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Central A/C Units and Runtime Efficiency

Posted by aceh (My Page) on
Mon, May 25, 09 at 9:15

I read that a unit which runs longer during cycles will do a better job because longer run times help dehumidify a house.

Here's the confusing part to me; regardless as to how high the EER is for a unit, wouldn't a longer run time still increase the operating cost? Logically it would seem that a shorter run time would save energy cost. (Assuming the unit is NOT grossly oversized causing it to cycle practically every 5 minutes or so.)

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RE: Central A/C Units and Runtime Efficiency

To achieve optimal efficiency & the highest operating SEER Rating long runtimes are essential.

It takes a lot of amps during startup & it takes around 5 minute to reach optimal cooling performance.

Optimal efficiencies:
We could cut residential heating and cooling equipment size in America by 30% to 50% if Contractor's would perform honest Manual J calculations, and provided full credit for every load reducing element or detail they could do prior to equipment sizing, when doing the initial load calculation audit.

Air infiltration rate, can be half the load, & should be checked & reduced. Ductwork & airflow must be checked & optimised for full nominal BTUH performance.

Additionally, load reduction remedial actions should be provided as options toward further reducing Air Conditioning and heating equipment sizing.

Then undersize equipment just a little, while optimizing the ductwork & thus reducing blower MTR HP & its heat, while optimizing airflow through the evaporator coil.

The comfort level is never as good with short cycling oversized units; & it is very hard on equipment.

Smaller units draw less electricity; I use a Half-Ton window unit for nearly 900-sq.ft., it uses around 500-watts or less, my brother has a 1.5-Ton central unit & the indoor blower draws as many watts as my entire window unit. - udarrell

Here is a link that might be useful: Efficient Ways I use Window Units

RE: Central A/C Units and Runtime Efficiency


I read your page and I am impressed by your experience and knowledge of Heating and Air Conditioning, which is why I came to this fourm in hopes of getting the Pros, such as yourself, to help a person like me with no training or knowledge of this field.

You said "it takes a lot of amps during startup and about 5 minutes for the unit to reach optimal cooling performance".

So once the unit reaches its optimal cooling performance and it's running a relatively LONG cycle, it's still saving money because it's using less amps than it did for its initial startup? I still can't quite make the connection between a system that's running but saving money compared to a system that's off using NO energy at all. (Unless the amps/energy used at startup and during those first 5 minutes are far more than the engergy used during a "normal" on/off cycle.)

RE: Central A/C Units and Runtime Efficiency

One of the problems is that the larger units many times are Over sized units, are tying to push & suck too much air through too small a ducts & filter grilles.

So, they use a lot of blower & compressor amps not don't cool any better than a smaller unit. Also, if there is much humidity hey won't pull it down.

To maintain a very narrow t stat setting with over capacity units, it will be constantly cycling on & off which is extremely inefficient & very hard on components.

The Lower cost of lower capacity equipment; Comfort & longevity of the condenser & compressor are important reasons for sizing down instead of up.

If you use floor type fans to circulate the air & a t stat with a wide differential setting to lengthen the cycles over a say 3 degree on/off range you could get by with a larger A/C. - udarrell

Here is a link that might be useful: Reducing Home's Heatload-Sizing equipment & Ductwork.

RE: Central A/C Units and Runtime Efficiency

I designed my house and was active in reviewing the contractors recommendations based on the Manual J analysis. When all is said and done the old adage of 12,000 btu (a ton) for every 500 sq ft of floor space is pretty accurate, except I would modify that for any known major variable that you know would cause a problem. In my case, I had shade trees all around so there's less direct sunlight load, but I have 9' ceilings which is more air to condition.
My split system has a 13 SEER that was sort of cutting edge for 1996, a single speed scroll compressor. The blower is a variable speed one, which is key to my system because I have a humidistat installed (inexpensive). During the heat of the S. Texas day it will run at higher speed for maximum cooling due to the heat load, but since it runs for like 12 minutes, humidity is totally wrung out. In the pre-dawn hours when the humidity is like 85%, but temp is 72, it doesn't run much cause there's no heat load. When it does run, the humidistat tells it its getting humid in here so it runs at slow speed so it wrings out the air, slowly getting to the selected temp setting. I have the humidistat set for like 63%, so if it achieves 64% or higher it automatically shifts to high speed. I like it alot. Last week it was 97 outdoors, the AC ran like 12 minutes then was off 18 minutes. Before you select your SEER, you should understand that number directly relates to your energy bill. Ex: Since a 16 is 33% more efficent than a 12 (diff of 4 divided by 12), an electric bill that would be $100 with a 12, will now be $67 with a 16 (of course a 16 costs more to buy, but rebates/tax credits help). Good luck.

RE: Central A/C Units and Runtime Efficiency

One other thing to remember about oversizing/humidity. an oversized unit will get the temp down to the thermostats setpoint quickly, but will not properly de-humidify. This is a total comfort isse here.. as the higher the humidity.. the warmer the conditioned space "feels". It may be 72 in a room.. but have a humidity of 70% and not feel comfortable.. and certainly not "feel as comfortable as a room of 78 degrees with a 50% humidity. so you see the lack of dehudification an oversized unit can cause.. can also cause one to run a lower stat temp to compensate for the higher humidity.. therby increasing the energy consumption.

RE: Central A/C Units and Runtime Efficiency

To be clear, if you run the SAME air conditioner for longer, it will cost more. The point here is that how long it runs when it comes on is related to how much cooling you need compared to how much the unit can give. A properly sized unit will run longer, using less energy, and keeping you more comfortable. An oversized unit will have many short runs, use a lot of energy, and the temperature will oscillate up and down. We have a 1.5 ton unit and we have this problem. (Well-sealed 1500 sq ft apartment.) The oversized equipment will probably break sooner too.

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