Using Setback T-Stat with Modulating Furnace (?)
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  1. #1
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    Using Setback T-Stat with Modulating Furnace (?)

    Is it fair to say that after upgrading to a York/Coleman/Luxaire modulating furnace previous experience with thermostat setbacks (on a 70% eff) must be unlearned since the modulating unit will not recover very quickly?

    100K LP9C furnace Heat Loss est at 17F (Current OD temp) = 52K. Furnace running over 1 hr to raise indoor temp by 2F (Remeber, the furnace only starts at 35% eff so it takes a while before the unit achieves ballance point...)

    Is there a general guideline that ballances comfort and efficiency for these newer units during cold spells? Based upon recovery time it seems that the ecconomic cost savings for turning the stat back 4F isn't worth waiting for the house to warm back up in the morning.

  2. #2
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    Its is recomended that you do NOT use set back with the York/Luxaire/Coleman mods.

    Because of it taking long to recover.

    If your stat has Adaptive Recovery enabled. It should figure out how many hours early to start heating though.
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  3. #3
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    setback is bad

    We have installed many luxaire modulating furnaces and the only complaint I have ever heard is that when "I come home from work my house is cold". These furnaces are designed for comfort and eff. They operate at their peak eff. when temp. is maintained in the space. If you turn it down a few degrees at night for comfort reasons then so be it. You will not save on your gas bills by turning it down during the day. I have expirimented in my own home where I have a two-stage with variable speed blower. I maintain same temp all day and pay less in gas bills than when I would turn it down during the day. Setback is BAD.

  4. #4
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    Thanks guys.

    I wasn't aware of the economics. I'm used to sleeping in a cold house (for comfort). and 4F takes forever to recover. Is this unique to the Johnson controls family of furnaces (e.g., would I have had the same issue with a 3 stage Bryant). Can't change anything now, Just want to know if I should kick myself for not understanding the limitations of a modulating furnace.

  5. #5
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    I cannot speak for the York/Luxaire/Coleman brand of operation but most of the multi-stage units, when operated by the proper t-stat, do limit operation at lower outputs to accommodate temperature recovery. They do this a couple of different ways. On some, if the deman for a call for heat is instituted simultaneously at both the 1st + 2nd stage terminals, it will immediately go to 2nd stage. But if the 1st stage precedes the 2nd stage by just 1/10th of a second, then it will operate on 1st stage for up to 10-minutes before stepping up to 2nd stage, assuming 2nd stage is calling. This is also true in slightly varied form on the 3-stage furnaces. Those intimate with the Y/L/C line are better suited to advise on what is proper operation for recovery in the morning. IMO, you should be able to have a nice cool bedroom and then have good, quick recovery in the morning. If you don't get a satisfactory answer from your installing company, it may help to place a call to the customer service line of the manufacturer and walk through the operation with them. There could be some hidden issue that needs to be flushed out.
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  6. #6
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    The long recovery time your having is due to the way the UPG line mod is controled.
    Since it uses a single stage thermostat.
    Its rate of modulation increase is based on previous run times.

    Other brands would recover quicker because the signal from the stat would force them to higher modulations quicker.

    The UPG mod, is the only mod that uses a single stage thermostat and still is a true modulating furnace.

    It is a draw back, and a plus at the same time.
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  7. #7
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    Quote Originally Posted by beenthere View Post
    Its is recomended that you do NOT use set back with the York/Luxaire/Coleman mods.

    Because of it taking long to recover.

    If your stat has Adaptive Recovery enabled. It should figure out how many hours early to start heating though.
    Old thread, I know, but do you have any links to the manufacturers of mods that state that setbacks should not be used?
    An answer without a question is meaningless.
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  8. #8
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    Quote Originally Posted by furnacedr View Post
    We have installed many luxaire modulating furnaces and the only complaint I have ever heard is that when "I come home from work my house is cold". These furnaces are designed for comfort and eff. They operate at their peak eff. when temp. is maintained in the space. If you turn it down a few degrees at night for comfort reasons then so be it. You will not save on your gas bills by turning it down during the day. I have expirimented in my own home where I have a two-stage with variable speed blower. I maintain same temp all day and pay less in gas bills than when I would turn it down during the day. Setback is BAD.
    Interesting.

    I wonder if this has to do with btu per inch surface area of heat exchanger ratio going UP at turn down? Energy transfer is about surface area.

    Or possibly due to slower airflow across the heat exchanger being able to pull more BTU from condensation?

    Certainly there is savings due to significant drop in ECM amp draw at lower static.

    Maybe there is also savings in that the duct is cooler. I suppose duct doesn't spend time delivering cold air as it's "warming up" at each cycle because there are no cycles.

    I know gentler, longer operation usually means much more even temps throughout the house.



    The wetheads seem to have known lower and slower wins the race.

    Holohan wrote about it 20 years ago: http://bit.ly/danholohan1992allshookup

    Bean has a great site that talks about comfort and efficiency: http://bit.ly/beanNthermostats
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  9. #9
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    Quote Originally Posted by tipsrfine View Post
    Old thread, I know, but do you have any links to the manufacturers of mods that state that setbacks should not be used?
    No I don't. The guy doing the training on them told us that at the course.
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  10. #10
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    Sounds like more of a controls issue that a difference in unit effciency. The modulating controls prefer "feed forward" requests and to follow it's loop control logic, rather than being reactive. I think Infinity controls are somewhat similar is that regard as well. The system works better with adaptive recovery than reasponding reactively to sudden setpoint changes. I think it tends to be less liekly to overshoot, especailly in a smaller home or one when the furnace is a little oversized.

    OF coruse as always, controls are influence greatly by thermostat placement and airflow characteristics.

  11. #11
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    I love that you guys are thinking about these things with me.

    Shortcircuit used setback last winter and has agreed to stop this winter. If we are lucky and have similar winters, we might have some better understanding of value.

    (If I know when and what a thing will save I can model it and sell it. If I don't, selling it with promises of "savings" is not ethical.)
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  12. #12
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    Quote Originally Posted by motoguy128 View Post
    Sounds like more of a controls issue that a difference in unit effciency. The modulating controls prefer "feed forward" requests and to follow it's loop control logic, rather than being reactive. I think Infinity controls are somewhat similar is that regard as well. The system works better with adaptive recovery than reasponding reactively to sudden setpoint changes. I think it tends to be less liekly to overshoot, especailly in a smaller home or one when the furnace is a little oversized.

    OF coruse as always, controls are influence greatly by thermostat placement and airflow characteristics.
    When a thermostat is coming out of set back, the York mod doesn't know its coming out of set back. So if runs based on the previous run times of set back. And recovers slowly. The thermostat doesn't know its controlling a mod that isn't going to come up to high fire quickly. So the stat doesn't know to start recovery as early as it should.

    The slow recovery is the only reason that they recommend against using set back with it.

    Setting the stat to aggressive recovery helps though.
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