New Thermostat Short Cycling A/C

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Tenderpaw

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I have a Bryant single stage heat pump with electric back up heat installed when my house was built in 1999. My thermostat from the beginning was a Honeywell Chronotherm programable that my wife has always hated because she is to short to read it.

Last weekend i installed a Lennox CS 7500 thermostat that i bought new in the box off of Ebay. I really like it's styling and it is easy to use. I installed it and programed it per the instructions and tested that it turned on the A/C, heatpump, and backup heat properly. Since installing, i have notice the A/C seems to short cycle. Today i did a test of the new thermostat and the old thermostat.

New Lennox CS 7500 thermostat:
set at 78 with a 2.5 degree temp differential
outside temp 95 degrees
Starting inside temperature of 80 degrees
Cycled on for 7 minutes and dropped the temp from 80 to 78 and then turned off
Stayed off for 10 minutes and then cycled on again for around 7-8 minutes. I continued to do this for the next hour
I verified temps displayed on thermostat with external thermometer

Old Honeywell Chronotherm:
Set at 78 with unknown temp differential
Outside temp 95 degrees
Starting temp 80 degrees
Cycled on and ran for 25 minutes and cooled the house to 76.5
Cycled off for 7 minutes and then cycled on again when the temp in the house was 78 degrees, ran for 15 minutes and dropped the temp to 76 degrees and then shut off again. It repeated this process for the next hour.

Setting aside that the Lennox is more accurate temperature-wise than the Honeywell, what gives? I noticed the Lennox would drop the temp to the setpoint and shut off immediately. The Honeywell ran longer.

Any advice on what i should change in the setup program on the Lennox stat?

Thanks,

Dave
 

Dana

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Independent of the thermostat issue, if it's pulling down the temp fully 2F in 7-8 minutes when it's 95F outside, the AC is ridiculously oversized for the cooling load (not that you can do much about that until it's time to replace it.) If right sized the system would run continuously (or nearly so) when it's that hot outside, taking several tens of minutes per on-cycle, but it would still keep up. Sounds like you are well under a 50% duty cycle, at a temp that may be near or even exceed your 1% outside design temp.

What model Bryant is this, how big is your house, and what are your 1% and 99% outside design temperatures?
 

Tenderpaw

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The house is a story and 1/2 with 1900 sq ft downstairs and 1000 sq ft upstairs. This is the downstairs unit I'm dealing with and they told me when i built the house that they installed a 4 ton unit downstairs and a 3 ton upstairs. The upstairs does NOT have short cycles. The model number on the sticker on the unit is 697CN048-A. Sorry, but i don't know what the design temps are. The house is in the Houston area where average high temps in the summer are mid to upper 90's. We probably see 5-10 days a year with temps above 100.

The new stat seems to come on at 2 degrees above set temp and immediately shuts off when it reaches the set temp. Setting aside that the old stat is cooling the house 2 degrees below the setting, it also seems to stay on for several minutes after it reaches what it thinks is the set temp temp.

Thanks.
 
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Dana

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Houston's 1% outside design temp is 94F (which means that for ~87 hours out of a year in an average year it will be hotter than 94F, sometimes much hotter.) It gets over 100F at my house too (in a location with a 1% outside design temp of 83F), but outdoor air temperature accounts for but a fraction of the cooling load. Humidity and direct solar gains through the windows & roof are major factors (it's never really humid in Houston, is it? :) ) In other words, the cooling load isn't directly proportional to the difference in indoor & outdoor temperature. In sticky-steamy gulf coast areas like Houston the air temperature difference doesn't account for more than half the peak load.

A typical first floor cooling load in a reasonably tight house would be about a ton per 1200-1500' of conditioned space, and for a second floor with a code-min insulated attic about a ton per 1000'. If the ducts are in an attic above the insulation it adds another ton if the ducts aren't insulated, maybe a half-ton with R8 ducts. Based on your 7-8 minutes-on / 10 minutes-off duty cycle the 4 ton unit for the downstairs is more than 2x oversized for the load but not 3x. A 2-tonner could handle your load (with margin!), and it would handle the humidity better than the 4 tonner, delivering higher comfort (really!) A duty cycle of 8 minutes on out of every 18 with a 4 ton compressor is an implied load of 4 tons x (8/18)= 1.55 tons, which for a 1900' first floor zone works out to a ratio of a ton per 1225'- a totally credible number for a 1.5 story first floor.

If you want to nail it down to the nth degree on sizing you could use a (relatively) cheap datalogger/timer to track duty cycles, using the methods outlined by this HVAC & building consultant based in Atlanta.

The 3-tonner upstairs is also likely to be sub-optimally oversized, even if there are uninsulated ducts in an attic space outside the insulation. That too could be determined by duty cycle analysis on the existing system.
 

Tenderpaw

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Thanks for that analysis. Since my system is still reliable despite its age, I'd like to delay replacement as long as possible. Based on the system size and how both stats are performing, what would you recommend I do? Set a 3 degree temp swing in the setup program and set the temp 1-2 degrees cooler? Thanks.
 

Dana

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I wasn't suggesting that your replace it soon, only that the oversizing is a strong contributor to the cycling problem. When you do replace it, installing something more appropriately sized would be the "right" thing to do. Somewhere between age 20- 25 years most are getting ready for retirement, (even if they're still working). That's still a handful of years or more off, but within in a decade it'll likely be nearing the top of the priority list.

The bigger the temperature swing you can program into it, the longer the cycles will be, so yes, start there.
 

Tenderpaw

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I wasn't suggesting that your replace it soon, only that the oversizing is a strong contributor to the cycling problem. When you do replace it, installing something more appropriately sized would be the "right" thing to do. Somewhere between age 20- 25 years most are getting ready for retirement, (even if they're still working). That's still a handful of years or more off, but within in a decade it'll likely be nearing the top of the priority list.

The bigger the temperature swing you can program into it, the longer the cycles will be, so yes, start there.
Thanks for your help. I'm going to put your analysis in my file for when I do replace the systems!
 

Dana

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You owe it to yourself to take the time to do more careful duty-cycle measurements rather than one web-forum napkin math exercise on an extremely small & imprecise sample set. With better data and a more precise number the risks of blowing it go down. While I'm pretty confident it will consistently measure at less than 2 tons, there is also a real possibility that it'll come in under 1.5 tons given multiple and higher precision duty cycle measurements.

It's counter intuitive, but smaller is generally better for comfort, as long as it still covers the peak loads.
 

WorthFlorida

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I looked up the installation manual for CS7500. I think you may have a program issue. Besure that you have it set for one stage cool. It might be optioned for two stage cool. But check all the settings. Below is one example that in auto mode when you want to cool it to 75 at 8:00am, around two hours earlier it will start to run to get the temp down to 75 by 8:00 not in 10 minutes after it powers up at 6:00am. It defaults to enable this feature. Disable this feature then a 8:00am the a/c turns on to run it to the set temperature.

Do you have the dehumidification set to enable? Check it and disable it.

Be sure you're running in cool only and not auto heat/cool. Your dead band might be kicking in and turning off cooling. Do you have the outdoor therm sensor? Just go through all the program settings again.

Your old thermostat might have have been a mercury bulb and the new one is not. Just check that when the A/C is blowing that there is no cold air running down inside the wall of the thermostat that might be affecting the temp sensor. You just need to check how things are different from the old and the new. Good job by at least comparing the two thermostats but now you need to know how what to look for what they're different. The user guide is very good and one of the best I've seen on explaining programming.


SMOOTH SET RECOVERY(SSR) Options are enable or disable. Default is enable. When enabled, smooth set back begins recovery up to two hours before the programmed time so that the programmed temperature is reached at the corresponding programmed event time. Assume 12°F per hour for first stage gas/electric heating and 6°F per hour for first stage compressor based heating or cooling. With Smooth Set Back disabled, the control will start a recovery at the programmed time.
 
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Jadnashua

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Some of the smarter programmable thermostats learn the response rate of the overall system, and will start to heat or cool at whatever time required to get to your set point at the time set. Some use a fixed 'before' time, some don't have that feature at all.

The location of your thermostat could be part of the reason why it is not working as you desire...it must be out of direct sunlight, preferably not on an exterior wall, and out of direct register (or whatever you're cooling outlet is) air stream so it can get a real sense of the space's temp, not the direct outlet. But, if your system is significantly oversized, it is hard to get your set point without either overshooting, or lots of inefficient cycling. Things work best if the system runs more than some minimum timeframe, as on/off cycles are bad for both wear and comfort. Some thermostats are adjustable for the amount of hysteresis to help prevent short cycling (some of that is setup based on selecting the type of system programmed).
 

Tenderpaw

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Thanks for the program advice. I'm running the old stat temporarily but will put the new one back this weekend. Regarding settings, I'm fairly certain it was set for the following:
Single stage cooling
Dehumidification off
Smooth set recovery off
Running in cool only mode
No outdoor sensor
5 degree deadband
2 degree temp differential
60 second fan run after shutoff

I don't think there is a draft in the wall. I'm pretty sure the old stat us non- Mercury as it clicks on start up.

Do I want the SSR on or off with my system? I'm not sure on that, but it was off for this install.

Thanks
 

DonL

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Do I want the SSR on or off with my system? I'm not sure on that, but it was off for this install.

Disabled or just set the temperature to Hold for testing. Enabled saves energy when the house is not occupied.

When the T-Stat is new, It may take a 1 week to learn its surroundings, and your life patterns.

I think you are worried about nothing. If your wife does not like it, Turn it off and see how she likes that. ;)
 
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WorthFlorida

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Your settings look good to start. My suggestion was to make it a dumb thermostat and see how it goes, then enable features as you like. Problem with these new smart thermostats you don't know what criteria is turning things off or on unless the display shows it. You might want to get an infrared thermometer. I use one at work to check the temperature reading of a thermostat (there are 11 of them). Usually the wall temperature and the face of the thermostat display are within 1 degree of each other. Just a quick read that the thermostat is displaying the real room temperature.

http://www.homedepot.com/p/Klein-Tools-12-1-Infrared-Thermometer-IR1000/202330832
 
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