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Thread: OT: Maximizing HVAC Efficiency via Cycle Times

  1. #11

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    The thermostat minimum on and off times are likely to protect the equipment from inadvertent short cycles. As an engineer I've never taken the time to understand why my window AC unit says not to restart it within 5 minutes or some such, but it does. These settings shouldn't effect your overall power consumption. Similarly I doubt the cycles per hour will either, since the dT between inside and outside is not really going to change. Setting the CPH to 2 vice 3 may gain you a little efficiency with fewer starts and longer run times, but I doubt it'll be much, and you likely will have larger temperature swings. Shading or reflective film on south and west facing windows, passive attic ventilation, and more attic insulation are all good suggestions. I'd also evaluate whether your air ducts are in conditioned or unconditioned spaces. Any ducts in unconditioned spaces should be insulated and all ducts should be well sealed.

  2. #12
    Join Date
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    way i understand it - the greater the dT the greater the energy loss, so letting the house go longer between temperature adjustments will save money - also, the equipment doesn't work harder if you let the house warm up - it works just as hard whenever it is running - it may run longer but if there is a long enough delay between cycles you will save money - thermodynamics

  3. #13
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    This was my understanding as well 01-7700.

    There are currently no discernible temperature swings, and as stated in the OP, some variation would be just fine. Unless the temps are extreme, you never even know the hvac exists. No noise, no swings, even temps, no noticeable air movement - the house is just magically at the exact temp you set (the thermostat never even comes off the set temp). The variable speed air handler is really nice when coupled with the well designed system.
    I have no problem tolerating a little temp swing if longer/less frequent cycling has a useful effect on electrical use and no detrimental effect on the equipment. It seems as if the improvement might be modest at best.

    The electric bill is low - the reason I am looking for a 'little bit more', is due to the unusual electric plan I signed up for. There were no good ‘per kilowatt hour’ plans, so I went for an oddball that has a crazy low rate if you stay under a set limit, but penalizes you heavily if you go over. It is very easy to stay under unless the temps are extreme (which is only 3-4 months per year), but summer came early and we are using right under the limit. Being able to save a few kWh would make it easy to keep from going over.

    ANYhow, thanks for the info everyone - lots of good advice!
    I did put an additional layer of insulation in the attic when I moved in and remodeled, have double panes (cheap ones though), and added roof vents and many additional eave vents. The house is quite tight, condenser/evaporator coils are kept clean and there is a ceiling fan in every room. Ducts are in the attic, but are insulated and well sealed. Someone is usually home, so it is infrequent that the thermostat gets set back.
    Having a choice of electric providers has done more than anything, cutting the bill by over 50%. There is no NG out here, so the house is all electric. Winters are mild and the heat pump runs much of the time on the coldest days of the year (so sizing appears ok), but the AC brings the temp right down and doesn’t run very long, even in the worst heat of the summer.

    Will think about what I can do about the dryer. I put a seal on the door, mostly for noise, and the door is usually closed when the dryer is running. I wonder if it is therefore starving for air somewhat.
    I have thought about the whole house heat exchangers, but never moved on it. Common up north, but very unusual down here.

    The 2 large main returns are high, the small ones in the MBR and guest BR are low. Not sure I can do much to change them in a useful manner at this point, but this is all the sort of advice I was hoping for.
    Last edited by Joel; 06-12-2018 at 06:55 PM.

  4. #14
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    Quote Originally Posted by wmgeorge View Post
    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.
    I would like to understand in more detail why changing the settings a bit would be catastrophic. I have not changed anything but the droop and extended fan run, and was hoping for someone with expertise to elucidate. As mentioned in the OP, I can only change the CPH to 2 and increase the on/off times by 5 min.
    As stated, longevity of the equipment is a priority, but I honestly don't know how increasing the cycle time by a few minutes and reducing the cycles per hour by 1 could harm the equipment. A little educating would be helpful.

    It sounds like you have a history rife with homeowners messing things up.
    Despite having been responsible for maybe a few dozen units, I have never had to call out a tech. The usual problems have been caps or fan motors. Once, I had a problem where the condenser fan would only run intermittently, but an hour or so later I isolated it to a cracked solder joint on the control board. Was pleased it could be fixed right then and for free! This was on MY (current) Trane XL unit, unfortunately. I did do my homework before choosing it, and it sounds like I made the right choice. Bought it right before they transitioned from R22 though.

    The blanket around the compressor is for sound deadening. If you do not mind annoying your neighbors or your family take it off.
    As stated in the OP, noise is not a problem. We are on a few acres and cannot hear the unit in the house. Longevity of the compressor is the primary consideration. We have hot summers and mild winters, and I thought that might give a nod toward removal.

  5. #15
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    Quote Originally Posted by Joel View Post
    The electric bill is low - the reason I am looking for a 'little bit more', is due to the unusual electric plan I signed up for. There were no good ‘per kilowatt hour’ plans, so I went for an oddball that has a crazy low rate if you stay under a set limit, but penalizes you heavily if you go over. It is very easy to stay under unless the temps are extreme (which is only 3-4 months per year), but summer came early and we are using right under the limit. Being able to save a few kWh would make it easy to keep from going over.
    Build 6 small bunk houses out back and bring electric service to each one of them. Sign up for the same oddball plan on all six and then siphon power from them to power your main house. Bingo. CHA--CHING.... Somebody StOp Me! WHAM, BAM!! thank-you MAM!! Low electric bills without going over on any of the services

    EDIT: Or better yet, convert your house into a 6 apartments (on paper, but not really). Then order 6 separate metered service for the "apartments". WHAMM! KAA_ZAMM!! Thank me later for the low electric bills!
    Last edited by 3 Phase Lightbulb; 06-12-2018 at 07:38 PM.
    Work hard play hard

  6. #16
    Join Date
    Apr 2002
    Location
    Central Iowa
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    When I was teaching the HVAC/R program I had access to the computer based ACCA Manual J load calculation software. My equipment in my home was old, 1988 era. After I had the house walls insulated (brick so from the inside) and replaced the windows I did the calculation and also had my students run for practice. I downsized my furnace from 80,000 to 60,000 Btu's heating (natural gas) but with a two stage burner and ECM motor, same size AC new unit only it was a 14 SEER from a 10. Filter was a Honeywell box with 4 inch pleated filter that is changed once per year.

    My heating / cooling bills nearly cut in half. Here is the part nobody will like, I set the digital thermostat at one temperature day or night and never change. It gets set lower or higher if we are gone for a few days other than that it never changes. Thermostat is set at the defaults. Set at 74 F in summer, 73 F in winter.

    My year around budget monthly billing cost for electric and gas is $146. Electric is 9 cents Kwh, gas is market value whatever in the Midwest. House and 4 seasons porch 1600 sf, garage is 24x24 heated to 45 except when I am working in it. Trane furnace, Goodman AC unit. Central Iowa.

    Insulate your house or space, get the right sized unit. NOT the same as existing and have it put in the right way. Btu's are Btu's do all the engineering you want but energy savings are in the basics. If your tech can't do a H&C load calculation, can not size equipment or size ductwork, get someone else.

    Oh and I also worked in the trade for nearly 30 years.
    Last edited by wmgeorge; 06-13-2018 at 08:05 AM.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - MakerGear M2 3D printer- 20 Watt Ray Fine Galvo fiber laser, LightObject 40 watt co2 Laser Engraver

  7. #17

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    Quote Originally Posted by 01-7700 View Post
    way i understand it - the greater the dT the greater the energy loss, so letting the house go longer between temperature adjustments will save money - also, the equipment doesn't work harder if you let the house warm up - it works just as hard whenever it is running - it may run longer but if there is a long enough delay between cycles you will save money - thermodynamics
    A greater temperature swing around a set point will not save money compared to a smaller swing due to difference in dT, the average dT is still the same. To impact dT you have to actually change the middle point of the range (t-stat setpoint closer to outside temp).

    Everything else you said is correct though, reducing cycling should result in a modest increase in efficiency and the equipment isn't running any harder. Equipment is made to run.

    For both of the above reasons, programmable thermostats setback for periods of in-occupancy absolutely will save money if they can be used.

  8. #18
    Join Date
    Apr 2002
    Location
    Central Iowa
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    If your running an air to air heat pump setting back is a bad idea. When they need to catch up for heat that means usually running the resistance back up heat. Best systems us gas heat for the back up to air source heat pumps. If you really want to save with a HP system, put in ground source or geothermal. Pricey but pay back is fast depending, depending on where you live. But natural gas heating cost is hard to beat per Btu and maintained costs.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - MakerGear M2 3D printer- 20 Watt Ray Fine Galvo fiber laser, LightObject 40 watt co2 Laser Engraver

  9. #19

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    wmgeorge, the OP is cooling not heating. From the sounds of it his system will recover in a "catch up" situation just fine.

  10. #20
    Join Date
    Apr 2014
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    I've seen software demoed on programs like This Old House or Hometime where they put the floor plan and orientation/location into it and let it direct what to put where. Is this kind of detail useful for getting an efficient HVAC setup?

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