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Thread: Heat Loss

  1. #1
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    Default Heat Loss

    Does anyone know what the normal rate of heat loss is for a contemporary house? Here's the situation:

    My house was built in 1995 and insulated to R-30 in the attic. Not sure about the walls probably R-21. My thermostat turns itself down to 58 at 10PM. With the outside temp around 40 degrees I lose about 2 degrees an hour. Thermostat may on come around 5:30 or 6am but its not scheduled to warm the house until 7AM where it brings the interior back to 68.

    Is 2 degree loss an hour acceptable? I certainly could add insulation but I have several vaulted ceilings and not sure what accessibility will be in those areas. So the cost benefit is a question.....

    //Rick

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  2. #2
    Ted Menelly's Avatar
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    Default Re: Heat Loss

    Quote Originally Posted by Rick Bunzel View Post
    Does anyone know what the normal rate of heat loss is for a contemporary house? Here's the situation:

    My house was built in 1995 and insulated to R-30 in the attic. Not sure about the walls probably R-21. My thermostat turns itself down to 58 at 10PM. With the outside temp around 40 degrees I lose about 2 degrees an hour. Thermostat may on come around 5:30 or 6am but its not scheduled to warm the house until 7AM where it brings the interior back to 68.

    Is 2 degree loss an hour acceptable? I certainly could add insulation but I have several vaulted ceilings and not sure what accessibility will be in those areas. So the cost benefit is a question.....

    //Rick
    2 degrees is nothing. You should be so lucky. As far as your walls being R21 then they must be six inch walls with an R19 at best for a six inch wall with some thin foam overlay at the exterior or 3 1/2 inch walls filled with foam.

    It also depends on whats going on outside as far as damp, dry, windy, calm. Also, you do know that a 58 degree temp takes a long time to heat back up and maintain 68 degrees. You could leave it warmer and still not spend anymore money. Keeping a stable temp in the home has a lot to do with savings. It is not just cooling down but the moisture content in the home is raising as well. When it gets down to 58 it is the entire envelope. Inside the furniture and wall cavities, the studs, everything. It takes a lot of energy to bring it back up.

    Of course you were not asking all those questions. 2 degrees an hour is not bad.


  3. #3
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    Default Re: Heat Loss

    Rick,
    Acceptable is your call. I don't know of a standard that measures heat loss per hour; most are energy used to maintain a comfort level. I would look at any penetrations in you vaulted ceilings. 1995 may or may not have sealed lights or electrical boxes. That would be a big heat leak. Seal up before adding insulation.
    Rick S

    Rick Sabatino
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  4. #4
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    Default Re: Heat Loss

    Quote Originally Posted by Rick Bunzel View Post
    Does anyone know what the normal rate of heat loss is for a contemporary house?
    There isn't such a thing. But if you want to increate the energy efficiency of your home there are two factors you need to be concerned about:

    1 - air sealing
    2 - insulation

    in that order. You might check with your utility company or state to see if they offer energy assessments. Thermal imaging and a blower door test will help you find areas to remediate.


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    Default Re: Heat Loss

    Quote Originally Posted by Ted Menelly View Post
    It also depends on whats going on outside as far as damp, dry, windy, calm. Also, you do know that a 58 degree temp takes a long time to heat back up and maintain 68 degrees. You could leave it warmer and still not spend anymore money. Keeping a stable temp in the home has a lot to do with savings. It is not just cooling down but the moisture content in the home is raising as well. When it gets down to 58 it is the entire envelope. Inside the furniture and wall cavities, the studs, everything. It takes a lot of energy to bring it back up.
    It is a myth that it takes more energy to heat the house back up than to maintain the temperature.

    Setback Thermostats | CMHC


  6. #6
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    Default Re: Heat Loss

    Setting a thermostat back too much such as a geothermal heat pump is not a good idea. The system could take up to an hour or just over that to bring the temp back up one degree.

    With geothermal its better to keep a constant temp rather than dropping it down more than 2 degrees.


  7. #7
    Bob Spermo's Avatar
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    Default Re: Heat Loss

    Here is a simple formula for heat loss

    BTUs/hr (lost or gained) = Delta T x Sq Ft (area) x U factor


  8. #8
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    Default Re: Heat Loss

    Quote Originally Posted by Bob Spermo View Post
    Here is a simple formula for heat loss

    BTUs/hr (lost or gained) = Delta T x Sq Ft (area) x U factor
    That formula only works for radiant heat loss (where U factor is known). In the US, where air barriers are not required in the code and air sealing requirements are rarely enforced, convective heat loss makes up a significant portion of a structure's total heat loss. It's probably easier to find and seal up those leaks than it is to determine the convective heat loss coefficient to derive a blended U factor.

    Dr. John Straube makes the point that rather than having tight houses with known air exchange points, we instead rely on building mistakes for our fresh air requirements. Put that way, it makes our current practices seem comical.


  9. #9
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    Default Re: Heat Loss

    Quote Originally Posted by David Wood View Post
    It is a myth that it takes more energy to heat the house back up than to maintain the temperature.

    Setback Thermostats | CMHC
    Notice that this study is for an empty house that is airtight and well insulated!
    From the study... Note that these savings are for two airtight, well-insulated, unoccupied houses. The savings in your home may vary but are likely to be in the same range.

    Once you add in the mass of furnishings, flooring, etc those items also cool or warm as the temp in the home dictates. They will radiate either cool or warm until they their temp equalizes. It does take more energy to cool or warm that increased mass over an empty home. It is best and more economical to try and maintain a level temp in a home. Granted, depending on what part of the country you are in this might change.

    We keep our 2000sf home around 65f in the winter and around 68f in the summer. My highest winter gas bill has been $107, that was last year when we had several weeks in the teens and single digits. My average gas bill is $42. The only gas appliance I have is heat so that is easy to track. My average electric cooling bill is around $200 to $230 during the summer months (June, July and August). In the winter my electric is around $90. My home was built in 2006.

    Last edited by Scott Patterson; 01-02-2012 at 08:55 AM.
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  10. #10
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    Default Re: Heat Loss

    Energy is lost through convection through the building envelope due to the difference between the inside and outside temperature and through air leakage. The greater the difference between the inside and outside, the greater the energy loss. Setting back the thermostat reduces the difference in temperature, as well as the heat energy in the air that leaks out through all the unintentional openings.

    Yes, it takes energy to heat up all of the contents of the house, but you get all of this energy back when the furnace is off and the house is cooling down.

    All of the utilities, the CMHC and NRCan up here advocate set back thermostats.


  11. #11
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    Default Re: Heat Loss

    Quote Originally Posted by Scott Patterson View Post
    Notice that this study is for an empty house that is airtight and well insulated!
    From the study... Note that these savings are for two airtight, well-insulated, unoccupied houses. The savings in your home may vary but are likely to be in the same range.

    Once you add in the mass of furnishings, flooring, etc those items also cool or warm as the temp in the home dictates. They will radiate either cool or warm until they their temp equalizes. It does take more energy to cool or warm that increased mass over an empty home. It is best and more economical to try and maintain a level temp in a home. Granted, depending on what part of the country you are in this might change.
    Scott, that's simply not true.

    First, objects don't radiate cool, they absorb heat. The "unoccupied" portion of the study was meant to control for behavior and heat sources that the researchers couldn't control.

    While it's true that the more mass you have to heat the more energy it will require to raise the temperature, it also means there is more heat energy stored to keep temperatures warm. This is the principle behind heat sinks and geothermal heat pumps. The heat energy you've used to heat the five tons of brick and concrete in the center of your house doesn't just disappear - that heat is radiated back to the rest of the house. As such, the only thing that matters is how fast the heat is escaping the house

    If it were true that keeping a constant temperature was more efficient, it would be true that it doesn't matter what either temperature is. Without getting into the math of it, you can see how this isn't the case with a simple thought exercise. If the exterior temperature is 68F and the interior temperature is 68F, you don't require any energy to maintain the interior temperature. If the exterior is 40F and the interior is 68F, you need more energy. If the exterior is -40F you need significantly more energy to maintain 68F. This is because the amount of energy required to maintain the interior temperature is a function of BOTH the U factor of the system AND the difference in temperature between the outside and inside.

    You can think of it like voltage: Air is a pretty good insulator between two conductors with a potential of 240V. However with a potential of 240000V air will be an insufficient insulator and you're going to get an arc.


  12. #12
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    Default Re: Heat Loss

    Quote Originally Posted by Corn Walker View Post
    Scott, that's simply not true.

    First, objects don't radiate cool, they absorb heat. The "unoccupied" portion of the study was meant to control for behavior and heat sources that the researchers couldn't control.

    While it's true that the more mass you have to heat the more energy it will require to raise the temperature, it also means there is more heat energy stored to keep temperatures warm. This is the principle behind heat sinks and geothermal heat pumps. The heat energy you've used to heat the five tons of brick and concrete in the center of your house doesn't just disappear - that heat is radiated back to the rest of the house. As such, the only thing that matters is how fast the heat is escaping the house

    If it were true that keeping a constant temperature was more efficient, it would be true that it doesn't matter what either temperature is. Without getting into the math of it, you can see how this isn't the case with a simple thought exercise. If the exterior temperature is 68F and the interior temperature is 68F, you don't require any energy to maintain the interior temperature. If the exterior is 40F and the interior is 68F, you need more energy. If the exterior is -40F you need significantly more energy to maintain 68F. This is because the amount of energy required to maintain the interior temperature is a function of BOTH the U factor of the system AND the difference in temperature between the outside and inside.

    You can think of it like voltage: Air is a pretty good insulator between two conductors with a potential of 240V. However with a potential of 240000V air will be an insufficient insulator and you're going to get an arc.
    OK.

    All of that makes sense but I just do not agree 100%.

    Scott Patterson, ACI
    Spring Hill, TN
    www.traceinspections.com

  13. #13
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    Default Re: Heat Loss

    I agree that temperature setback makes sense, physically and empirically. It's been done for decades; if it weren't more efficient than keeping a house at a steady temp, we'd have known about it long ago.

    Raymond, I'm wondering how geothermal came up, and why it's different. And are you talking water or air heating?

    I can understand how you wouldn't want to make big sudden changes or go to temp extremes with in-floor radiant, but don't understand why small changes are a problem with geothermal heat in general.

    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
    - James Burgh, 1754.

  14. #14
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    Default Re: Heat Loss

    Quote Originally Posted by Kristi Silber View Post
    Raymond, I'm wondering how geothermal came up, and why it's different. And are you talking water or air heating?

    I can understand how you wouldn't want to make big sudden changes or go to temp extremes with in-floor radiant, but don't understand why small changes are a problem with geothermal heat in general.
    Geothermal systems typically have a much slower recovery rate, so a large demand may engage the secondaries which are usually electric. However if you invest in a high quality programmable thermostat that can control geothermal heat pumps or one made specifically for geothermal systems you can avoid this problem.


  15. #15
    Ted Menelly's Avatar
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    Default Re: Heat Loss

    58 is uncomfortable, to most, even when you are sleeping. Your arms out of the blankets and you will move in your sleep to cover up repeatedly during the night.

    I can understand wanting heating savings but my comfort level rates a little higher on the heat scale.

    As far as the furnace heating the furniture and the 2x4s etc and costing you to heat but you gain that heat back when the furnace is shut down? Nice in theory but you are still losing heat that you have to heat back up again.

    I am not arguing with anyone but if you want to maintain reasonable comfort then the thermostat needs to be higher and not touching it, in almost all homes, with the exception of the perfect homes will cost you more money.

    As stated by many. Comfort is in the individual. A restless sleep being too hot or too cool when a steady thermostat to maintain a modicum of comfort is what I call less expensive.

    Heat loos is heat loss. In both the body and home. It still costs to heat it back up. Bring it back up when awake? You must be uncomfortable in the cold.

    Just an opinion.


  16. #16
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    Default Re: Heat Loss

    Quote Originally Posted by Ted Menelly View Post
    I can understand wanting heating savings but my comfort level rates a little higher on the heat scale.
    You're talking about temperature, not heat here.

    As far as the furnace heating the furniture and the 2x4s etc and costing you to heat but you gain that heat back when the furnace is shut down? Nice in theory but you are still losing heat that you have to heat back up again.
    It's nice in both theory AND practice. Now the 2x4s cost you in heat transfer, because they work as a thermal bridge. That's why a 24" stud wall has better thermal performance than a 16" stud wall. But if you put a giant concrete cube in the middle of your house and heated it to 68F, you could set your thermostat back and your temperature would drop a lot slower than if your house was completely empty.

    As stated by many. Comfort is in the individual. A restless sleep being too hot or too cool when a steady thermostat to maintain a modicum of comfort is what I call less expensive.
    You may prefer to sleep at 72F or 68F, but that's your individual comfort. I get too hot and prefer a cooler sleeping temperature. Were we in identical houses and all else being equal, I'd save energy dollars setting my thermostat back at night.

    Heat loos is heat loss. In both the body and home. It still costs to heat it back up. Bring it back up when awake? You must be uncomfortable in the cold.

    Just an opinion.
    My thermostat brings the temp back up to 68 an hour before I wake so I don't have that problem.

    Last edited by Corn Walker; 01-02-2012 at 08:45 PM. Reason: clarification

  17. #17
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    Default Re: Heat Loss

    However if you invest in a high quality programmable thermostat that can control geothermal heat pumps or one made specifically for geothermal systems you can avoid this problem.
    The thermostat will control when the geo unit goes into 2nd stage and 3 rd stage electric resistance back up or gas.

    Again it is not wise to set a geo unit below a 1 or 2 degree set temp regardless what thermostat is used. Otherwise you will spend more on electricity to bring the temp back up quickly.

    My geothermal unit has a set back thermostat and I keep the temp at constant 70F. It runs 99 percent of the time in stage one. I have never had the electric resistance (10kw) come on or needed it.

    My unit is a closed ground loop. It pumps a water/glycol mix through the ground picking up the latent heat in the ground. The temp of the incoming line is approx 45F.
    After going through the heat exchanger the water exiting and going back out to the ground loop is 40F. So 5 degrees has been picked up and used to heat the house.


  18. #18
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    Default Re: Heat Loss

    Rick,
    You post an interesting question on heat loss in a contemporary home.
    The problem is that your home may or may not have been built to the standards (with out errors) for 1995.

    You could perform a study on your home to see how your home is performing.

    • Using a Manual-J which will give you the heating requirements for your home.
    • Then you can use the actual temperature drop in relation to the exterior temperatures to develop the data.
    • Then create the formula equation to determine if your home is performing to the expected heating requirements.
    • By plotting the a graph, may be easier to visualize than formula equation differences for comparison.
    This probably has been done, but I can not find the equation nor a program to accomplish the task. I admit I have not looked to hard. The project would be interesting along with the results.

    Thought of having an energy audit done, but do not think that they would give you the answer that you are looking for.


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