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  1. #1
    Join Date
    Jul 2008
    Posts
    33

    Most energy efficient setback point for house in Dallas?

    We have a 1 story 2000 sq. ft. ranch-style house in Dallas with a 3 ton A/C that I beleive is about 12 years old. I would appreciate your advice on the best setpoints for the programmable thermostat to minimize energy usage. Specifically, whether to turn the A/C completely during the day when we are both at work.

    I have searched the forum but found conflicting advice and I think part of this may be due to homes in different climates (e.g. we are not as humid as Houston nor do we get much a break in the temperature at night as northern cities). For example, the high during the afternoon is usually around 98-100, and it may stay above 90 degrees until well after midnight. Even at 3am it may not get below 87.

    Here is my t-stat program currently:

    7:30AM - we leave house, A/C is set to 88 (highest it will go).
    6:00PM - A/C to 82. We usually arrive home between 6:30-7:30 and if the A/C has been off all day, the temp will be 86-87. If the A/C comes on at 6 then it will be a few degrees cooler when we walk in the door.
    8:30PM - A/C to 77. The house will then take until 11PM to reach this temperature, and will start cycling after midnight (it's still 90 outside). It will stay set at 77 until we leave at 7:30AM the next morning.

    On the weekends when we are usually home, I set it to 82 during the day and 77 at night. So we are ok with the comfort level at 82 right now. So am I making a mistake by setting the t-stat so high while we are at work? I don't mind coming home to a warm house, my main focus is on cutting our outrageous electricity bill.

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Roughly speaking, I think your meter may do the most spinning when you're trying to pull your house down from 88 degrees to 82, then 77. 88 degrees is a high heat load for an a/c system sized to hold interior temperatures in the mid-seventies, generally. A system holding a house at 75 degrees is working less hard than a system trying to pull a house down from 88 to 77. I imagine, going by your schedule above, that the system probably does not stop running from the time it kicks on at 88 to pull down to 82, or is just starting to cycle at 82 when suddenly it gets bumped down to 77. Then it runs flat out until late in the evening, when 77 is reached. So, say, from 6 PM to 12 midnight, about six hours, it is running flat out, and doing so with higher return air temperatures for most of that pulldown period.

    Since you are comfortable with 82 and subsequently 77, try lowering your setback (unoccupied) temperature to 84. This knocks off four degrees of pulldown. Watch the system's behavior over the next several days after you make the change. See if it reaches 77 sooner than midnight, with comparable outdoor conditions to what we're having now. See if the system begins to cycle sooner than midnight. A few things I'm thinking about...reduced pulldown time, although slightly counterbalanced by perhaps a few more run times during the day when you're not home. But...pulldown time will be shorter, and system will likely run less overnight once 77 degrees is attained in the PM.

    Setbacks work best with houses that do a good job of holding heat in during the winter, and holding cool air in during the summer. If your house is marginal at best toward this end, you'll be money ahead to tighten, insulate, and reduce heat gain to the attic, if possible.

  3. #3
    Join Date
    Mar 2003
    Posts
    7,314
    A study done by a major manufacturer indicated that one degree of setback per hour of time gone is in most cases the best answer. Its conflicting though, because some older homes have tremendous inefficiencies due to leakage. Depending upon humidity levels, it may be more beneficial to not setback that far. Allowing your unit to run every so often will keep humidity in check, thereby reducing the total load later. remeber it will cost you to dehumidify your entire house each time you setback if you go too far. All of your interior sirfaces like drywall, carpet ,wood etc will suck up moisture, and let it out when the ac is runnig. It is a delicate balance that you must learn to deal with in your own home.

  4. #4
    Join Date
    Jul 2003
    Location
    Texas
    Posts
    457
    This does not deal with setback, but with basic rates. In Dallas county, a basic kwh rate is around .158 cents per kwh.(not comitted to a term contract).
    Get signed up for a 1 or 2 year contract and it can drop to around .127 cents per kwh. In Carrollton, I use Cierro, not TXU, to get the lowest rate. The diference in a 2000kwh month is $316 vs $254.
    In Texas, you get to chose your electricity provider, in case you were not aware.

    I only use a 1 degree setback, preferring not to let the house contents heat up.

  5. #5
    Join Date
    Jun 2008
    Posts
    63

    forget setback

    I have gone through the same thing in Houston. Once the temps get close to design loads, around 95 for us, setback is non-efficient. You spend more energy getting the heat and humidity out of your house than maintaining a set temp.

    Setback does work in milder months when your system is technically oversized and can catch up quickly. The power companies tout setback in the hottest months as a way of saving energy. It's really a way to reduce their peak loads, and the consumer pays for it.

    Don't setback for a few days, read your meter daily and see which setting uses less kwh.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,758
    Shop. Your better at psychrometric charts then me.

    Whats the BTU content of a pound of dry air at 80F 50% RH.
    And what is it at 89F 40% RH.

    OP. Your doing about the best your going to. Changing your set back to less will provide quicker comfort, but not more savings. Raising it much more then you already set it back, won't have much more of a savings.

    To save more. You will need to make improvements to your homes insulation, window shading, and air infiltration.
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  7. #7
    Join Date
    Jul 2008
    Posts
    33
    Don't even get me started on the electric rates here. We were month-to-month and watched our rate go from 10cents/kwh to 12, to 15, to 18, and then to 20.5. When I saw the 20.5 coming I decided to lock in a year at 15. Of course, two weeks later, the price of natural gas has crashed and we could've paid 12.5. So now I have to decide between keeping 15 cents/kwh or paying $100 to break the contract and lock in 12.5. Ridiculous.

    Thanks for the other tips, I check the meter with the A/C set to 88 during the day and then try it set at 84 to see which is better. Though I feel that with daily variations in temperature, wind, and humidity, this may not be scientific enough to draw any conclusions. Currently, the A/C gets from 87 to 82 by the time the program runs from 6pm to 9pm. Then it takes until midnight to get to 77. So bascially it comes on at 6pm and runs continuously until midnight. I agree that if we didn't let it get so hot during the day, it would have less work to do in the evening. But I can also see the argument that all the "cold" we put into the house during the day is just going to leak out by 6pm and so it would be wasted.

    As far as the house goes, I have tried to make it more energy efficient. I installed a radiant barrier on the rafters, replaced the recessed lighting with airtight insulation rated cans, added more soffit ventiliation to the attic, and this weekend I'm going to blow cellulose insulation over the existing rockwool insulation. Windows all have blinds but as for the attic, even with the radiant barrier it still gets quite hot up there (I've measured air temp of 130 when it is +100 outside) and that's where the air handler and ducts are.

  8. #8
    Join Date
    Mar 2008
    Location
    Central Maryland
    Posts
    246
    Quote Originally Posted by soupcxan View Post
    As far as the house goes, I have tried to make it more energy efficient. I installed a radiant barrier on the rafters, replaced the recessed lighting with airtight insulation rated cans, added more soffit ventiliation to the attic, and this weekend I'm going to blow cellulose insulation over the existing rockwool insulation. Windows all have blinds but as for the attic, even with the radiant barrier it still gets quite hot up there (I've measured air temp of 130 when it is +100 outside) and that's where the air handler and ducts are.
    Sounds good. Make sure, if you have soffit vents, that the insulation is not blocking them.

    If you haven't already, consider getting a checkup/maintenance that includes- cleaning of inside and outside coils, static pressure check, and subcool/superheat check under daytime operating conditions. It will cost more than a regular checkup but less than a monthly electric bill, but may pay off big-time in electric savings.

    -HF

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by beenthere View Post
    Shop. Your better at psychrometric charts then me.

    Whats the BTU content of a pound of dry air at 80F 50% RH.
    And what is it at 89F 40% RH.
    80/50 = 31.2 btu/lb, dewpoint 59.7

    89/40 = 34.2 btu/lb, dewpoint 61.5

    Dewpoints are a bonus, no charge.

  10. #10
    Join Date
    Aug 2007
    Location
    Dallas, TX
    Posts
    43
    Our previous 12-month contract expried and we paid 24.9 cents for the month of June , shame on me for not keeping on top of it.

    Along the same lines, we keep our house 73 all day long (we like it cool), and then around 8pm we're all on the bedroom side of the house unitl the morning. Two zones on a single dampered unit, and we turn up the non-occupied side up to 80. So we're supplying just the occupied portion of the house (call it 40% by square footage), but the returns are pulling air from the entire house. There's two sets of double-doors seaprating the two spaces, and we've got positive pressure on the occupied side at night that wants us to keep them open a bit (and we do). Also, the occupied side is west-facing, all windows have shades or blinds and most have 80-90% blocking woven screens, but it's definitely got a heat loading issue. Same story with the radiant barrier, venting, and getting more blow in at the end of the summer, sealed my registers, and airtight baffles on most recessed lights.

    Anyway... turn the unoccupied side back down to 73 at 7am, takes no more than 60-90 minutes to cool down. 5ton Trane XL19i, 300 sqft, Dallas. The VS fan will not go too fast during this cool down.

    Since it's no more than 80 at night, and still <=80 during the cooldown, am I being smart here?

    Thanks for humoring me.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,758
    Cool.
    So a 2000 sq ft house, with 8 foot ceilings would have 16,000 CF of air, if empty.
    At sea level, using the posted BTU per pound of air.
    The higher drybulb, lower humidity air, would only have 3,609 more BTUs of heat in it.

    I pulled up a 14 SEER A/C.
    At 105&#176; OD temp, 80&#176;F DB 67&#176; WB, it pulls 3.05KW, at 32.9 MBH, 10.78 EER
    At 105&#176; OD temp, 85&#176;F DB 71&#176; WB, it pulls 3.17KW, at 34.9 MBH, 11.00 EER
    Sorry, can't find any 90&#176; ID temp listings.

    So actually, at the higher setback ID temp, the A/C would be more efficient.
    And as long as the return gas was cool enough for the compressor, be better for the compressor.

    JMO
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  12. #12
    Join Date
    Jul 2008
    Posts
    33
    Oh, I don't know if this affects the recommendations but we have 12 ft ceilings in most of the house, so even more air mass to cool.

  13. #13
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,062
    Quote Originally Posted by soupcxan View Post
    WHere is my t-stat program currently:

    7:30AM - we leave house, A/C is set to 88 (highest it will go).
    6:00PM - A/C to 82. We usually arrive home between 6:30-7:30 and if the A/C has been off all day, the temp will be 86-87. If the A/C comes on at 6 then it will be a few degrees cooler when we walk in the door.
    8:30PM - A/C to 77. The house will then take until 11PM to reach this temperature, and will start cycling after midnight (it's still 90 outside). It will stay set at 77 until we leave at 7:30AM the next morning.

    On the weekends when we are usually home, I set it to 82 during the day and 77 at night. So we are ok with the comfort level at 82 right now. So am I making a mistake by setting the t-stat so high while we are at work? I don't mind coming home to a warm house, my main focus is on cutting our outrageous electricity bill.
    You are lucky to have a/c that will do the quick pull down your scheduling. On part of this group thinks long runs are more eff. and make the last longer. It's interesting to here them talk against the longer run you have with the setup and setdown. Their fear that more customers will want a/c systems that are able pull down the temp 6-8^ in a few hours. Most here are recommended small systems that are undersized because of inability to control humidity during cool wet weather.

    Temperature setup saves energy provide the time is long enough. If the outside temperature is 20^F warmer than inside, a given heat transfer is established. When the inside temperature is raised 10^F, the heat transfers at 50% slower rate. The heat gained going rising up 10^F will have to be removed at nigh and is not a savings. But the slowed rate of transfer once the higher temperature is reach is all savings. So, a couple hours are probably not worth it. But 10 hours is different story. Learn to read your electric meter. Do it both ways and compare the results. A 4 ton a/c uses 4-5 KW per hour. Saving several hours 15-20 kw/day. I feel you will have better humidity control with an over-sized a/c like yours by seting the temp up during the day and getting a long run instead of short cycling through mid day. The equipment will last just as long if installed properly. The major limitation is your equipment ability to pull the temp down when you arrive and to have several hours of high temperture stability. I am doing some data logging but I only get to about 95^F in WI. I can cut my a/c by 50% by using temp setup during a hot day. It be less for your. Read your el meter and post the results. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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