-wire heat-only) is at "rest" and not calling for heat? The thermostat appears to function normally with a zone valve opening and an end switch making when a call goes out for heat.

The reason for the question is a "hot" transformer smell, similar to the old Lionel train sets as an example, that I don't recall noticing before. The thermostat was disconnected while replacing the zone valve and doing some plumbing work.

When calling for heat I'm getting 24-volt readings which is what I expect. When not calling for heat I get a 24-volt reading at the transformer and would expect 0-volt readings further "downstream" since the thermostat is open but instead I'm getting 12-volt readings.

  1. Is that normal?

  2. Is that a possible reason the transformer smells "hotter" than I remember?

  3. Is the transformer just going bad?

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  • 2
    "12 volt current" will have a lot of people burrying their heads under a pillow, whilst dribbling profusely. Do you have a diagram of the setup that we can see? 24V DC? As a blind guess, it should be 0V if the thermostat is closed, 24V indicates open. 12V suggests a wiring problem. But without a diagram we're guessing. Possibly better in DIY se as there doesn't seem to be a project here. If it seems to be getting hot and smelling it may be a good idea to throw the switch, turn it off and get someone in to look at it. – user3190797 Jan 18 '18 at 23:38
  • 1
    Lets not close this so fast. OP could get some sense of action to take, etc. – user51490 Jan 19 '18 at 1:02
  • you failed to include a very important piece of information, all available info about the transformer. ... it is not possible to help you if we have to guess about the transformer output – jsotola Jan 19 '18 at 2:36
  • Your thermostat probably has a "heat anticipator". I should be set to match the zone valve current. The anticipator being a wire wound resistor will have a voltage drop when the thermostat connection is closed, adding a small amount of heat to the temperature sensing apparatus. I hope this helps. – Optionparty Jan 19 '18 at 4:15
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    Not nearly enough information for us to troubleshoot. Need a rough wiring diagram of the 24V-side system, photos if possible, also need to know the point you are measuring and whether you are on AC or DC on the meter. – Harper Jan 19 '18 at 16:30

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