A/C efficiency question


OK, it was chilly last night, went down to 47F.
House stayed above 72 with no heat.
Temps will be up in the 80's today.
I normally keep the windows and doors closed due to allergies.
Is it less expensive to run the AC now when the house is 72F and the outside temp is still below 60? (taking into account I don't mind the house being down to 65 for the first half of the day) Or is it better to wait until the house gets above 72F and the outside temp is 80F?
Whoa! Gotta turn off the ac now, it went down to 64 quite fast!
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Tony wrote: ...

...
It will be slightly more efficient running w/ lower exhaust temperatures but it'll not be enough to be noticeable until the outside temp's are 100F or thereabouts. For "close enough", it'll simply be the amount of time the unit runs that controls cost.
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Tony wrote:

There's no way to answer that question. Humidity has a huge effect.
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....as are other factors. Adding an attic fan will greatly increase the efficiency --and lower the costs-- of running an AC.
nb
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Takes a bit less electricity when it's colder out. How much, I'm not sure. Probably not enough to worry about.
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Actually I think this is incorrect. The colder the ambiant air around the condensor the more heat that the condensor will shed upping the subcooling of the liquid leaving the condensor and reduce the flash gas in the evaporator.
Also if the nighttime temp is 47F the humidity will be low when the heat of the day peeks at 80F.
Also if the max temp outside is 80F the house is not going to gain much heat as the heat transfer is greater when the temp difference is higher.
Pehaps a better approach would be to open windows in the early morning letting the cool morning air into the house, and then shut the drapes and windows on the south side of the house in the morning as the sun hit them.
By doing this you can avoid any AC use.
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Roger Shoaf

About the time I had mastered getting the toothpaste back in the tube, then
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Roger Shoaf wrote:

It works for me so well, I've never needed to install A/C. The inside temp will lag the outside by 15 or more degrees F, keeping it quite confortable.
In Seattle, admittedly.
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Roger Shoaf wrote:

I install a lot of units in businesses where AC has to run year round. The OP mentioned allergies and an aversion to outside air. For units meant to run in cold weather, I install head pressure controls on the condensing units, it's hard to get a lot of HVAC techs to go to all that trouble in order to make a unit work better in cold weather. A while back I started seeing dual compressor units for homes that would drop out one compressor during low ambient outdoor temps and reduce the capacity of the system. Now I'm seeing inverter controls for the compressor motors to vary the capacity of the system. I haven't had an opportunity to install one of those yet but it is interesting.
TDD
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For low temp refrigeration, I've heard of a "headmaster". Aparently, someone makes a bypass valve, that allows some hot gas to bypass the condensor. I'd be thinking a snap disk operated fan, instead.
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Stormin Mormon wrote:

I use pressure switches and electronic temperature sensors for turning on/off the condenser fan for head pressure control on AC units. Of course a crankcase heater is a given if it's not installed at the factory.
Electronic head pressure control:
http://www.icmcontrols.com/products/product.php?prod_pkY
http://preview.tinyurl.com/37khjkq
I've used this type for refrigeration systems:
http://www.frigodesign.ru/sale/sporlan/HeadPressControl_Sporlan.pdf
http://preview.tinyurl.com/34czv93
The least expensive is a fan switch:
http://www.grainger.com/Grainger/items/4E041?Pid=search
http://preview.tinyurl.com/35zcftu
TDD
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I was thinking that the colder outdoor temps leads to lower discharge pressure off the compressor, and lower amperage draw.
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Tony wrote:

Hi, If you have an intelligent learning thermostat(programmable digital), it'll help you quite a bit. How is your humidity? Our blower is going at lowest speed setting most of time. When heat ir collis called for it witches to it's proper speed. I think this way it gives best comfort level.
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On 5/9/2010 7:18 PM, Tony Hwang wrote:

I recently had my HVAC guy change out my Honeywell thermostat to a different model which has a built in humidity sensor. I've only had one or 2 days experience, but it seems great. Here in the western mountains of NC the temperature usually doesn't get real high during the day. But, the humidity can. This thermostat will (over) run the AC to remove moisture. I have it set for 76 degrees and 45%. On the 2 days where is ran, it did exactly as advertised. It will let the temp go up to 3 degrees low to control humidity. However, in my case it held the 76. It probably also helped that the AC unit is a 2 stage and of course it was running on only the 1st stage. Now, I suspect if you live in the real high humidity areas like Florida, the results might not be as good. BTW, you can also hook up a whole house dehumidifying system to this thermostat if it is needed. I don't think I'll need that here. I'll check it out during this coming summer and report back. So far, I'm impressed.
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Art Todesco wrote:

Climatouch makes an even better multistage tstat which controls humidity inside a 4 RH (+2 and -2) variance.
I use it on a dual compressor heatpump in deepest part of S.Florida and it does not change the inside temperature when running a humidity control cycle.
For example, I set RH to 50% and system comes on at 52% and runs until RH is 48%. When it runs due to humidity it engages stage1 compressor and runs variable speed (DC motor) blower at ultralow speeds allowing the coil to collect all moisture very effectively.
No other tstat comes close to this ability.
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wrote:

In the long run it's more efficient to leave the controls alone. Your house has good insulation.
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