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Thread: OT: Maximizing HVAC Efficiency via Cycle Times

  1. #1
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    Jun 2001
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    North Central Texas
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    Question OT: Maximizing HVAC Efficiency via Cycle Times

    Some years ago I installed a Trane AC/heat pump unit with a variable speed air handler. I added a return duct and put dampers in all the supplies. The unit never has any problem ‘keeping up,’ air balance is perfect and everything works extremely well.
    We keep the house pretty warm as summer intensifies, and summer has come early and with a vengeance. Since this is a pretty sharp group and several even have HVAC experience, I was hoping to get some thoughts on minimizing our electric usage.

    The thermostat has many changeable parameters. Sticking with applicable AC settings, I can change:
    Cooling cycle rate (default is 3 cph, and 2 or 5 are options)
    Minimum on time (d=10 min, 0 thru 15 op)
    Minimum off time (d=10 min, 0 thru 15 op)
    All non-default settings are explicitly ‘not recommended’
    There is also a ‘cooling droop’ setting that keeps the unit on longer, but brings the temp well below the set-point.
    I have the extended fan operation set for 60 seconds, IIRC.

    As I understand it, it is the starts that really cost you, and I would accept some temperature variation in exchange for lower electrical use. I do not want to do anything that would negatively affect the lifespan of the HVAC.

    So, should I maximize the off and on times, minimize the cycle rate, and will this make a meaningful difference in electric use?
    Any other thoughts on ways to minimize electric use are also appreciated.

    One other thing - the compressor has an insulating blanket and I am not confident that is the best idea for longevity in the summer heat. Additional compressor noise is NBD.

  2. #2
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    Aug 2003
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    Southwest Georgia, USA
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    Consider installing a high/low return. The house will fill from the top down with warm air in the winter, and from the bottom up with cool air in the summer. The house will be more comfortable, and you'll use less power. You can use a blanking panel to put in place of the filter when changing modes.

    The insulation on the compressor may be there to reduce the power requirement of the crankcase heater, if it has one.
    Last edited by winchman; 05-30-2018 at 02:37 AM.

  3. #3
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    Central Iowa
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    The blanket around the compressor is for sound deadening. If you do not mind annoying your neighbors or your family take it off. Frankly as its also a Heat Pump I would leave it on for compressor warmth in the winter. As a guy with over 30 years in the commercial HVAC/R trade, my free advice, leave the damn thermostat settings alone.

    OR If you want to create a $100 plus service call go ahead and mess all you want, but pick a week or so of really hot weather so when you mess something up your AC will be down that entire time.
    That Trane unit is one of the best in the business, I have one and I could have purchased other brands. I also have a Honeywell digital thermostat I just set and forget.

    If you want to save money on your AC, insulate more in the attic, ventilate the attic, put in new windows in and shade trees. The high - low return would help with your comfort. You folks in the south should be using high returns in the summer anyway.
    Last edited by wmgeorge; 05-30-2018 at 07:55 AM.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Fine Line Automation 4x4 Router, MakerGear M2 3D printer- 20 Watt Ray Fine Galvo fiber laser, LightObject 40 watt co2 Laser Engraver

  4. #4
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    Missouri
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    Why do you think that starts "really cost you"?

    There can be a certain time per start which is not effective for cooling, while the system is getting balanced again, maybe that is what you mean?

    What MAY "really cost you" is a cooling plan that allows the house to warm up, and then takes a long time to drag the temperature down again. Often it is better to keep the temp a bit more "even". You deal with a hot house, and the system works hard more of the time.

    It does not matter what the cooling plan is, the house takes a certain energy to keep at a certain temp. Insulation lowers the cost......
    Last edited by J Tiers; 05-30-2018 at 07:54 AM.
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    Hashim Khan

  5. #5
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    May 2013
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    I heavily insulated my house thirty years ago and bought a Trane furnace and air conditioner. Both were high efficiency at the time and rated well. I have enjoyed low energy bills and a comfortable house the entire time. The heat gain and loss surveys guided my purchase size of both the air conditioner and furnace. They both run long cycles during peak times Winter or Summer. I was given good advise by a relative in the furnace business and I took it. Mike

  6. #6
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    Joel I feel for you, it's getting hot down here too, though it's still cooling down to the low 70s overnight at least. I can't speak to HVAC specifics like wmgeorge can, but one thing to consider is a smart thermostat like a Nest. Our electricity company has a deal where they can use it to manage peak demand by precooling the house before peak hours in exchange for a $60 up front rebate off the thermostat and $30/year off your bill. It also switches to a lighter load (less cooling in summer, less heating in winter) when you're not in the house. Might not save you a huge amount, but it adds up.

    I'll also repeat what others have said - insulate! Extra insulation in the attic, fresh door seals (especially patio doors), blinds in the windows and ideally, external window shades (that's on my list of things to do). Plus make sure the filter is clean and give the indoor and outdoor coils a clean a couple of times a year. You'd be amazed at how scuzzy the coils can get even with regularly changed filters. the previous owner wasn't so good with that and had cats and dogs, so there was a disturbingly thick layer of dust and hair on the indoor coils. Cleaning that off dropped our electricity bill in half last summer! We haven't been here a full year yet, but last summer our bills were ~$170/mth, about $100/mth during winter and about $50/mth in the months in between when the AC or heating wasn't being used. We'll see how this summer goes!

  7. #7
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    Quote Originally Posted by Joel View Post
    Some years ago I installed a Trane AC/heat pump unit with a variable speed air handler. I added a return duct and put dampers in all the supplies. The unit never has any problem ‘keeping up,’ air balance is perfect and everything works extremely well.
    We keep the house pretty warm as summer intensifies, and summer has come early and with a vengeance. Since this is a pretty sharp group and several even have HVAC experience, I was hoping to get some thoughts on minimizing our electric usage.

    The thermostat has many changeable parameters. Sticking with applicable AC settings, I can change:
    Cooling cycle rate (default is 3 cph, and 2 or 5 are options)
    Minimum on time (d=10 min, 0 thru 15 op)
    Minimum off time (d=10 min, 0 thru 15 op)
    All non-default settings are explicitly ‘not recommended’
    There is also a ‘cooling droop’ setting that keeps the unit on longer, but brings the temp well below the set-point.
    I have the extended fan operation set for 60 seconds, IIRC.

    As I understand it, it is the starts that really cost you, and I would accept some temperature variation in exchange for lower electrical use. I do not want to do anything that would negatively affect the lifespan of the HVAC.

    So, should I maximize the off and on times, minimize the cycle rate, and will this make a meaningful difference in electric use?
    Any other thoughts on ways to minimize electric use are also appreciated.

    One other thing - the compressor has an insulating blanket and I am not confident that is the best idea for longevity in the summer heat. Additional compressor noise is NBD.
    Sounds like you're already setup well enough. If you're worried about AC electricity cost I think the best thing you can do is just raise the set temperature.
    Work hard play hard

  8. #8
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    One thing few people take into account is your clothes drier. When running it pulls unconditioned air from outside through door seals and other air leaks, heats it and pumps it outside. You have to cool or heat all that new air unless your drier has an outside air source. We try to dry clothes when the outside temp is closest to the thermostat setting. During the winter we direct the drier exhaust back into the house

  9. #9
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    Quote Originally Posted by Rex View Post
    One thing few people take into account is your clothes drier. When running it pulls unconditioned air from outside through door seals and other air leaks, heats it and pumps it outside. You have to cool or heat all that new air unless your drier has an outside air source. We try to dry clothes when the outside temp is closest to the thermostat setting. During the winter we direct the drier exhaust back into the house
    Same with bathroom exhaust fans, kitchen range hood, gas water heaters that don;t have a fresh air intake, etc. It's also better to dry cloths using the low humidity air in your house than the unconditioned air from outside.
    Work hard play hard

  10. #10
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    You can also put a "whole house" heat recovery unit at the outside air source. They are now required by code here for new construction.

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