Which A/C Cycle is Most Cost Effective?

pawprint1August 16, 2009

A: 1 hour on & 1 hour off

or

B: Every 15 minutes (thermostatÂs default setting)

I believe A is the most cost effective because the unit powers on 1 time every 2 hours, even though run time is the same.

Is it true that A/C start up is the most expensive & the thermostat default setting is for comfort control?

I donÂt care if the temp fluctuates 3 or 4 degrees if itÂs cost effective.

Thanks in advance!

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ryanhughes

What kind of thermostat do you have? Usually, there is an average CPH (cycles per hour) setting, which I recommend leaving at 3 (on average 3 cycles per hour). I'm not sure your thermostat would let the temperature rise that much regardless of its setting. True, consistent starting every 15 minutes carries with it higher electricity costs as well as more wear and tear on the system itself.

    Bookmark   August 16, 2009 at 3:10PM
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creek_side

It sounds like you have the wrong thermostat, or the stat is wired wrong. The cycles per hour setting is for a furnace. An air conditioner or heat pump should run until the thermostat's temperature setting is satisfied. In hot weather an air conditioner or heat pump in cooling mode should run almost continuously if it is correctly sized.

Unnecessary cycling is inefficient, provides poor humidity control, and shortens system life.

    Bookmark   August 16, 2009 at 3:42PM
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pawprint1

ItÂs a Honeywell programmable thermostat. There are 4 different settings to change the CPH to, for the a/c and separate for furnace.

Yes, the unit is oversized, which IÂm stuck with. It can cool the house down in 5 minutes. When at home I manually override the system. Running every other hour is fine with me.

Without downgrading my unit, I donÂt know what else to do.

    Bookmark   August 16, 2009 at 4:18PM
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creek_side

Can you post the thermostat model number? I'd like to familiarize myself with it?

    Bookmark   August 16, 2009 at 5:10PM
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pawprint1

RTH230B

    Bookmark   August 16, 2009 at 5:14PM
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ryanhughes

Creek side, air conditioning side of operation also has a CPH setting, at least on Honeywell thermostats. Both TH8320's I installed, I set both heat and cool to 3 CPH. That to me seems to be the best balance between comfort/economy.

    Bookmark   August 16, 2009 at 5:42PM
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creek_side

I have personally only had one HoneyBucket thermostat, which I jettisoned. It was controlling a Carrier gas pack, not well, I might add, and it didn't have a CPH feature. I really don't like that concept when it comes to AC or heat pumps.

    Bookmark   August 16, 2009 at 6:45PM
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srercrcr

"In hot weather an air conditioner or heat pump in cooling mode should run almost continuously if it is correctly sized"

This is total bunk. First, nobody does this. Second, if it were sized to run all the time under typical conditions, then when a heat wave hits you have no way to keep it cool. Your source readily admits that and recommends one TURN ON A FAN!!
How's that for comfort control?

    Bookmark   August 16, 2009 at 10:38PM
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creek_side

srercrcr, you are fighting an uphill battle on this subject, again.

Correctly sized AC and heat pumps do run almost all the time when it is HOT. The generally accepted standard is to size for 95 degrees outside, 75 degrees inside, with some obvious regional variations.

From your prior posts, it is pretty clear you have an over sized system. They are all too common. They cost the homeowner money and have poor humidity control. Grossly over sized, which isn't all that unusual, they can contribute to or be the cause of health problems in the home, including inducing and supporting mold growth.

As to "nobody does this," do you know everybody? Obviously not. Our system runs continuously when it is in the low 90s and above. It does that because it is sized correctly. It may be sweltering out, but we are cool and comfy inside, with low indoor humidity and reasonable power bills. That is the way it is supposed to be.

    Bookmark   August 17, 2009 at 8:04AM
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ryanhughes

In very hot weather (design conditions) a properly sized system better be running near constantly.

    Bookmark   August 17, 2009 at 11:08AM
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pawprint1

creek_side: I when I replaced my old unit (which couldnÂt keep the house cool) the salesman suggested a bigger unit. I wanted a cool house so I agreed.

Now I learn that I didnÂt need a bigger unit "IF" my old unit, with the correct tonnage, was working properly, it was enough.

IÂm in Columbus Ohio; our summers are upper 80Âs (sometimes 90Âs) with average humidity of 70-75%. On a 90 degree day, humidity 70%, my unit can cool 1 degree every 6-7 minutes.

Last month a technician was out for my annual tune-up. My conductor was replaced. My static pressure was too high so he installed a small vent in the basement and I downgraded my thick air filter. Now static pressure is on target. Inside and outside coils have been cleaned. No Freon was needed, levels are perfect. My blower motor was on high speed since installation. The technician now has it on low, for better humidity control. It now blows 57 degree air.

The unit is 5 years old and in pristine condition and only used for our 3 summer months a year. The unit is just too big.

I thought I would save money and life on the whole system if I manually override the thermostat. I donÂt mind a 4 degree variance. Doing that would make it run about 25 minutes consistently. Thoughts?

    Bookmark   August 17, 2009 at 11:36AM
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udarrell_2007

I'm a retired longtime HVAC contractor & Tech, my 1937 rural farm home does not have central A/C, - just oil heat.

I bought a 6,000-BTUH Half-Ton room A/C, it's only supposed to cool 225-sf, it cools perfectly even in 104-F Heat Index weather, with the use of a WindMachine 3300 fan around +850-sf - or the entire first floor area.

Thats 7-BTUH per one SF, vs their recommended 26.6-BTUH per one SF, or 3.8-times less BTUH per SF.

I am for wide adjustable on/off differential TH settings of up to 4 or 5-F degrees; e.g., on at 77 off at 72.

For oversized systems you'd need a 10-F 80>70-F. swing.
more BTUH per SF.

In your case, I'd be looking for a smaller A/C. - udarrell

Here is a link that might be useful: My Room AC

    Bookmark   August 18, 2009 at 9:13AM
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energy_rater_la

posters are giving you good info for the most part.
(hey Ryann..how ya doing?)

the less stops and starts your unit makes..benefits you
in more dehumidification, lower operating costs and longer life of unit.

one way you can help your system out since it is not running long enough to dehumidify, is to add a stand alone
dehumidifier. place it as centerally as possible in your home and be prepared to empty out the water it removes as the RH in your home is lowered.

btw if hvac companies did load calcs most homes would have correctly sized units instead of oversized units.
sizing for the 10 hottest or coldest days of the year assures that the ho will not be calling hvac company on those few days when outdoor temps exceed the 20 degree difference that ac is desigend to achieve from indoor to outdoor.
this is a shoddy way to do business, but happens daily.

hvac company can charge more for larger unit and ho
pays the cost monthly in utility costs plus upcharge for larger unit. also they get to replace sooner.

doesn't take a rocket scientist to figure that one out does it? pay now for load calc..save on cost of size of unit and monthly utility cost
or pay now for larger unit and every month for excessive operating costs..and do it all over again in 10 to 12 years.

best of luck.

    Bookmark   August 18, 2009 at 11:52AM
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srercrcr

"longer life of unit."

So we have here a smaller sized unit that runs 22 hours a day and you're promising a longer life for the system?

And your blower runs 22 hours, mine runs 11 hours...your fan operating costs are double mine and yours wears out in 6 years instead of 12 years.

Geez

    Bookmark   August 18, 2009 at 5:43PM
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ryanhughes

Srercrcr, I'll give you my thoughts on the topic, and you may disagree and think your setup is superior to a properly sized and designed system (as I've gathered that yours is oversized - wrong?). Most energy is pulled when the compressor starts (blower motor may be different), and starting is what produces wear on the compressor. Running once the compression ratio/pressures/etc. are all where they need to be is no big deal. That happens about 10 minutes or so into the system's operation, when it's at its peak capacity and efficiency.

Have you ever noticed, that light bulbs almost always go bad when you turn them on again, and not while they're currently lit?

    Bookmark   August 18, 2009 at 9:20PM
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ryanhughes

And energy_rater_la, haven't heard from you here in a bit, or maybe we're posting in different threads. Hope all is well with you.

    Bookmark   August 18, 2009 at 9:22PM
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srercrcr

bump...could someone respond to both points?

Assume you have a smaller sized unit that runs 22 hours a day and you're promising a LONGER LIFE for the system?

And your blower runs 22 hours, mine runs 11 hours...your FAN OPERATING COSTS are double mine and yours WEARS OUT in 6 years instead of 12 years.

(I don't think I would call my sytem oversized...2.5 ton, 1320 sf w/ 9 ft ceilings...highest summer bill in S Texas...$135)

    Bookmark   August 19, 2009 at 12:33PM
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energy_rater_la

srercrcr...
this was what I read:

A: 1 hour on & 1 hour off
or
B: Every 15 minutes (thermostatÂs default setting)

somehow we got from there to here:
"your blower runs 22 hours, mine runs 11 hours...your fan operating costs are double mine and yours wears out in 6 years instead of 12 years.

Geez"

an excellent response was posted to your post.

OP's unit is oversized, as he stated, and as many of us
figured out from his run times and questions.

your unit based on run times and sq ft of home is closer to
being sized corecctly. the only assumption in that statement is that the statement is based on what little I know about your house, unit and run times.

chill man, no one is attacking you.

    Bookmark   August 21, 2009 at 5:35PM
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