# Thread: How to calculate chilled water usage

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## How to calculate chilled water usage

Large condo building that gets chilled water from Austin Energy used for cooling. Each condo has their own HVAC unit. Is it possible to determine chilled water usage based on CFM? For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.

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Originally Posted by seththomas

Large condo building that gets chilled water from Austin Energy used for cooling.

Each condo has their own HVAC unit.
Is it possible to determine chilled water usage based on CFM? YES

For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.
___ ___ ___ Q. Total = 4.5 * CFM * Enthalpy Difference { ED }
____ Enthalpy in and Enthalpy out are dependent on the temperature and humidity
______ ED = E in - E out

Presume the difference in Air energy = Water energy

...
Or determine the annual energy use based on local Coolng Degree Days (CCD)
and an applicable ACCA Manual J calculation for each condo unit.
However, the room temperatures are not going to be the same.

I presume you are investigating a utility billing method for each tenant.

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Yes, I am trying to determine if there is a better way to improve the current utility billing method. The chilled water is billed based on Ton Hours. To simplify the discussion, let's assume ED = 0. So using the formula for a user operating their unit for 1 hour that is 1,100 CFM, the result would be 4.5 * 60 * 1,100 = 297,000. What does this represent in terms of Ton Hours?

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Originally Posted by seththomas

Yes, I am trying to determine if there is a better way to improve the current utility billing method.
The chilled water is billed based on Ton Hours.

To simplify the discussion, let's assume ED = 0.

So using the formula for a user operating their unit for 1 hour that is 1,100 CFM,
the result would be 4.5 * 60 * 1,100 = 297,000.
_______________ ...... 0 = ______ ... 0

What does this represent in terms of Ton Hours?
Assume ED = 0

Q. Total = 4.5 * CFM * Enthalpy Difference
Q. Total = 0

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T room = 75'F
R.H. = 50%
E in = 28.149 BTU/lbm from Psychrometric Chart

T supply = 55'F
R.H. = 90%
E out = 22.205

ED = 5.944
Q. Total = 4.5 * 1,100 * 5.944
......... = 29.423 BTU/HR
............ = 2.45 Tons

A/C runs ~ 300 hours per month
........... _______ > 100 & < 500 depending whether
........................................ 1. Condo is in Vancouver, Phoenix or Miami
........................................ 2. Size and construction of the condo
...………………………………………...……...…….. Condo built in 1918 or 2018
........................................ 3. T-stat set-point at 71'F or 79'F

............ 735 Ton Hours / Month ... > 245 & < 1,225

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AUSTIN TX
MET DATA
COOLING DEGREE DAYS / MONTH

CDD MONTH RUN TIME
.................. HOURS / MONTH
CDD ____ ..RUN TIME .. TON HOURS
10 JAN 9.25 __ 22.7
21 FEB 18.93 46.4
58 MAR 50.52 123.9
154 APR 129.76 318.2
348 MAY 285.21 699.3
517 JUN 418.05 1025.1
593 JUL 477.28 1170.3
616 AUG 495.15 1214.1
429 SEP 349.10 856.0
198 OCT 165.42 405.6
47 NOV 41.23 101.1
14 DEC 12.80 31.4
3005 ANNUAL 2452.70 6014.0

IF T-STAT set at 72'F, Add 25% - 40%
IF T-STAT set at 78'F, Subtract 20%

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## Supply Air temp and RH

Originally Posted by dan sw fl
T room = 75'F
R.H. = 50%
E in = 28.149 BTU/lbm from Psychrometric Chart

T supply = 55'F
R.H. = 90%
E out = 22.205

ED = 5.944
Q. Total = 4.5 * 1,100 * 5.944
......... = 29.423 BTU/HR
............ = 2.45 Tons

A/C runs ~ 300 hours per month
........... _______ > 100 & < 500 depending whether
........................................ 1. Condo is in Vancouver, Phoenix or Miami
........................................ 2. Size and construction of the condo
...………………………………………...……...…….. Condo built in 1918 or 2018
........................................ 3. T-stat set-point at 71'F or 79'F

............ 735 Ton Hours / Month ... > 245 & < 1,225
How is the supply air temperature and relative humidity determined when the chilled water is pumped through the coils of the HVAC unit and most of the supply air is from inside the unit with only a small percentage of fresh air.

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## How does the temperature of the chilled water factor into the equation

Originally Posted by seththomas
How is the supply air temperature and relative humidity determined when the chilled water is pumped through the coils of the HVAC unit and most of the supply air is from inside the unit with only a small percentage of fresh air.
Doesn't the temperature of the chilled water that runs through the coils factor into the determination?

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Originally Posted by seththomas

How is the supply air temperature and relative humidity determined
when the chilled water is pumped through the coils of the HVAC unit
and
most of the supply air is from inside the unit with only a small percentage of fresh air.
Equipment Performance data is available from vendors though their AHU Selection software.

+ KNOWN Real Life situations.

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Originally Posted by seththomas
Doesn't the
temperature of the chilled water (CW) that runs through the coils factor into the determination?
YES, Standard CW temperature = 45'F.

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

-------
http://www.ref-wiki.com/technical-in...mperature.html

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EERE Technical Fact Sheet
__________ ______ SIZING
________________ Equipment Selection
……………………………….. Life-Cycle Analysis

JANUARY 2002
_____________ is or is becoming OutDated in several aspects.

12. Originally Posted by seththomas
Large condo building that gets chilled water from Austin Energy used for cooling. Each condo has their own HVAC unit. Is it possible to determine chilled water usage based on CFM? For example, one unit spec is: 1,100 CFM on low fan speed with a 1” filter.
Probably easier and more accurate too just install flow meters for each user!

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## How does the temperature of the chilled water factor into the equation

[QUOTE=dan sw fl;25805305]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?

14. [QUOTE=seththomas;25805394]
Originally Posted by dan sw fl
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!

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Originally Posted by pecmsg
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.

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## Flow meter

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

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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.

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[QUOTE=pecmsg;25805397]
Originally Posted by seththomas

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.

19. [QUOTE=dan sw fl;25805435]
Originally Posted by pecmsg

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?

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Originally Posted by seththomas
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.

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