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Which of these two setbacks is more economical?

sunlight33 Member Posts: 311
Like what the title says, one is to setback the heating curve by 3 degrees at night and keep the thermostat constant, the other one is to set the thermostat by 3 degrees while keeping the heating curve constant. I have all baseboard heating, boiler is Vitoden 200 with outdoor reset function, and Vitotrols which will give boiler an indoor temp feedback. Thanks!


  • Zman
    Zman Member Posts: 6,562

    Why do you want to change the heating curve?Do the heat loss characteristics of the building change at night?

    Why not just leave the t-stat and curve alone? That would  be efficient.

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • kcopp
    kcopp Member Posts: 3,717
    I agree...

    if you have the vito 200 why mess w/ the boiler reset.
  • Gordy
    Gordy Member Posts: 9,514

    Setback obsession, set it and forget it.

    You are defeating the purpose of the whole big V logic with either choice.
  • remodel
    remodel Member Posts: 68
    so night setback, talking about a curve adjustment

    not an actual high limit (room stat setback).  If the boiler is sized right wouldn't a shift in the curve produce lower temp water thereby producing more condensation and lower BTU so everyone can sleep a little cooler.  Once night set back is off in the morning the boiler produces warmer water and more BTUs and a warmer room.  If the boiler is not oversized she should still not cycle on and off, just a way to trick some degrees out of the boiler, or do you guys just run cooler water to bedrooms and be done with it.   
  • RobG
    RobG Member Posts: 1,850

    That is what God invented TRV's for :)

  • remodel
    remodel Member Posts: 68
    TRVs are great but

    don't they just regulate the room, i.e. throttle back the BTU output for that room.  So what you are saying is that bedroom stays cooler all day and night due to TRV throttling back flow and if I want a warmer room I turn TRV up.  Yeah but still seems like the curve setback is doing that at the control, instead of at the radiator and when I wake up I have same flow but warmer water,  at night I have dt =130-110 and in the morning I have dt = 160-140 same btus from the boiler just less btus from emitter. 
  • sunlight33
    sunlight33 Member Posts: 311
    edited February 2014
    Here's my motivation

    I thought that if I could lower the return temp by a few degrees by decreasing the curve then it would make boiler condense even more efficiently.
  • Gordy
    Gordy Member Posts: 9,514
    oversizing emitters

    Should be part of the deign goal to achieve lower average water temps.

    If emitters were in place, and this is a new boiler added to old emitters did you calculate emitter size? They may be already over sized IF envelope improvements were made after initial emitter sizing. Sometimes installers errored on the side of oversized.

    Remember condensing is not the major part of a Mod/Cons efficiency, and savings. Its the ability to throttle the burner to the load. Same goes for TRVs. Dont get all wrapped up in the condensing end.
  • archibald tuttle
    archibald tuttle Member Posts: 801
    how much baseboard do you have?

    are you really running the baseboard under 140 much? 

    So here is the complication, if you use setback, you actually need to set the reset curve up - ideally it would do this itself automatically by judging the difference between conditioned space temp and conditioned space target temp compared to outdoor temp. so the comeback from the setback will be no condensing except maybe a couple minutes for the first pass of cold water.

    to figure out whether steady state operation might be more or close to as efficient, you have to figure out if you are actually condensing and at what outdoor temps and how often you experience those temps.

    it is still more btu efficient to turn your heat down at night.  sorry for all the eurofinatics out there.

    what is your temp. zone, i.e. where do you live? Unless you're super insulated and got uninhibited long walls of all high capacity element or double element, i just don't see you condensing much anyway so setback the thermostat -- my no-radiant areas we use a 10 degree setback (who knew ladies got cheaper to keep with maturity) although you got to have really good insulation and no sheathing air leaks in area of baseboard or run glycol if you're going to do that.

    don't know the algorithm on the vitodens.  but if it doesn't have conditioned space sensor as part of input and doesn't know you are way below setpoint, it will not adjust curves up so you have to start the recovery much earlier than you need.  mine is set for 4 AM so its warm when i roust at 6:30.
  • Gordy
    Gordy Member Posts: 9,514

    If you are setting back 10 degrees, and actually achieving a 10 degree drop in temp for guessing 6 hour setback time frame. You either need some serious insulation up grades, or you never hit that 10 degree drop in temp until your system comes out of set back.

    Either way the btus you think you saved during that period of set back will get added to the backside coming out of setback as the system will run longer, and at a higher water temp to attain the higher set point.

    If your desire is to have cooler sleeping temps (not for everyone) , and no occupancy in the sleeping areas during the day I would suggest zoning for the said areas, and still maintain one set point.
  • sunlight33
    sunlight33 Member Posts: 311
    edited February 2014
    I see

    I am in zone 5A. So it does seem more complicated than I thought. It's a 20+ year old house, walls are 4'' (R13 fiberglass), attic has about R50 of cellulose. By the way, I do prefer sleeping in a slightly cooler temp (about 3 degrees cooler), so that's why I asked if I should drop the 3 on thermostat or the Vitotrol (heating curve)?
  • archibald tuttle
    archibald tuttle Member Posts: 801
    Gordy's right . . . eer and wrong i think

    You're right. We don't always make the bottom of our setback if its shoulder season and we don't always make the top of our setups when its friggin cold out, but i set the times involved with a good deal of anticipation.

    But insofar as i understand the laws of thermodynamics, the closer the temp. of the conditioned and unconditioned spaces the less the heat loss.  ergo you lose less btus when it is cooler inside. and you don't start losing the same amount of btus again until the space is fully out of setback.

    yes you spend some input btus coming out, but those inputs are strictly related to what you lost and you lost less when the building was colder.

    the key exception would be how the efficiency of your burner responds to different water temps.  the obvious is at the low end but if your target temp is above condensing, your system water is not going to be condensing temp except the first pass through when come out of setback.

    the main relevance to not setting back, as i understand it, is if you could run at a low mod in condensing mode the whole time, but i just don't think that is as likely with the emitters you have. do you know what temps you run, taken some sample target temps vs. outdoor.  is your target keyed to feed or return temp.

    the real issue is: can you get much done with 135 deg or cooler water and baseboard?

    i run baseboard in half the house with a fairly aggressive reset and doubled element in the larger spaces. the pick up is noticeably slow. its 27 out right now and my return target is 135.  so if you have enough element and insulation maybe when its above 25 you're condensing somewhat to more as it gets warmer. but setting back won't take you out of condensing unless your controller sets the curves up for quicker make up.

    so you can do a lot of anticipating, start the setback as early as insulation and house characteristics allow, then bring it back at 4 in the morning.

    i think the main knock on setback is you can't get that stuff right and outdoor conditions may prevent ideal comfort if you don't recover in time, and with limited savings why bother.  On the other hand, I'm not convinced you have a bad idea about just lowering the reset curves if your controller allows you to do that by hours.  This could well bring more condensing operation.  Mine are not time sensitive. 

    as far as the mod in mod-con goes, again, i disagree a little bit about where the major efficiency lies. My understanding of thermodynamics here is that the mod alone mostly saves short cycling on less than full load, it doesn't raise efficiency of op without employing condensing mode.

    even with low load on a non-mod, it ramps more quickly towards target temp, stack temps are going to be a little higher while the water temp is higher so that is slight loss of efficiency but the assuming same target water temps and modulated fire the boiler is going to run at those less efficient water temps longer. with non-mod it not only goes through the cooler but also the warmer water part of the cycle more quickly.

    the issue, it seems to me, might be if the cycle is so short there is a purge and maybe post purge which sends heat up the chimney for maybe 20 or 30 secs total when no heat is being made.  so those safety purges are sources of wasting heat and short cycles would add to that waste.

    summary,  as long as you don't set your curves up for quicker response and undermine condensing, turning the thermostat down saves input btus.

    very low temp, hi-mass radiation doesn't change temperature fast enough to make it as logical a candidate for setback, but that is because most of the controller strategies I've seen to date seem to drawn on the steady-states-ville model from europe. but i don't thinnk it is that difficult to imagine a controller hooked up to a little weather station feeding it wind, light/sun as well as temp, and to internet forcecast data that actually sets back low temp hydronic - as well as anticipating effects of those weather condtions on conditioned space temp when not in setback.

  • SWEI
    SWEI Member Posts: 7,356

    > The main relevance to not setting back, as i understand it, is if you

    could run at a low mod in condensing mode the whole time

    The "main relevance here" is that recovery from setback requires a boiler sized larger than the design load.

    > My understanding of thermodynamics here is that the mod alone mostly

    saves short cycling on less than full load, it doesn't raise efficiency

    of op without employing condensing mode

    Modulation more closely tracks the load, reducing standby losses.  As the return water temperature drops, efficiency increases.  At some point along that curve, flue gas condensation begins.  The exact point at which that happens is a fraction of a degree below the last point at which it was not occurring.  There is nothing fundamentally different about those two points (the way there is for something like a phase change.)
  • archibald tuttle
    archibald tuttle Member Posts: 801
    the SWEIt spot

    I think we are saying approximately the same thing.  I said the efficiency will go down as return water temp climbs.  You said efficiency will go up as return water temp goes down.

    But, interestingly, his original question about whether to setback temp or setback his curves gets at the point that, short of a perfectly insulated house running water at the desired conditioned room temp, the key decider of needed water temp is the difference between indoor and outdoor temp. And forgetting the condensation point question which has a particular temp or range around a particular temp associated with it, this is really a differential temp question.

    So if you lower your curves at night or your target lower indoor temp with a thermostat, that would require lower temp water thus more efficiency. if you raise temps or raise the curves back only to your normal operating, recovery will be slow but efficiency no worse than normal for any given outdoor temp.  If you raise them higher for recovery, you lose some efficiency on recovery as Gordy was implying , but i doubt you lose more than you gain.  maybe it gets close enough that you won't find convenient equipment with this capacity designed in. But that is different than contending that you actually don't save energy by turning down at night.

    I have taken this all by rule of thumb that reducing the difference between indoor and outdoor temp (as in setback) saves more than the temps required for recovery, if one is patient and if condensing is taken out of the equation.

    I know that is qualitative and instinctual, not quantitative and scientific. Maybe somebody with the numbers can chime in as to the changes in efficiency over the range of non-condesning return temps and for that matter through the range of partial to fully condensing temps.

    But for any given target temp, the advantage of mod is still to prevent short cycling in accomplishing that return temp. . . no?

    Don't get me wrong, my instinct is that load matching is a desirable characteristic, just as load matching circulators seem a great innovation. But right now we are paying a pretty serious premium for these technologies and mod has been all but married to con, so folks who could benefit from mod but don't need con are paying for both. Maybe I'm overly fixated on this, and I think the cons will have longer life and need less maintenance in non-condensing use so maybe there is an adequate tradeoff.

    This is also maybe epitomized by the long arguments here recently about using on demand hot water heaters as boilers. There you get mod, on the edge of con, regulated by flow, and a price point that works.  I know all the boiler manufacturers  at AHRExpo were bemoaning the market is too full of competition and they can't get price points with return.  To me, boilers seem to cost twice what they should, yet boiler manufacturers aren't making any money. Something is askew here, regulations, liability, I don't know if what all plays into this, but if we got all these people chasing a sweet spot boiler and I'm not finding anyone delivering an entry level 100,000 btu mod con noticeably under 2 grand. I though that was where we were headed but the market seems to moving the opposite direction. Making me wish i could just keep buying the old heatmakers for a thousand bucks and be done with it.

  • sunlight33
    sunlight33 Member Posts: 311
    holy ****

    This is some deep stuff.
  • Gordy
    Gordy Member Posts: 9,514
    edited February 2014
    As deep as

    People want to make it. ;))

    Honestly if you take your heat loss program, and plug in theoretical set backs. Theoretical set points for the indoor temp you will see that at design conditions it's not enough to get excited about unless you are on oil, or LP this year, possibly electric depending on your kW rates.

    Theoretically Brian may be correct, but it's such a small amount. As far as set back goes lots of variables.

    In my case for 2000 sf home with a set back of 5 degrees for 6 hours is about 100 bucks a 6 month season on NG for fuel. It's actually less because the heat loss program is basing everything on a design day which is not all the time so there you have it. I did not get that anal about the calculations so it's probably a lot less than 100 bucks a season.

    SWEI, and Brian are speaking different tongues. Brian is assuming a boiler is generously sized to pick up that deep set back probably still using boost to get there. On the other hand SWEI (Kurt ) is banking on just enough boiler to off set the heat loss there fore maximizing efficiency with a closely matched heat source. I won't speak for Kurt though.

    Edit: this is enough said unless you want to go deeper? A mans home is his castle, and the MRT makes it a castle, or a cave. I shall relish in the fact I can sit in my recliner with my tightly whites, or wear sweats with a blanket? Hmmmmm.

  • SWEI
    SWEI Member Posts: 7,356
    Well spoken

    And I'm actually not opposed to setting back the water temp from the boiler to see how it all works and feels.   My experience is that setting back a thermostat with a tight ODR curve is generally an exercise in futility.

    Note that there's still more stuffing inside a mod/con boiler than a tankless.  The methods of modulation are quite different on most gas tankless as well.  Then you factor in the sales volumes and the distribution channels and it starts to make some sense.
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