AC thermostat fan delay

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Old 03-19-06, 06:01 PM
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AC thermostat fan delay

A few newer thermostats provide the option to keep the fan (air handler) on for a few minutes after the thermostat cycles the compressor off. This scavenges a bit more heat out of the house instead of letting the evaporator just warm up by cooling the attic. Are there any aftermarket devices that could add this delay to an older thermostat? I have Honeywell Chronotherm III thermostats. If something like this isn't available, it seems that someone could make some money by selling one - it's just a simple timer and electronic switch that performs the same function as the fan (on/auto) switch, parts couldn't cost more than a few bucks - but I'm sure that there are idiosyncratic thermostats out there that would make it a challenge.
 
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Old 03-19-06, 07:00 PM
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Check with your heating/cooling pro. There may be a fan limit switch that can be installed, similar to the heating systems. "Same thing only different"
 
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Old 03-19-06, 07:15 PM
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Yeah, it won't be a part of your thermostat, but a relay in your air handler. I had a system once where the fan just dropped to almost an idle speed after the a/c shut down. The theory was to keep the air moving over the filters to clean the air constantly.
 
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Old 03-19-06, 07:24 PM
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Thanks, I like the idea of a direct fan switch - probably easier than building a small electrical circuit to stuff into the thermostat.

But I've heard mixed views on constantly circulating air. I actually have electrostatic filters installed in the air handlers so it would continue cleaning/filtering. But I've talked with folks that didn't like the constant circulation because it takes any moisture that hasn't drained out of the evaporator and puts it back into the air. I live in Houston and humidity control is at least as important as cooling.

Your thoughts on constant, low speed air circulation and humidity??? Maybe there's not enough water left at the end of the cooling cycle to worry about? (I keep the drains clean.)
 
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Old 03-19-06, 08:19 PM
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Sorry, I wish I had your problems!!!!! Made it all the way to 28d F today, Not counting wind chill!!!!!!!! Check with your HVAC guy/Gal, They would know best.
 
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Old 03-19-06, 08:28 PM
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Thanks for the great ideas. I've gotten some additional info from the Air Conditioning forum and am ready to try some stuff. THANKS!
 
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Old 03-20-06, 07:32 AM
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Most newer furnaces and air handlers come with this option built into them. Often it simply needs to be set on the system board. The system only runs for a minute or so after the call for cooling is satisfied. Not enough to put the humidity from the coils back into the air, but if they ran much longer that would be a problem. Since newer thermostats also have the option built into them, there probably isn't much market for yet another device, but in the 60's I would have said there wasn't much market for a thing that sits on the counter and heats food by use of radio waves either...

Doug M.
 
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Old 03-20-06, 08:57 AM
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OK, I'm now spurred on to new queries.

Both furnaces are 15 year old Lennox units: GS18Q3/4-100-7 and GS18Q3-75-7.
I'll start with the first (larger) unit. It has a 3 speed fan motor and is controlled by a Honeywell Chronotherm III thermostat. The red (low speed) wire is not connected to anything - just an empty wirenut.

I assume that the existing fan limit switch is only operative for the heat cycle, so what I need is either a constant-on low speed or a few minute delay of the fan after the cool cycle ends. Given its age, I suspect that the Honeywell thermostat doesn't have the ability to control this delay, so it sounds like my option is to either pursue the cooling-cycle limit switch or just have the fan run at low speed constantly. What is the necessary connection for the latter?

(I'm posting this here because of the comment about the thermostat - I'll also go to the AC forum. Thanks!)
 
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Old 03-21-06, 06:54 PM
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The AC forum may be best, MOST of those people are well versed in controls/electric associated withe their equipment. But Keep us posted aswell please. Its always nice to know whats out there.
 
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Old 03-21-06, 07:41 PM
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The two GREAT suggestions I got from the Air Conditioning forum were:

1) just find a newer thermostat with the delay capability and install it.

2) replace the fan relay with a "time off delay" relay - one that has a built in time delay.

That whole train of thought was spurred by some of the comments on this forum - Thanks!
 
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