I have an aging Bard split A/C unit … using an old-fashioned Honeywell (round heat/cool) thermostat.

Thermostat set to “Cool”
Thermostat fan set to “Auto”

When rising temperature turns A/C on, the outdoor compressor unit runs but the air handler circulation blower fan does not turn on.

If I set the Thermostat fan to “Fan On,” the air circulation blower fan runs … and everything is fine except, of course, the blower runs constantly.

Is this a thermostat problem (?), a faulty relay / switch in the air conditioner unit (?) … or (?).

Thanks for counsel …

 

The same circuit from the thermostat to the indoor unit and the fan relays, etc. in the indoor unit is used to operate the indoor fan in both ON and AUTO mode. The exception is an internal connection in the thermostat/subbase itself from cool to fan which comes into play when auto fan is used. [the subbase is the part that is mounted on the wall and contains the switches] Try taking the ring off the thermostat and look for 3 itty bitty screws that hold the thermostat to the subbase. Make sure they are tight. Next, put the thermostat in cool/auto and see if you have 24 volts from G terminal to C terminal in the indoor unit. The terminals will be where the cable comes into the indoor unit from the thermostat. It's unusual to have that kind of failure on a round Honeywell, but I guess anything can happen. Most likely the problem is in the fan switch on the subbase. Let us know what you find. Thanks.

 

Over the years, ive run across quite a few Q539A Honneywell (round) subbases where if you slightly wiggle the 'Fan' plastic lever on the subbase, you can make the fan cut in and out in the 'auto' position while the thermostat is calling for cooling...which of course indicates a poor connection requiring the subbase be replaced. However, instead of putting on a replacement subbase, I would go with a digital thermostat which will be more accurate and easier to read.

 

Sincere thanks for counsel thus far.

I disassembled the thermostat and did not see anything awry.

After thermostat reassembly, in reply to one of the points by daddyjohn:
When I put the system to “Cool / Auto” the voltage from G to C is nil ... and, of course, the blower fan is not running.
When system in “Cool / Fan On” the voltage from G to C is 24 volts, and the blower fan runs fine.

Does this lead to any other recommendations or diagnostic requirements, or is it definitely a problem in the fan switch portion of the thermostat. If affirmative, I will replace the thermostat. Thank you …

 

It 's the subbase switch. Time for a new thermostat.

 

Thermostat replaced (w/ a digital Honeywell RTH110B), and all systems functioning properly. Mission accomplished. Sincere thanks for counsel and recommendations ...

 

i woke up this morning, to a beautiful day (at least it was OUTSIDE my home).
inside, it was 80 degrees and balmy and i decided today was as good a day as any to do a yearly inspection of my Lennox system. the findings were atrocious.
as a property owner, i had rented the house out to tenants for 3 years. sadly, they did not respect my home.
i went outside, to clean the fins and exterior unit. all was good and well after that with the exterior unit.
i came inside, ostensibly to change the filters, and found instead a plastic window weather sheet on top of a think heavy sweat shirt on top further of the filter, which was filthy. (now i understood the lack of airflow that i thought nothing of having never dealth with central systems.)
so i removed them, cleaned the permanent washable filter, and reinstalled.
i went down under to the coils. HORRIBLE.
used a soft brush carefully in downward strokes to clean all that off. success.
drain hole was also clear.
checked the lines by applying water for leaks, and found no bubbles.
no icing that i've ever seen, inside or out.
while doing all this, the outside power control box was pulled out (have you ever seen a small handle on a flat face with 4x 1/2inch long flat bars that insert like the electric meter?) and ALL the inside breakers were turned off (no sense electricuting myself, right?)
so i did all the cleaning, reinserted that weird power control face outsde leading to the exterior unit, came inside, set my thermostat, and flipped on all the breakers.
increased air flow followed.
slightly cooler air followed.
"apparent" normal functionality resumed for an hour.
then, i noticed something awry.
the external unit was on and working as i assumed properly, but was not shutting off.
the internal blower unit WAS NOT operating with the thermo set to "Cool, Auto fan".
somewhere there is an interruption causing a problem.
out comes the handy dandy techno gadget known as a meter.
all read correctly: 24.
yet, the blower is not blowing on auto.
all wires were uncapped, cleaned, mechanically twisted in a sound manner, and recapped. power was turned on yet again, and the problem remained: outside unit will turn on as normal, condensors and compressors and fan motor alike, but still no auto-functioning of the interior blower.
at this point, i am remiss to further make attempts to touch anything at all myself.
having searched Google for more answers and turning up this page, i decided to make this post.
here's my specs: old 1980's central heating/central air unit, Lennox, in what i would assume as being good shape (evidenced by professional installation of a new compressor and fittings on the external) and serious lack of compassion for your best buddy as a home owner (all the aforementioned lack of maintenance that has resulted in my maintenance worker being fired), which sadly, is myself, lol.
my thermo is an White Rodgers Digital tht features two buttons for temperature up/down, Fan setting switch for On, Off, or Auto. nd a selector switch for Heat or Cool, or Off.
having read countless pages and not turning up a solution, i humbly request more educated opinions on my problem.
i looked deeply into the workings of the thermo and see no corrosion, no cobwebs, no loose wires, no improperly connected wires, and what i would call pristine condition.
is there some step involving resetting between indoor/outdoors units that i am completely unaware of?

 

First, don't run the AC without the inside fan running. Take the stat off and jumper the R and G wires together and see if the fan runs.

 

Did you try it in FAN ON mode ?

Like skaggsje mentioned you can try jumping R to G with a little jumper wire.
If that does nothing... try it at the air handler end.