A/C blower fan controller?

So the compressor runs and then at some point the A/C decides to turn on the blower fan. Mine works fine for awhile, but then it starts turning the fan on and then off again, within a few seconds, as if it can't decide whether the coil is cold enough.
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On 6/18/2014 10:16 PM, Davej wrote:

What does your AC service tech say? Why did you tell the class this information? What would you like for us to do?
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On Wednesday, June 18, 2014 10:16:26 PM UTC-4, Davej wrote:

Are we talking about a central AC system that's part of a furnace? A window unit? A mini-split?
IDK what the latter does, but in all the first two types that I've seen, the coil temp doesn't control the blower. The blower is turned on and the compressor start at the same time. When it shuts off, the blower typically stays on for another minute or two after the compressor goes off.
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On Thursday, June 19, 2014 6:39:38 AM UTC-5, trader_4 wrote:

Yes

Hmmm, you're right. I thought it worked identical to the furnace but apparently not. I've had this strange blower fan operation ever since the furnace was installed and have used continuous fan to avoid it, but now even continuous fan has strange behavior.
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On Thursday, June 19, 2014 7:50:37 AM UTC-4, Davej wrote:

ce was installed and have used continuous fan to avoid it, but now even con tinuous fan has strange behavior.
You need to put a meter on it and see if it's getting power but not running or if the control board is not keeping it powered.
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On Thursday, June 19, 2014 8:25:22 AM UTC-4, jamesgang wrote:

nace was installed and have used continuous fan to avoid it, but now even c ontinuous fan has strange behavior.

Agree, that would be a good place to start. Also, if it;s a modern unit, they have LEDs that light to indicate OK or identify faults if the CPU board knows there is one.
Running the fan constantly may not be such a good idea either. In furnaces from 25 years ago, putting the fan on all the time put it on high, the same speed as for cooling. Which is OK, except that those motors were not efficient and it's going to run up $$$, running when it doesn't need to .
Newer furnaces, the blower when set to run all the time, doesn't run at high speed. Mine runs closer to the heating speed. That isn't going to produce maximum cooling. With mine, if the AC kicks on, then it goes up to high. With the OP system with the problem, without knowing how it operates no way to know what it does.
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trader_4 wrote:

tried?
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On Thursday, June 19, 2014 8:16:15 AM UTC-5, trader_4 wrote:

The symptom has cured itself for the past few days. When the symptom was oc curring I could hear a relay clicking on the furnace board, so it was defin itely deciding to turn the blower fan on and off. Also the blower fan motor was cool to the touch. The furnace controller terminal voltages never chan ged: Red had 26VAC and Green had 20VAC continuously.
I think the first thing I should do is get the thing wired correctly becaus e I suspect that it has never been wired correctly.
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Davej wrote:

On most control board you cn adjust blower torun off delay. You an increase it. Mine is set at 120 secs. to get the last drop of hot or cold air out.
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The way the companies dick around with settings to meet certain seer ratings, it would not surprise me that the blower is set up to only come on when a certain head pressure or coil temperature is reached. Probably not your case, but could be.
That is one reason why the fans ramp up in speed instead of comming on and just have one speed for heating and anther for cooling in a heat pump.
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On Thursday, June 19, 2014 11:39:52 AM UTC-4, Ralph Mowery wrote:

I've never seen such a thing and doubt it exists. It certainly wouldn't be based on temperature or pressure, because the controller in the furnace isn't measuring either of those. It could have a time delay, but every system I've seen the blower starts at the same time as the compressor. My new Rheem ramps up over about 30 secs, but it starts same time as the compressor. The coil is cold very quickly. Without knowing exactly how they calculate the SEER, it's just speculation that a 30 sec or 1 min delay would even have any effect at all on the SEER.

Your source for that? I thought the reason variable speeds ramp up is because they can and it makes the blower far less noticeable when it starts up.

IDK about heat pumps, but my Rheem has multiple jumper selectable speeds for both heating and cooling. The heating speed in every furnace I've seen has always been less than for cooling.
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