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05-12-2012, 07:27 AM   #1
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## Theoretical question

Under the same conditions of (indoor) temperature and humidity, which will cause an air conditioner to run a longer cycle: a higher or lower coil temperature?

Last edited by veesubotee; 05-12-2012 at 08:34 AM. Reason: wording

05-12-2012, 08:16 AM   #2
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I don't understand your question, a home AC unit is generally setup to controll temperature, therefore one needs a temperature differential from the controlled space, the larger the differential the shorter the cycle.

Realitive humidity allows the air to carry more heat, but this changes as the air is cooled through the coil, condensing water out of the air stream thus reducing pounds moisture, but raising the realitive humidiy at the coil face.

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05-12-2012, 08:56 AM   #3
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Quote:
 Originally Posted by Jackofall1 I don't understand your question, a home AC unit is generally setup to controll temperature, therefore one needs a temperature differential from the controlled space, the larger the differential the shorter the cycle.
What I'm really trying to get a handle on has to do with the issue of CFM, i.e., setting airflow < or > 400/ton. While it has been stated that > 400 will result in higher efficiency, the supply air is higher in temperature. Intuitively, I would think that moving more air mixing with room air (even at a higher temp), would result in a shorter run time (less electricity = higher SEER).

An example of this would be, when I enter my sun-baked car. I usually run the a/c at 1 or 2 (airflow). Normally, this would take a long time to cool things off. Turning the fan up to 3 will shorten the cooling time. If, as I assume that auto a/c's run at a single stage, the airflow temp would be higher even though it feels about as cold (due to high velocity??).

So, I'm trying to nail down how in general a +/- adjustment to airflow would affect cycle time. Comment please.

Quote:
 Realitive humidity allows the air to carry more heat, but this changes as the air is cooled through the coil, condensing water out of the air stream thus reducing pounds moisture, but raising the realitive humidiy at the coil face.
Not really up on psychrometrics, so can't comment.

 05-12-2012, 09:25 AM #4 Member   Join Date: Oct 2009 Location: Nebraska Posts: 2,362 Rewards Points: 1,000 Yes,lower cfm equals longer run times. An AC removes X BTU's per hour with some being temp and some being moisture. A thermostat only reacts to temp. With that said the extra electric used for longer run run times is offset by the thermostat being able to set 3-4 degrees higher and have the same human comfort because the humidity level is lower.
05-12-2012, 11:21 AM   #5
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Quote:
 Originally Posted by Marty S. Yes,lower cfm equals longer run times. An AC removes X BTU's per hour with some being temp and some being moisture. A thermostat only reacts to temp. With that said the extra electric used for longer run run times is offset by the thermostat being able to set 3-4 degrees higher and have the same human comfort because the humidity level is lower.
Thanks, that was what I was looking for (and confirms my suspicion). If I can increase my latent cooling and comfort level, I'll be happy. Energy costs in past (hot) summers has not been excessive, so a little more won't hurt.

05-12-2012, 12:53 PM   #6

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Unfortunately its not as cut and dry as we might like it to be. While a higher air flow rate will lower house temp quicker. It will also remove less moisture from the air, causing you to feel warmer.

74° at 46%RH feels just as cool as 72° at 50%RH. But running higher CFM through your coil, you could end up with 70° at 54%RH.

You need to see if the lower air flow will dehumidify the house enough that you feel just as comfortable at a higher temp then you do at the higher air flow.

Your A/C will get a higher SEER with a higher air flow and a room temp of 80°F. Then it will with a lower air flow and a room temp of 74°F. But A/C is about comfort, so chose which one you want. high efficiency, or comfort.

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