Strange Humidity Spike After AC Compressor Cycles Off


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Old 05-08-24, 07:05 AM
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Question Strange Humidity Spike After AC Compressor Cycles Off

Details:
  • New construction, 3000 sqft home.
  • AC wasn't reducing humidity adequately (60-70% RH in spring).
  • Installed Aprilaire E100 Pro dehumidifier with the return connected to the AC's return plenum and the supply connected to the AC's supply plenum.
  • Placed thermostat in one of the supply ducts to monitor dehumidifier effectiveness.
Problem:
  • Dehumidifier works as intended but I'd like to wire it to control the AC blower for increased efficiency.
  • After the AC compressor stops, the fan continues for 1-2 minutes. During this time, humidity in the duct spikes up to 10-15% before the blower turns off.
  • Dehumidifier then takes 10 minutes to restore previous RH levels.
  • See attached thermostat screenshot.

Does anyone know why the humidity in the ducts increases so drastically after the AC compressor cycles off? I'm also worried that if I wire the dehumidifier to the AC, the blower will push this humid air through the entire house. Any ideas or solutions for this potential issue?

 
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Old 05-08-24, 07:15 AM
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Dehumidifier is set to 50% RH and using around 20 kWh/day which means it's running non stop 24/7
 
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Old 05-08-24, 07:17 AM
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Where is your ducting located? If it passes through unconditioned spaces and there are leaks in the duct than unconditioned, humid air could enter the ducts.

More likely I think you are seeing the results of the change in temperature within the duct. Relative humidity is based on humidity and temperature so when temperature changes, and it changes a lot when the AC cycles on and off, it will change the relative humidity even though the amount of moisture in the air is the same.
 
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Old 05-08-24, 07:28 AM
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The blower is bringing comparatively warm air into a cool duct via your cold air returns. So the dewpoint inside the ductwork is simply closer to the temperature of that return air. That's how relative humidity works. This effect is typically even more pronounced in summer months when the humidity is higher, and is one reason why uninsulated ducts will sometimes drip with condensation. The duct is cold (below dewpoint), while the air around it is comparatively warm, so the humidity nearest that cold surface will be higher than anywhere else. Nothing surprising about that... and nothing wrong in your case... Just physics.

Because anytime you bring warm air into a cool space the RH will go up. Similar to how if you compress air it will wring the moisture out if it.

If your humidity line showed a 15 minute average rather than minute by minute, it wouldn't appear so alarming.
 
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Old 05-08-24, 07:39 AM
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Thank you for your reply Pilot Dane and XSleeper, I appreciate it. I'm in the Houston area, and my furnace and distribution plenums are in the unconditioned attic. My ducts are insulated (flex R8), but I will check for leaks when the temperature outside drops. It's too hot in the attic now.

Additionally, I noticed a lot of moisture on the AC coils, and even water splashes in the supply plenum. I monitored the drainage for about 5 minutes and didn't see any obvious clogs.

Given this situation, do you think it would still make sense to wire the dehumidifier to the AC to activate the blower? The wiring instructions seem straightforward.
 
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Old 05-08-24, 10:56 AM
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After the AC compressor stops, the fan continues for 1-2 minutes.
Logically it sounds like a good idea..... it's not.
The non cooled air is pulling the moisture off the coil.

Try having the blower and condenser stopping at the same time.
 
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Old 05-08-24, 11:50 AM
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Thanks, I won't wire in the dehumidifier.
It looks like the thermostat's extended fan runtime is already off (Honeywell allows 15 seconds to 15 minutes).
The cycles per hour (CPH) setting is at 2. For CPH, in my case, would a lower setting reduce the chance of high humidity air blowing after the system stops? Should I change it to 1 (the default was 3)?
 
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Old 05-08-24, 02:00 PM
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Not following.... extended blower time is off but it still is running after condenser shutdown ?
 
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Old 05-14-24, 05:14 PM
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Yes, youíre right. I have no idea why itís still running when the Extended Fan Runtime setting is off. I talked with hvac company who installed it, and they are coming this Friday to check it.
 
 

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