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Old 06-10-2009, 02:00 PM   #1
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Additional Blower Control


I have two HVAC units, one for upstairs and one for downstairs. They work well and both are controlled by programmable thermostats. I do have one slight problem and that is the air in the bedroom gets a little too humid. This occurs in the wee hours of the morning. The quick fix (when it awakens me) is to turn on the upstairs blower for a couple of minutes to replace the air in the room with air from other parts of the upstairs region of the house.

The thermostats are programmed to lower the temperature in the house by a couple of degrees F just before bedtime. If I were able to put some sort of timer on the upstairs blower motor to run it for a couple of minutes every hour or two, it would solve the problem.

Is there any device that I can purchase that would start and stop the blower at predetermined intervals. I'm thinking about an ordinary timer and wiring it to the blower using the power supplied to the unit itself.

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Old 06-10-2009, 05:02 PM   #2
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Additional Blower Control


You need to make sure that your "fan" command does not conflict with a controller command telling the blower motor to do something else.
I'd tap into the "fan" switch on your 'stat, but the elec. arrangement then gets a little more complex, involving a 555 timer and relay and . . .

Maybe you can find a reasonably priced timer that just offers a contact closure. Some places offer kits that do this.

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Old 06-10-2009, 05:10 PM   #3
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Old 06-10-2009, 06:10 PM   #4
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Additional Blower Control


Honeywell Th8320, TH8321, and YTH9421, have fan recirc ability.
It will run the blower for 35% of the time, if there has been no call for heat or cooling.
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Old 06-11-2009, 10:51 AM   #5
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Additional Blower Control


The thermostat has a blower on and blower auto positions which enable one to run the blower continuously if desired. When I get the time (have an automobile apart at the moment), I'll look at the back of the thermostat and see how the blower switch controls the blower. If I'm lucky, there might be only two wires coming off the switch and going to the blower relay on the unit or to the brain box of the unit. The wires are colored coded so I can trace them back to the unit. I have a digital multimeter and know how to use it.

I have an old hot water recirculation pump with a very nice timer. The pump motor is bad so I figure I can remove the timer and use it. For a mechanical timer, the number of on/offs is very high and so they can be of short duration. There are at least 30 slider switches on the dial, possibly more.

I don't want the blower to run 35% of the time. I figure about 5% would be good enough considering the prodigious air flow at the two registers.

What do you think? Does this sound reasonable?
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Old 06-11-2009, 04:20 PM   #6
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Additional Blower Control


5% would be 3 minutes.

35% 21 minutes. If the system hasn't called during that hour.

Is your timer going to be wired not to bring on the blower if there was an A/C call within an hour.
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Old 06-12-2009, 06:53 AM   #7
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No, it'll run at set time regardless of what the AC has done. As it exists now, if I move the blower switch to on position, the blower will run continuously. If the AC is called, it just starts and runs until the preset temperature is attained. What I'm thinking will do this but do it at prescribed intervals throughout the night.

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