How Long to Cool Nine Degrees?

13 years ago

During the day, starting at 6 a.m., our thermostats are set at 77F. At 6:30 p.m. the thermostat for the MBR, Guest Room and Library is programed to go down to 68F. It seldom gets below 70F by 5 a.m.

There is a second system for the rest of the house that's set to go down to 70F.

We run the blowers continuously. The supplies are in the floor. The returns are high on the walls.

It has been in the 90's and humid during the days for weeks, but shouldn't the system overcome that?

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Comments (5)

rufusdoofus
13 years ago
I don't think there's a definitive answer to your question. Too many variables are involved. In any case, you're apparently trying to surpass the limit of what your system can do.

68Â°F and 70Â°F is quite chilly for residential air conditioning considering that systems nowadays aren't sized for a design-temp that low. High humidity puts a hit on the effective cooling capacity. Latent heat removed to condense moisture doesn't lower the temperature.

Just sayin' ... unless you have a very low power rate, I'd hate to have your electric bill. My last bill was $202 and that's running at 78Â°F to 79Â°F. I live near two of the cities listed among the top 10 in the U.S. for high humidity per 2008 NOAA statistics, and it's hot here as well.
tigerdunes
13 years ago
chisue

depending on condenser sizing,design temps both outside and inside, and recent abnormal outside temperature in much of the country, I think your programmed setback and recovery is unreasonable. I am actually surprised that it recovers as much as it does. And if operating costs are important, then I suspect any savings from the setback are more than offset by a system running continuosly for so long.

I would rethink your strategy.

IMO
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david_cary
13 years ago
chisue - Here in nc I get to 65 in our bedroom. Our setback is 77 during the day. Our stat gradually steps down and it is 65 when we go bed at 9pm. Our bills are very reasonable so I'm just posting to give support to your strategy.

If you want it colder at night and it is unseasonably hot, then you need to lower your setback. Your system may be working fine and be sized appropriately. It just is hot and you are asking for a temp colder than design.

I have a staged 3 ton system wuth a manual J that came in at 2.1. So I am a bit oversized and I am also zoned and not looking for 65 everywhere. For anyone that cares my humidity is 38-42% - the magic of VS and 2 stages.
veesubotee
13 years ago
"High humidity puts a hit on the effective cooling capacity. Latent heat removed to condense moisture doesn't lower the temperature."

Running constant fan in a high humidity situation is counter-productive. When your a/c shuts off, your inside coil needs time to drain. Running the fan at that time will cause re-evaporation of the water present.

V
chisue
Original Author
13 years ago
Thanks, all! I suspect we're just asking too much due to the constant heat over the last two weeks. We've had a bit of a break this weekend, and last night the system in question did reach 68F by early morning.

I'll also revisit having the blower on 24/7, lest the coil be thwarted from draining, veesubotee. I should put the fans on 'auto' overnight? (Prefer to avoid the "on", "off", "on", "off" overnight, but if that's how the room gets and stay cool at night, we'll do it.)

Do y'all really think this is an vast waste of energy? When the temp in the house is 68-70F in the morning, the A/C doesn't kick in until about 5 p.m. It then maintains at 77F until it's set to start back down to 68F at 6:30 p.m. Only the blower fans are running during the day to circulate the air in a sealed house.

I'm asthmatic and sleep better in a cool, dry room. I *could* just install a casement unit in the MBR. Would that save a lot of electricity?

Our furnaces are two-stage, but that's moot when the A/C isn't running and just the fans are on constantly. Does that even apply outside of the heating season?

How Long to Cool Nine Degrees?

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rufusdoofus

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How Long to Cool Nine Degrees?

Comments (5)

rufusdoofus

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