Fan Doesn't Work When Set to Auto

For one unit of my heating system, the fan doesn't come on when set to AUTO. It works fine when set to ON. Replacing the thermostat didn't solve the problem. (At first the boiler was also not coming on but this was solved by replacing a relay.)
For my other two units, I had the same problem but worked around it by setting a switch on the existing thermostats to ELEC instead of GAS, even though my boiler is run with gas.
Does anyone know what the problem might be?
Thanks
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Yup. Replace the thermostat, and the subbase.
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and it needs freezone

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Sorry for my ignorance, but what is freezone?
Thanks
62BT409 wrote:

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It's an integral part of the heating systems that NYC slumlords operate.
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It is what a lot of customers tell me to "just throw some in". It comes in 3.5 oz bottles. I usually stand back about 20 feet from the outdoor unit and start tossing from there. Bubba
wrote:

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as long as you have been doing the work, you must be damn good a hitting the little hole :o)

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With the switch in auto, the fan comes on when the furnace is hot enough to be blowing hot air. Not all the time.
--

Christopher A. Young
You can\'t shout down a troll.
  Click to see the full signature.
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Yes.
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Does the thermostat energize the blower relay?? or does the the furnace control board energize the blower relay?? Without the most simple, basic, information, there is no way to tell. Can you tell whats missing??
Try.......
Furnace; Make Model number Serial number
Thermostat; Make Model number
Without this information, your wasting our time.
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