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I have a question? How much current does it typically take to close the blower relay on a air handler. I recently got a new thermostat which requires an external transformer and it doesn't appear to be closing the blower relay.

Brian Weaver
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The thermostat doesn't supply current to close the AC relays; it only acts as a switch. Power for the relays comes from the heating system, through the thermostat (when it has turned itself on) and back down to the relay in question.

Anything in that loop -- the wiring up to the thermostat, the connections to the thermostat, the relay itself -- could be a failure point.

First thing to check is your connections at the thermostat. You can check the relay by reinstalling the old thermostat (if you've still got it) or by shorting the right pair of wires (I don't remember which offhand) and seeing if you can get it to turn on that way. If that works, reinstall the new thermostat again, carefully checking your connections both for correctness and to make sure you have good electrical contact, and try it again. If it still fails, it is possible that you have a defective thermostat (or an intermittent wire...).

keshlam
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  • Yea. My problem is that my there is no c-wire or an extra pair in the wall. I purchased a 300 mah transformer last week in order to connect to Rc, and C to power thermostat and cooling but when it goes into cool mode the thermostat relays click but thats about it the unit doesnt turn on. I check the voltage between Y, and C and its 24 volts like it should be. – Brian Weaver Sep 07 '14 at 19:32
  • The OP is connecting the wall wart to Rc and C. When the thermostat calls for cool, the wall wart is supplying the power to pull the relay. The system works when Rc is connected to the transformer in the air handler. See the OPs first post. – Tester101 Sep 07 '14 at 21:52