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  1. #1
    Join Date
    Apr 2005
    Location
    NE PA
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    698
    I recently read a study that using a night setback on a heat pump can have a significant efficiency impact. The study found that a 2 degree night setback can reduce overall efficiency of an air source heat pump by 25%. When the stat calls for the morning warm up, it will force the auxiliary resistance heat to turn on. This will result in a period of resistance heat being used daily, even if it is not needed due to low outdoor ambients. The result was up to a 25% reduction in average COP. It was noted that an outdoor lockout stat reduced the impact.

    We should advise customers of this (I have for years told owners to not set turn the temp down at night - but I never realized the impact it could have).

    Any of you out there ever seen a programmable stat that either walks up the setpoint 1 degree at a time, until the final setpoint is reached, or have the second stage call up disabled on a setpoint change of more than 1 degree ?

    Honeywell, if you are listening, here is idea to improve the VisionPro line.

    paul

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,381
    A good stat like the VP does try to keep the backup off. But there's such a thingie called the balance point. If the heat pump is near or below the balance point, the heat pump alone can't even maintain let alone warm the house back up. Some claim their stats will warm a house without backup even in bitter weather. Ain't possible! So I tell customers in mild weather, a backup may save money. In cold weather, no, no matter how good the stat.

  3. #3
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,209
    Baldloonie, you are right in a lots of ways. But if one has smart heat recovery feature, then they setback is not as much of an issue. Smart recovery raises the setback temperature in 1 degree increments, so the back-up heat does not activate. Whether, this is acceptable or not to the home owner, may have to be determined by actual performance. I'm going by my own experience, though, as I don't really know the balance point of my house. Last night it went down to around -5 F and my heat pump brought my house up to 70 by 10:30 AM. I have my aux heat locked out to 5 F so I must have been using some of the aux, but how much I can't determin. I am using intelligent heat staging and have 15KW of strip heat staged in 3.

    Thorton

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,381
    Pretty simple, plot your heat loss on graph paper opposite the output of the pump. Where they meet is the balance point. That's where the pump's output meets the home's heat loss.

    I have my backup locked out too. Once it gets down in the teens, my heat pump can't keep up let alone warm the house back up. My thermostat is plenty smart, it just can't do the impossible. My pump is about 1/2 ton oversized for the heat gain and the house is tight as a drum.

  5. #5
    Join Date
    Mar 2004
    Posts
    1,042
    Smart recovery can help a bit, but only when you're far enough above the balance point that the heat pump alone can raise the temperature as fast as the thermostat expects. For example, if your balance point is 30 and you set back 10 degrees overnight, come morning if it's 30 when the system starts trying to recover, you just aren't going to make it there without some aux heat. Under those conditions the heat pump would only be enough to break even, not to warm up things from where they are. Aggressive use of outdoor thermostats or auxiliary heat lockouts can help, but then a setback just results in missing your target temperature completely.

    The shame of setting back a heat pump is that you end up cycling off the heat pump- sacrificing your more economical heat source- only to have to catch up later with expensive strip (or fossil fuel) heat.

    To me the real feature the thermostat makers could consider is something to watch daily trends of indoor and outdoor temp to figure out where the balance point is, and then get aggressive about running the heat pump during above-balance-point conditions when it knows that there will also be below-balance-point conditions in less than 24 hours. Or simpler logic, even, like "run the heat pump unless indoor temp exceeds 72, but only use auxilary heat if indoor temp goes below 70". That would work sorta like having a wide staging differential but keeping the setpoint on the high side, except you wouldn't have to lie to the thermostat to stay comfortable.

  6. #6
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    Bald:

    I am fully aware of balance point. I do not think that is the issue. For example: I have a conventional heat pump in an addition of my house (separate zone). It has a balance point of about 12 to 15 deg, depending on wind. I can go 90%+ of the days and never see any strip heat. If I set back 2 degrees, the strip will come on every morning. If I diasble the strip, and keep the setback, it will still recover on the majority of days, just a bit slower. The ideal would be like wyounger suggested, a smarter stat that really knows when to call the second stage.

    Again, independent of balance point, a setback will generally cost people more money to operate due to the second stage call up each morning. I am just surprised that most "heat pump" designed stats do not have a "smart recovery" mode as standard.

    paul

  7. #7
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,209
    Originally posted by wyounger
    Smart recovery can help a bit, but only when you're far enough above the balance point that the heat pump alone can raise the temperature as fast as the thermostat expects. For example, if your balance point is 30 and you set back 10 degrees overnight, come morning if it's 30 when the system starts trying to recover, you just aren't going to make it there without some aux heat. Under those conditions the heat pump would only be enough to break even, not to warm up things from where they are. Aggressive use of outdoor thermostats or auxiliary heat lockouts can help, but then a setback just results in missing your target temperature completely.

    The shame of setting back a heat pump is that you end up cycling off the heat pump- sacrificing your more economical heat source- only to have to catch up later with expensive strip (or fossil fuel) heat.

    To me the real feature the thermostat makers could consider is something to watch daily trends of indoor and outdoor temp to figure out where the balance point is, and then get aggressive about running the heat pump during above-balance-point conditions when it knows that there will also be below-balance-point conditions in less than 24 hours. Or simpler logic, even, like "run the heat pump unless indoor temp exceeds 72, but only use auxilary heat if indoor temp goes below 70". That would work sorta like having a wide staging differential but keeping the setpoint on the high side, except you wouldn't have to lie to the thermostat to stay comfortable.

    wyounger you have a great idea there. Ah, the one about the thermistat trending the temperature of the indoor temperature and the outdoor temperature and then using these trends to raise or lower temps using aux or just heat pump or which one would do the job within the alloted time and the most economical. I think this would be called fuzzy logic. It wouldn't be hard to to this especially, if there is an outdoor sensor, right? Although, I agree with a lot of what your saying, I don't think that setback temps especially when a heat pump is involved is designed to be set back more then a couple of degrees or so. This being said you were talking about a 10 degree swing and definitely, a heat pump would not bring the house up to regular temp within a hour and half which is what smart heat recovery is all about.

    Nevertheless, bravo on your observations

    Thorton



    [Edited by thorton on 12-13-2005 at 07:31 PM]

  8. #8
    Join Date
    Sep 2002
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    For dual fuel systems, it would make a lot more sense to have two setback temperatures during one time period - one for heatpump and one for the furnace/electric heat strips.

    We all know that conventional gas and electric heating systems work well with more aggressive setbacks. When the outdoor temperature falls far below the balance point, it doesn't make sense to only setback by one to two degrees when the heatpump isn't going to run anyway.

    With outdoor temperature sensors becoming more common on high end thermostats, I don't see why it would pose a problem.

  9. #9
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,209
    Originally posted by amd
    For dual fuel systems, it would make a lot more sense to have two setback temperatures during one time period - one for heatpump and one for the furnace/electric heat strips.

    We all know that conventional gas and electric heating systems work well with more aggressive setbacks. When the outdoor temperature falls far below the balance point, it doesn't make sense to only setback by one to two degrees when the heatpump isn't going to run anyway.

    With outdoor temperature sensors becoming more common on high end thermostats, I don't see why it would pose a problem.
    Amd it makes sense to not set back the thermistat back more then a couple of degrees if it makes the aux come on at set-up temperature. Ten degrees is a way too much of a temperature swing for a heat pump to recover from, if it did, the heat pump would be a way oversized. Heat pumps are made to run for long periods of time at a lower supply air temperature. Actually any piece of heating or cooling equipment should run almost all of the time if its been properly sized for the coldest temperature that it will encounter or as in the case of cooling equipment, the hottest temperature that it will encounter.

    Thorton

    [Edited by thorton on 12-13-2005 at 08:10 PM]

  10. #10
    Join Date
    Jan 2005
    Location
    Perth - Western Australia
    Posts
    775
    i think you guys are totally missing the mark, most customers want it warm when they go to sleep, but when setteled in tend to get warmer & actually want the indoor temp cooler , it beats having to get up & turn it down, & as loonie said the t-stat will handle recovery when set up properly the vp 8000 does a good job of this & allso a 2 degree recovery is negligible even 4 degree a few cents worth of juice for the sake of comfort is nothing to worry about. should they be lucky enough to have a good 2 stage system such as a 16 seer, they can easily afford a little strip heat. sometimes overanalyzing the situation just doesnt help, in the end the customer will set it how they want anyway.i believe in the most efficient system first & second set up to satisfy the customers comfort level,
    just my 2 cents worth & it makes my customers really happy & they rave about there savings

  11. #11
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    As an HO, that is my biggest heatpump-specific worry about switching from gas to a heat pump. I would like a couple of degrees setback for comfort at night, not a lot - probably 2 degrees. The larger issue is that I'd like my side of the bed about 4 degrees cooler than my wife's. Which isn't really possible, so we meet in the middle with a 2 degree setback.

    Not to derail things, just to note that from an HO perspective setback is indeed a comfort thing too. I understand that I can't do the massive day setback that I can do with gas; the question then becomes figuring out how long the house needs to be unoccupied before a setback to 55 + using aux heat when I get back is worth it. My guess is that it's only when the house is empty for a weekend or more, but I don't have any numbers to back that up. It's just a guess.

  12. #12
    Join Date
    Apr 2005
    Location
    Kingston Ontario Canada
    Posts
    1,209
    Originally posted by captainrat
    i think you guys are totally missing the mark, most customers want it warm when they go to sleep, but when setteled in tend to get warmer & actually want the indoor temp cooler , it beats having to get up & turn it down, & as loonie said the t-stat will handle recovery when set up properly the vp 8000 does a good job of this & allso a 2 degree recovery is negligible even 4 degree a few cents worth of juice for the sake of comfort is nothing to worry about. should they be lucky enough to have a good 2 stage system such as a 16 seer, they can easily afford a little strip heat. sometimes overanalyzing the situation just doesnt help, in the end the customer will set it how they want anyway.i believe in the most efficient system first & second set up to satisfy the customers comfort level,
    just my 2 cents worth & it makes my customers really happy & they rave about there savings
    Hi Captainrat! I'm not quite sure what you are refering to about missing the mark. What I think we are trying to determin is whether it's advantagous to set back a thermistat so much that it will make the back-up heat activate when the set-up temperature is called for, which is usually the more expensive heat, or not. In a heating system where there is a gas furnace or an oil furnace being used, set back is a no brainer. You set it back with a set back thermistat as much as you want and then have it set-up to the temp you want, when you want it. But when a heat pump is involved there are different considerations. One being that heat pumps take much longer to recover, and two is if there is smart recovery available with a particular thermistat should it be used and how agressively, should it be used.

    Thorton

    [Edited by thorton on 12-13-2005 at 11:29 PM]

  13. #13
    Join Date
    Sep 2002
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    2,897
    Originally posted by thorton
    Originally posted by amd
    For dual fuel systems, it would make a lot more sense to have two setback temperatures during one time period - one for heatpump and one for the furnace/electric heat strips.

    We all know that conventional gas and electric heating systems work well with more aggressive setbacks. When the outdoor temperature falls far below the balance point, it doesn't make sense to only setback by one to two degrees when the heatpump isn't going to run anyway.

    With outdoor temperature sensors becoming more common on high end thermostats, I don't see why it would pose a problem.
    Amd it makes sense to not set back the thermistat back more then a couple of degrees if it makes the aux come on at set-up temperature. Ten degrees is a way too much of a temperature swing for a heat pump to recover from, if it did, the heat pump would be a way oversized. Heat pumps are made to run for long periods of time at a lower supply air temperature. Actually any piece of heating or cooling equipment should run almost all of the time if its been properly sized for the coldest temperature that it will encounter or as in the case of cooling equipment, the hottest temperature that it will encounter.

    Thorton

    [Edited by thorton on 12-13-2005 at 08:10 PM]
    By conventional gas/electric, I was referring to gas furnaces and electric resistance heat strips, not heatpumps.

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