Best Program For Thermostat for Best Efficiency - AVS Forum
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post #1 of 15 Old 03-18-2013, 09:07 AM - Thread Starter
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I didn't want to hijack Bud's thread, so I thought I'd ask here. Does anyone know the best practice for programming a thermostat in the Southeast? This sounds like a simple question, but here are some details that make it a little different (I think)

  1. New home
  2. Spray foam insulation
  3. 2-speed air handler
  4. 2-stage compressor
  5. 7-day programmable thermostats
  6. Summers have extended periods near or in excess of 100 degrees during the day
  7. Zoned System

Last summer I originally programmed our thermostats to let the house warm up 6 to 8 degrees during the day. Normal was 74 (which feels warm to me) and I think I let it warm up as much as 82. At 5:30 it was programmed to cool back down to 74. Our cooling bill was higher than expected, so I asked our HVAC contractor for recommendations. His response was that program was bad in this part of the country because we were asking our system to cool off the house in nearly the hottest part of the day. Still upper 90's by 5:30. His thoughts were that with a 2-stage compressor, and foam insulation, we were better off leaving the temperature at 74 all day. I tried that and our bill dropped by $50 or more the first month.

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post #2 of 15 Old 03-18-2013, 10:05 AM
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I've been told the same thing by HVAC contractors in NC. I think it's more efficient to let the unit maintain a steady temperature than to try to let it rest during the day and run wide open all night.
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post #3 of 15 Old 03-18-2013, 10:14 AM - Thread Starter
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I really started wondering about this whenI saw the new NEST thermostats at our local Lowes. I wonder how well they do here?

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post #4 of 15 Old 03-18-2013, 07:00 PM
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Can't wait to get my Nest installed. My thermo is a POS.
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post #5 of 15 Old 03-18-2013, 07:14 PM
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The thermostat I installed in our old house had learning or predictive logic in it. Instead of setting a time to begin cooling, it asks for the desired temp at a certain time. The unit would begin cooling or heating in advance of that.

I couldn't really compare cost-effectiveness, but it's in the middle ground in terms of functionality between the automated predictive capabilities of the Nest unit and the more traditional programmable thermostat. I was happy with the way it kept the house. In the winter, I would sometimes wake up when the heater kicked on about 30 minutes before the "be out of bed time" but I never was home when it would begin to cool in the summer afternoons.

Another thermostat I installed came pre-programmed with an EPA pattern, which the instructions explained were most efficient. I just let that program run - no complaints.
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post #6 of 15 Old 03-18-2013, 07:21 PM - Thread Starter
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What temp do you have your summertime cooling temp at? The more I read, the more it seems like 78 is the norm. That seems really warm to me, but I think it would make a BIG difference in how hard it is for the unit to catch back up in the afternoons.

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post #7 of 15 Old 03-19-2013, 09:27 AM
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Interesting thread......

Do you have any idea what your humidity level is like at 74? I bought one of those 3M thermostats and they come with a pre-defined setting.. it calls for 78,85 and 82(lat at night). I dont use it.

First year I ran 78 during the 3PM-6AM time. Came on at 3PM so that by 5 it could hit target. Downstairs was fixed at 78. All other times it was set at 85
Last year I ran 76 during the 3PM-AM time. Downstairs fixed at 76. All other times it was set at 82

I saw a very minimal increase in costs last year. This is in FL.

This year I am not sure what it will be, I have since added a return and balanced the upstairs, so I believe the the upstairs could go back to 78 and be comfortable.

Do you have progress energy? They have a cool little feature on the website where you can see your bills and the highs/low for the month. This way you can evaluate your changes with reference to outdoor temp/humidity.

Also another point of reference as you mentioned- insulation. Upper ceiling temps will hit 90 in the smack of summer for us. Attic temps I have recorded 125.
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post #8 of 15 Old 03-19-2013, 09:48 AM - Thread Starter
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Do you have any idea what the energy usage difference was compared to keeping the temp at a 78 or 76 all day? It's interesting that what you are describing is pretty much what our "learning" thermostats did on their own. Our house typically did not reach the high setpoint during the day. It would usually go up 4 or 5 degrees, and by 3 or 4 o'clock the unit would come on to start cooling things back down to meet the 5 o'clock setpoint.

Humidity was a tad high last year in the house. I'd say it hovered around 55% most of the time. Keep in mind that's based on the thermostats which are notoriously inaccurate. We've installed a dehumidifier (last week actually), but would prefer to let the AC handle dehumidification during the summer as it will be more efficient. I.e., pump the heat outside rather than inside.

Last year I was uncomfortable at 74 if I moved around at all. Since we've put in the dehumidifier, 74 feels a lot better with humidity levels around 50%. I'm going to bump up the t-stats to 75 and see how we like that for a while. I may focus a little more on humidity control this year, and see what happens.

We're on Alabama Power. I need to look into their data logging capabilities. That would really help with making changes to the system.

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post #9 of 15 Old 03-19-2013, 11:03 AM
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Hmm. I like the house kept around 70, winter or summer. 78 would feel like I was broiling in the Nebraska sun...
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post #10 of 15 Old 03-19-2013, 11:14 AM - Thread Starter
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+1.

I think the big difference that may not be reflected in the current studies is the two-stage compressor. Is it better to have a ~3 ton unit run for off and on throughout the day, or have a 5 ton unit run for a couple hours continuously. It no longer boils down to just the amount of time the unit is on, anymore.

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post #11 of 15 Old 03-19-2013, 12:22 PM
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JPA - I think you and I had talked about this before in your theater thread as you were building your house....what kind of cycle times are you getting with your system? Sometimes the efficiency of the spray foam and windows makes it so that your system runs long enough to get to the correct temperature, but not long enough to dehumidify. There could be some adjustments made to the system itself or in the more sophisticated areas of thermostat programming that will allow your system to run longer to dehumidify.

One of the challenges in our house that has 3 separate systems is that we have an open floor plan that acts like a chimney. We literally have to keep all the doors closed in the upstairs hallway to allow the heat to permeate on the main level when the MAIN level system is running. If we didn't shut the doors, all the heat simply fills the bedrooms and the first floor works a lot harder in the winter because it is essentially heating both the first and second floors. The reverse is true in the summer where we keep the doors closed yet again to keep the cool in the bedrooms. The only bad part about this is the builder didn't put returns in every room, so having a room constantly pressurized with only supply doesn't really move or mix the air well....at all.

I think the Nest is cool if you are in a single-system home where the Nest's sensing capabilities were able to fully work by having the thermostat in a prime location in the house that sees any of the traffic. But like Fred says, I think you can do just as good of a job by properly programming the right kind of programmable thermostat that has 4 set points throughout the day. Since you have a multi-zone system, I would probably just stick with the programmable thermostats specifically designed to run that system.

As for temps....we usually set the thermostat at 76 or 78 over the summer and 68 or 70 over the winter and we never heat/cool more than 5 degrees from our daily max / min temperature. Anything more than 5 degrees and you are wasting energy. Ideally you want the humidity right around 30%-55%, but most aim for 45% as "comfortable". Since you are in the deep South, you probably ONLY need dehumidification year round I am guessing. That is what we have here in Charlotte. Up in Pennsylvania we had a humidifier and it was definitely needed, that's for sure.
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post #12 of 15 Old 03-19-2013, 12:37 PM - Thread Starter
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We have an open floor plan that sounds similar to the situation you have. I don't remember what the cycle times were like during the day last summer, but I do remember that the humidity was always near 55% despite setting the t-stat's to 45% or 50%. The unit eventually reached its temp lockout, and quit running. I'm hoping that between the dehumidifier and the 2 stage compressor, we'll be able to deal with the humidity this year.

I'd like to setup the appropriate schedules as well, but it's an expensive science experiment if I get it wrong. I was just testing the waters to see if anyone had a similar experience regarding leaving the temps set the same all day. Of course, it would probably help quite a bit if I went on a diet, but it's easier just to turn down the AC smile.gif

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post #13 of 15 Old 03-19-2013, 01:04 PM
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You want to talk about expensive??? My wife loves to sleep in a cold room...and when I say cold I mean optimal white wine temperature cold. It is not uncommon during the winter for our bedroom to be 10 to 15 degrees above ambient temperature. So if it gets down to 25 degrees at night, our room will hover in the 39-40 degree range as an example. I've literally had to turn our system OFF because 50 degrees is the minimum low temperature our thermostat would allow. Without shutting the system off it will be producing heat while the windows are open and drawing in sub-freezing air.

And noooooooooooooooooooooooooo....don't think it gets better during the summer either. I've walked in when she has the thermostat in our bedroom set to 55 degrees just so she can sleep under a pile of covers. It goes well beyond not sweating while you sleep to excess IMHO. I foolishly invited her wrath by locking out changes on the thermostat with a password and thereby locking in very reasonable 24 hour settings on the thermostat, including a chilly 65 degree nighttime setting. You guessed it...that lasted all of the first night when she snuck over to the thermostat while I was sleeping, saying she couldn't change the temperature.

II may not know much, but I do know if I ever build a home again I will have a dedicated system specifically for the master bedroom and make the room highly insulated for efficiency reasons.
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post #14 of 15 Old 03-19-2013, 01:17 PM - Thread Starter
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Bahaha! That's funny! Maybe you should have a window unit put right above her side of the bed. Then you could hang a sheet over that so she has her own little refrigeration unit smile.gif I'm only partly joking. Sounds like it would be cheaper.

I have to admit, though. Sleeping with the window open when it's COLD outside is such a fond memory! I haven't been able to do that in a while, though. With a little guy in the room, it's 70 degrees, year round, when the baby is sleeping!

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post #15 of 15 Old 03-19-2013, 06:20 PM
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I read somewhere (might have even been on the power company's website) that for a programmable thermostat to it's best work the temp swings should not be more than 4-6 degrees. Now following that guide line I have my thermostat set as follows. Nighttime temp at 65 until 0530 when it kicks on to 70 for a warm up while everyone is getting up and ready for work school then kicks off about 0830 when everyone is out and gone. Back down to 65 until 1600 when it kicks up to 68 for the evening and then at 2200 goes to 65. This seems to work really well for me and the family. The summer times are the same but obviously cooling and the nighttime temp is 74 with cooling kicking down to 70 then back to 74 during the day and 70 for the evening until bedtime then back to 74. I have a tighter temp swing in the summer as I have found it is more expensive to cool my home. I have actually had to modify my temps with my new system as it is much more efficient than my older system. I found that on my old system in the summer 72 felt warm while with the new system 72 actually felt "chilly".

With regard to the temp swings I stated earlier. I think the more efficient your system is the more of a "swing" you can have to a point as it takes less energy to make up the temp difference. I should also say that while we are away all the interior doors are shut, and the windows that have blinds/drapes are pulled/closed. I think this makes a big difference as well.

Oh and it sounds like that TMcG's wife would be happier in an igloo!

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Now a Certified Carpet Counselor and Plumbing Counselor (Self given titles - pay no attention).
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