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  1. #14
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    [QUOTE=seththomas;25805394]
    Quote Originally Posted by dan sw fl View Post
    YES, Standard CW temperature = 45'F.

    Coil Approach temperature is in the ballpark of 10'F
    depending the number of coil rows.

    -------

    I checked with Austin Energy and the CW temperature is 44'F. I assume this factor does not directly enter into the equation, but only affects how long the blower needs to run for the temperature in the unit to reach the set temperature. In other words, if the CW temperature were warmer, the blower would need to operate for a longer period of time to reach the set thermostat temperature, correct?
    That depends on the difference between return air and supply air temperature and CFM's!

    How accurate is that 44°F? 10% deviation could be a lot!

  2. #15
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    Quote Originally Posted by pecmsg View Post
    Probably easier and more accurate too just install flow meters for each user!
    This.

    If somebody was to charge me for something I'm using, I'd be upset if they can't show me how they measure how much I'm using. Especially if the billing method changed.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  3. #16
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    Thread Starter

    Flow meter

    Quote Originally Posted by pecmsg View Post
    Probably easier and more accurate too just install flow meters for each user!
    Do you have any recommendations for flow meters?

  4. #17
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    Thread Starter
    Apparently the "standard" in Austin is to allocate based on total electrical usage in each unit. This makes little sense to me since other appliances use so much more electricity than the HVAC when cooling. So someone using their 30 amp 240 v dryer a lot will get a larger allocation for chilled water.

  5. #18
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    [QUOTE=pecmsg;25805397]
    Quote Originally Posted by seththomas View Post

    That depends on the difference between Return Air and Supply Air temperature and CFM's!

    How accurate is that 44°F? 10% deviation could be a lot!
    CW Supply Temperature is generally within ~0.8'F of design.
    _____________________________________

    THE BIGGEST UNKNOWNS are the Condo Room Temperatures.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  6. #19
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    [QUOTE=dan sw fl;25805435]
    Quote Originally Posted by pecmsg View Post

    CW Supply Temperature is generally within ~0.8'F of design.
    _____________________________________

    THE BIGGEST UNKNOWNS are the Condo Room Temperatures.
    I agree but without knowing the exact temperature (Supply, return, CFM’s) how can you calculate usage accurately?

  7. #20
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    Quote Originally Posted by seththomas View Post
    Do you have any recommendations for flow meters?
    CW flow meter use needs to be validated
    based on review of the CW Flow Diagram
    to determine how the Secondary CW loops are actually arranged and controlled.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  8. #21
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    Quote Originally Posted by seththomas View Post
    Do you have any recommendations for flow meters?
    That depends on how accurate you have to get. If your billing they better be real accurate that = $’s.

    I like Dwyer Instruments.

  9. #22
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    What type of and how many circuit setters ( denoted by GREEN symbol) are installed?
    Attached Images Attached Images  
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #23
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    Quote Originally Posted by seththomas View Post
    Large condo building that gets chilled water from Austin Energy used for cooling. Each condo has their own HVAC unit.
    ___ Utility __

    https://austinenergy.com/ae/commerci...strict-cooling

    44'F - 45'F SUPPLY

    https://austinenergy.com/ae/commerci...mplementations

    https://austinenergy.com/ae/rates/re...and-line-items
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  11. #24
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    First, the answer to your question about ton-hours is that it is 12,000 Btu's of cooling provided for a period of one hour. It is measured with a btu meter, which is a device that measures instantaneous water flow and entering/leaving water temperature difference to calculate the Btu's provided at that moment and sums them up. These devices are expensive, but are the only way for a district chilling plant to accurately bill customers.

    Second, you can only achieve the necessary level of accuracy to bill individual tenants by adding a Btu meter to each fan coil. I guarantee that you will find that the expense will be enormous and you will choose not to pursue the idea.

    Before I expand on this answer, I will give you some of my background: I worked for the fan coil group of a large air conditioning company for a number of years and continued to consult for them after moving on to other roles. My area of expertise was coil design and application. I was also responsible for our selection software. I also spent a lot of time reviewing competitors' ratings. I was a very active participant on the AHRI Room Fan Coils Engineering Committee. Over the years, I was asked many times by customers how they could achieve individual-unit level billing, and when I explained why a Btu meter was required the idea was inevitably abandoned due to cost (and that was for new construction - retrofit would be worse).

    1. Your fan coils are not operating at the rated condition. When the engineer gave the salesman the external static pressure and entering air temperatures, I guarantee the pressure was different than the actual and very probably the rated entering air temperature was 75 degF dry bulb/ 63 degF wet bulb. Further, the airflow rating was with a clean filter, and with small forward curve fans the airflow drops rapidly as the filter loads up.

    2. Unless your building has non-modulating pressure-independent valves on each fan coil (unlikely), the water flow will vary depending on demand for chilled water in the rest of the building and the signal from the thermostat.

    3. This is one that I did not tell customers: Performance ratings of fan coils, particularly at low air and water flows, are not very accurate. AHRI currently* only requires testing with the fan at high speed, 80/67 entering air (unrealistic), and 45/55 entering/leaving water. Pretty much all fan coil companies have only tested their thermal performance at those conditions and count on coil performance computation to figure out the performance at other air flows, water flows, and water temperatures. Unfortunately, most do a pretty bad job with this, as they do not account for uneveness of the airflow across the coil at lower flows - my company included. This is particularly true for the low fan setting, as fan coils are never specified at the low setting.

    *AHRI recently released a comprehensive update to AHRI 440 that does require much more extensive testing at realistic conditions, as many in the industry understand accuracy of ratings has been a big problem. I do not think they will begin certifying to the new standard until 2021 or 2022. Even then, the accuracy will be far from what is necessary for tenant billing, and AHRI can't do anything about the rated vs. actual conditions issue.

  12. #25
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    [QUOTE=pecmsg;25805439]
    Quote Originally Posted by dan sw fl View Post

    I agree but without knowing the exact temperature (Supply, return, CFM’s) how can you calculate usage accurately?
    An appropriate, FAIR Billing procedure would be to
    treat each Condo using the same method.

    Given : A/C unit capacity ~= CW GPM through Circuit Setter

    ___ Run time / month * unit capacity = Energy Use / Month

    FOR EXAMPLE
    K.I.S.S.

    __ ONE OPTION: Simply Monitor { monthly record } A/C operating time

    1. 10 hours * 3.0 tons = 30 Tons * 20 condo units = 600 ton hours / day
    2. ..8 hours * 2.5 tons = 20 Tons * 25 condo units = 500 ton hours / day
    3. 12 hours * 2.0 tons = 24 Tons * 30 condo units = 720 ton hours / day
    _____________________________Total ___ .. = 1,820 ton hours / day

    _________ % of Total Austin Energy charge
    1. 30 / 1820 = 1.648 %
    2. 20 / 1820 = 1.099 %
    3. 24 / 1820 = 1.319 %

    In practice, make a detailed summary of ALL 80+ [ # ? ] condos
    to determine actual % CW use for Each condo.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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