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Energy Efficiency - HVAC

  1. Jun 1, 2008 #1
    As I fine tune our VAV HVAC system (506,000 sq.ft. facility), we'd like to determine the effectiveness of our adjustments over time. As we both heat and cool portions of the building at the same time (electric VAV reheat), we need to apply degree days to the analysis. How is that done? Sometimes we're in heating degree days - sometimes cooling degree days. I'm guessing sometimes we're in both heating and cooling. I have access to total monthly KWHr & Therms of gas used.
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  3. Jun 1, 2008 #2


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    Hi Greg'sDad!! You're son has done some incredible things, such as create this community.

    Uhm, he won't give you an honorary contributorship? Greg!!! Turn your dad gold, come on, you can afford it. :smile:
  4. Jun 1, 2008 #3


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    Greg'sDad, I have no idea what the answer is to your question.

    But I can tell you that your son has created a unique and fantastic site for those of us who like to talk to scientifically-minded people. You have every reason to be very, very proud!
  5. Jun 1, 2008 #4


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    A degree day is a day's worth of a certain number of degrees above or below the 65F equilbrium temperature (the temperature at which no heating or cooling is required). Ie, if the temperature is a flat 55 F for one day, that's (65-55)*1=10 heating degree days. Note: depending on the type of facility, the 65F equilibrium temp or even the heating degree day method itself may not be valid. Some parts of office buildings, for example, require air conditioning year-round. But here it is:

    Since heat transfer is directly proportional to temperature difference, you can directly proportion degree days to calculate energy use. For example, if your usage one month is 1 MBTU and there were 1,000 heating degree days that month, if the next month has 500 degree days, your usage should be 500 MBTU.

    With the amount of money at stake for such a large building, you may want to consider hiring an engineer to do a study like this. They can quickly identify efficiency issues with your systems.

    [edit] Oh, you're that Greg's dad? Yeah, he's created quite an internet phenomena here.
    Last edited: Jun 1, 2008
  6. Jun 1, 2008 #5
    Welcome, Greg's Dad. Thanks for creating the creator. :wink:
  7. Jun 2, 2008 #6
    Hello there, I might be this this forum's next one-post wonder, but I've got a background in Physics and energy-efficiency so I think I might be able to offer some useful advice on this thread...

    Anyway, it looks like I can't post links until I've posted to 15 or more threads, so please message me if the links I was going to post sound like they might be helpful.

    I think the most important thing to realize is that degree-day-based calculations are generally very approximate, as there are far too many factors that it's very difficult to account for, and that can affect results considerably. I wrote an article about the problems with degree days that I think you'd find useful. (message me for the link)

    Anyway, the standard thing to do would be to correlate kWh energy consumption against degree days to determine a "baseline" consumption against which you'd compare future values. Once you have a past-performance-based formula that describes how kWh depend on degree days, you can plug in new figures to see if they show improvement compared to the "baseline". The article I referenced above describes this in quite a bit more detail.

    It's all rather complicated by the fact that you have both heating and cooling going on. Again, the article above has more on this issue, but essentially you should do your best to separate out the heating and cooling consumption (ideally you'd have them both metered separately), and perhaps just ignore any periods when both are operating together. Degree-day-based calculations are inaccurate at the best of times, but they become much more so when outside temperatures are near to the base temperature of the building, and when you have heating and cooling operating on the same day.

    If your heating and cooling is set up well you'll need heating and cooling degree days with different base temperatures. That's because ideally the temperature below which the heating comes on would be a few degrees lower than the temperature above which the cooling comes on (so that the two don't come on together, battle it out, and consume huge amounts of energy unnecessarily).

    I'd suggest you estimate the base temperature of your building (or ideally the separately-metered areas of your building) by starting with the temperature that your building is heated to, and then knocking off a few degrees for "internal heat gains" - the free heating that you get from people and equipment. Once you've estimated the base temperature (sometimes called the "balance point"), try correlating kWh with degree days for a few base temperatures around that point. It's impossible to estimate the best base temperature exactly, but you can find it experimentally by correlating with different base temperatures until you find the best fit.

    My company recently developed a free website that you can use to calculate degree days to any base temperature, and for loads of locations worldwide (particularly loads in the US). Again, please message me and I'll send you the link. This probably sounds like a plug... and it is... but to be fair I'm not aware of any other source of degree days that offers all the options you get at our site - options that are rather important for energy-efficiency-related calculations.

    If you've got weekly or monthly meter readings, you can use our site to generate weekly or monthly degree day values. Or, if you have less regular meter readings, you can get daily degree days from our site, and then sum them together into blocks that correspond with the periods over which you have measured kWh values.
  8. Jun 2, 2008 #7


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    Are you limited to using the numbers from the monthly bill? Or can you get a meter reading yourself, any time you want? If you can do that, you can monitor performance daily or weekly, rather than having to wait a whole month to evaluate things.

    Good luck.
  9. Jun 2, 2008 #8


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    Sounds like a homework question to me......

    And define your terms young man! What on earth is a VAV?

    Ok. Enough joking around. Your kid has created the only web forum that I thought was worthy of financial contribution. And I've seen plenty.

    Ok. Back to the original question.

    And the answer is; I don't know.
    We'll need to know the square footage footprint of the building.
    Is it 700ft x 700ft single level factory, or is it a multilevel building with a much smaller footprint.
    Are there heat sources making some parts unbearably uncomfortable (aka; the kitchen)?
    While other parts of the building filled with sedentary secretaries are freezing cold.
    Have you determined the heat capacity of the building?
    Where on earth is this building?


    There are many variables involved in complex problems.

    And Welcome!
  10. Jun 2, 2008 #9

    Welcome Geg'sDad:smile:

    The company in the link below can supply you with all of the meteorological records from your area including degree days. It is a pay for service company.


    Most local Electric and Gas utilities keep degree day records as a part of operational functions and they may be available for free.
  11. Jun 3, 2008 #10


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    I don't think any of that matters. He just wants to see if adjustments to the HVAC system make for a more efficient heating and cooling process.

    It may be as simple as dividing kWH by Cooling Degree Days as a measure of cooling efficiency, and dividing Therms by Heating Degree Days as a measure of heating efficiency.
  12. Jun 3, 2008 #11
    Thanks for all the comments. VAV = variable air volume system. We provide tempered air year round to the areas of the building, and reheat with electricity. Electricity is used for cooling and heating all year round. It's a 4 story mostly office building which is 2 1/2 years new.....
  13. Jun 3, 2008 #12


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    Hmm..... Brand new building..... This sounds more like a human comfort level than an efficiency question. Just start turning down the heat in the winter and the cooling in the summer. When the level of phone calls complaining about the temperature exceed your tolerance level, adjust the temperature 2 degrees the other way. If the phone calls persist, adjust the temperatures accordingly. When the phone calls drop to an acceptable level, you have reached your optimal settings.

    I should restate two things before I get blasted for improperness:

    The original problem:

    This had two potential answers: The human one, and the mechanical one.
    Human: Keep the temperature at around 72'F everywhere at all times.
    Mechanical: Turn the entire system off.

    Well, I'd never heard of a degree day, or a VAV until yesterday, but I think I understand them now.

    I think neither of them are really critical to the actual problem; human comfort level.

    So I guess my answer to the question is that you should just look at your electric bill at the end of each month, and compare the volume of voices of the bean counters vs occupants.
  14. Jun 4, 2008 #13


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    VAV systems provide ~55 F air because that is necessary to dehumidify the air. But if it is using a lot of reheat in the spring-summer-fall, that means that the minimum air volume is set too high (people often set them too high for some unknown reason).

    In winter, a little bit of outside air mixed in with the return air gives you cooler air (perhaps 55 F, but the controls should have a reset schedule to bump that up if it can work at a higher temp). Then the VAV box goes to perhaps 1/3 or 1/2 flow and the reheat does the rest. Or, you can have fan-powered boxes on the perimeter, where heat is most needed - those require only a small amount of air from the rooftop unit, but have a fan to circulate room air through the heating coil.
  15. Jun 4, 2008 #14
    The usual reason to do a regression analysis with degree days is to see if the heating control is actually working as well as it should. In theory, the better the system is operating, the tighter the correlation between energy consumption and degree days.

    Poor control can be due to mechanical problems, or human ones (people opening windows when the heating makes it too hot, or thermostats set wrong).

    You can get free degree days, both heating and cooling, at Degree Days.net
  16. Dec 28, 2008 #15
    Happy New Year to all!

    Here's an update....

    We've lowered our electric bill by $250,000 this year (last year's bill was $1.3M) even with rising rates! I attribute the bulk of the success to lowering the minimum VAV cooling cfm (much less VAV reheating) and raising the rooftop HVAC unit supply air temperature (again, much less VAV reheating). We also cut back on exhaust fan usage when the building is only very lightly occupied.

    We also installed photovoltaic solar cells just recently.

  17. Dec 28, 2008 #16
    Not sure why I was not successful logging in as Greg'sDad, but the post above was from Greg's Dad......
  18. Dec 28, 2008 #17


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    Outstanding! Glad to see you were able to get some good savings with those tweaks. As I'm sure you can now imagine, there are a lot of improperly set-up VAV systems out there that are wasting huge sums of money.
  19. Dec 28, 2008 #18
    For sure. I have some of the best vendors in Milwaukee supporting me, but no one really seemed to see this as a quick and easy tweak which could save lots of energy. Maybe I should start a business.....
  20. Jan 3, 2009 #19
    to increase HVAC system efficiency:
    1. reduce the service volume (reduce service height by lower the outlet; since u are in a place with obvious weather change, lower the decoration roof is the only solution)
    2. reduce air lose from door's gap (it causes huge lost to conditioned air )
    3. increase zones for different work load (reduce unnecessary air interchange)
    4. review the necessary for HVAC (e.g. is it piratical to install HVAC system in a Chinese restaurant kitchen?)

    I believe the parameter is as follow:
    1 location of your site
    - sun light data
    - weather (ambient air) data
    2 expected air condition
    3 No of ppl and their activities
    4 configuration of your site
    - glass penetration
    - glass and wall conduction rate
    - size of service volume

    Find a CISBSE Guide for calculation.

    P.s. the attach is one of my past approach to find the rating and efficiency in HW.

    Attached Files:

  21. Dec 11, 2010 #20
    I have seen this but do not understand it.
    Oil-fill radiator electric heater is in a 10’x12’ room =120sqft set at 71 degrees. The setting switch is set at 900watts. In a 10hr period it only used 5.7kWh! ( 5.7kwh is recorded on a meter)
    The oil-fill heater is then set at 1500watts in the same condition. What is the kWh use? And why is it not (1500w x 10hr = 1.5kWh)?
    What formula are they using?
  22. Dec 11, 2010 #21


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    I'm not sure I understand - it has a thermostat and a power output switch? If it has a thermostat, then the amount of energy used is dependent on how often it runs.

    For your second case, the energy should be the same, but it should run less often. Btw, 1500w x 10 hr is 15 kWh.
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