Central Air System oversized for our house


Old 06-20-14, 08:22 AM
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Central Air System oversized for our house

We purchased our house a few months ago and have just started using the AC. The previous owners had the central air installed last year but I believe the units may be oversized for the house (it's 4.0 ton system.) I did some research and based on the Manual J calculation, that's about right for the size of our house (1750sq ft.) but based on more detailed calculators, it should be around a 3.0 ton.

I wanted to leave the thermostat programmed at 75deg but the system seems to turn on and then shut off within maybe 5-10mins. I know the system is more efficient when it runs longer (and takes out more humidity) so does it make sense for me to lower the temp to say 71deg (where my wife actually wants it.) Anyone have an idea of which would keep the electric bill the lowest? Thanks!!
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Old 06-20-14, 08:49 AM
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The less it runs, the less it costs.
The smaller the delta-T is between indoors and outdoor temp will run the unit less.
The larger the delta-T, the more it's going to run.
This of course depends on other factors such as sun load and insulation.
Old 06-20-14, 06:11 PM
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Oversizing does kill efficiency and reduce dehumidification - turning it down would make the machine itself run more efficiently (amount of cooling vs power consumed), but you'll end up using more energy overall since more cooling would be needed.

One thing you can do to reduce short cycling is use a programmable t-stat (increase setpoint by 5+ degrees F when house is unoccupied) - works for great with inefficient, oversized furnaces, should work fine with an oversized a/c too. If your house was well built, it won't come on at all when the house is unoccupied, but will take at least a couple of hours to pull back down.
Old 06-21-14, 06:41 AM
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If U slow the blower CFM down to 350-CFM per Ton it will lower the sensible Btu/hr & thus increase the runtimes per cycle.

If U had a programmable 'SWING Adjustable' room T-stat, U could set it for long runtimes, as U can increase the temp-differential 1, 2 or even 3F which both increases runtime & off-time.
Two of them are the LUXPRO PSP511LCa or, an ACE ATX 1500 4115176.

Other than that the new system should have been sized right using a ACCA Manual J type load-calc.
Old 06-21-14, 10:08 AM
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Yeah. I would just set the temp a little lower to get longer runtime and more humidity out of the house. But first it would be nice to know the humidity in the house. So get yourself something to measure humidity then we will know i your levels are high or not
Old 06-21-14, 07:18 PM
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Does your wife really want 71 degrees? We just put in a all new system (no central ac before) We find it comfortable at 77 degrees/53% humidity. If we are doing any physical work in the house, we lower it to 76, or 75 on rare occasion.
Set it to 78 at night.
Old 06-23-14, 10:56 AM
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hvac retired mentioned using a adjustable swing t-stat to lengthen the cycles -> you can do something similar with the pro grade honeywell t-stats. (adjusting cycles per hour)
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