Heating, Air Conditioning, Fridge, HVAC/Question about Short Cycling

Advertisement


Question
Thanks for sharing your expertise, both here and on your informative website.  

I have a question about the on/off cycle of High Efficiency central air conditioners.  Iíve been searching online but canít find a definitive answer.  From what I found, 2-3 cooling cycles per hour is considered acceptable, with a longer run-time beneficial, but I couldnít find info about a minimum run-time to avoid damaging the unit and get the benefit of high efficiency technology.   

Hereís the background:  our 18-year-old A/C was replaced last week as part of a govít program.   It still worked, but was replaced along with the air ducts to help reduce our energy consumption.   Also, more attic insulation was added and air leaks were sealed.  

The old unit was a 4-ton with 12 SEER rating, but the HVAC contractor said it was over-sized and installed a 2.5 ton High Efficiency 14 SEER model based on the specifics of our house.   We have 1600 sq feet of living area with a vaulted living room ceiling, covered porches, and double pane windows.  Layout is a standard ranch-style 3br/2ba.   We live in East Texas, and our summer temps are in the 80-100+ range.

Following A/C installation, we noticed a possible problem.   After we cooled the house to 78 degrees, the compressor started cycling on & off frequently.  We raised the house temp to 82 degrees and still had the same problem.   We currently have the thermostat set on 82, and the outside temp is 85.  

In the past four hours (late afternoon/ early evening), the unit has cycled on/off three times every hour, staying on 7-11 minutes each time.   FYI, the thermostat is in the hallway between the bedrooms, which isnít ideal because of minimal ventilation, but there wasnít a better location.   Temp variation throughout the house when the A/C is running is only +/- 3 degrees, so the location of the thermostat is probably not a problem.  
  
My research about central A/Cs indicated that the unit needs to run a certain length of time to avoid damage and offset the cost of starting up --- but I couldnít find out how long that should be.   Our ďonĒ times have been as short as 3 minutes and are often 6-12 minutes, but the compressor has been "on" for an hour or longer during the heat of the day.     

The contractor believes the unit is working properly, and he isnít concerned about possible damage due to the short run-times, saying power surges are more of a problem.  We want to make sure the A/C isnít short-cycling, which can damage the unit and increase our energy cost.

My questions:   
Is there a minimum amount of time that the unit should run after the compressor comes on?  
Can you refer me to a site with this information?

Thanks for your help.

Answer
Sandi:

Don't know about your thermostat, but a lot of them operate at three cycles per hour (3 CPH), a cycle being 60 minutes/hour divided by 3 CPH = 20 minutes. A cycle is the sum of the on and off times, so if the unit runs 7 minutes it won't restart for 13 minutes.

What's with the 78F to 82F setpoints? You do understand that will exacerbate the oversizing and cycling, right? You should try 75F, the required indoor design temperature.

"East Texas" is a pretty big place, I picked Lufkin (outside design 95F), so at 85F outside it was 10F below design. With your 82F setpoint, the TD was 85 - 82 = 3F as compared to the design TD of 95 - 75 = 20F. Your unit "cycled on/off three times every hour, staying on 7-11 minutes each time", so it was running about 45% of the time. I'm quite surprised the oversized unit (640 SF/Ton!!!) ran that much. Something's not right.

Another problem: If the duct system is balanced, room temperatures are within +/- 2F of thermostat setting; you're +/- 3F, so the system hasn't been balanced.

There is no minimum time, the run time is a function of outdoor temperature, thermostat setpoint and degree of oversizing. Short cycling increases energy use and shortens service life.

I don't like the location of the thermostat: There's got to be a lag between when you need cooling and when the thermostat figures that out. Please try to relocate it into a heavily-used room. That might improve the short cycling.

Heating, Air Conditioning, Fridge, HVAC

All Answers


Answers by Expert:


Ask Experts

Volunteer


Fred Weldin

Expertise

I can't help you if your whatsis won't work, people (Especially if it's a refrigerator); I have no experience with appliances, and I haven't been involved with H&AC service and repair since March 08, 1996 (Thank the Lord); I always send a "standard" reply to appliance questions and H&AC "Service and Repair" questions (about 20 a month), so if you want to know why your whatsis isn't working like it used to, ask someone else. A lot of my answers arenít read by the questioners; in that event, I always send reminders to read the answers for a month or more (the word "idiots" comes to mind - actually there's another more appropriate word, but discretion prevents its use here). If you have questions about how big a unit you need, if one room's warmer than the others, if you want an opinion oas to scope of work on bids received, etc. just ask. Do visit www.askweldin.com, there's a lot of good information on sizing, ductwork, efficiency, as well as some techniques accomplished DIY-ers can use to troubleshoot and improve their H&AC. Finally, I want to thank all of the kind, serious people whom I meet here: I enjoy working with you. My wife says I'm "snarky" from time to time; as for you others, please use your imagination as to the responses swirling on the snarky side of my brain when I read your questions.

Experience

53 years in the business. See www.askweldin.com

Organizations
ASHRAE, ICC.

Education/Credentials
B of ME U of D 1965, numerous classes, seminars and a lot of "Hands On" learning since.

©2016 About.com. All rights reserved.