Hi all,
I am in the process of breaking out the fan energy from my HVAC system per
ASHRAE 2004 G3.1.2.1, following G3.1.2.9. The amount of energy coming from
my fans seems massive and I'm not sure if that is typical, or if I've messed
something up. I started by setting the temperature difference of 20 degrees
between supply and desired temp (55/75 cooling and 90/70 for heating) and
specified 62.1-2004 ventilation for outside air. I then ran equest and used
the SV-A reports to get supply CFM for each system. From that number I then
used Table G3.1.2.9 to get BHP for each system, which I then converted to kW
using the formula in ASHRAE. I then took that number and divided by CFM to
get kw/CFM and entered this number on the system->fan tab. When I go to the
SV-A report the system kW is exactly what I calculated it to be, but the
energy required for my ventilation fans is about the same as my heating and
cooling load combined, which seems outrageous.
Is this typical? Have I entered it In the wrong way? I would think fan
consumption would typically be a fraction of the actually heating and
cooling energy. Thanks for any thoughts.
____________________________
Michael Shields
Check your cfm/sf number in your systems, that would definitely cause this
if it's set too high. Also what field are you using for your OA ventilaton?
Carol
I looked at the ACH based on the SV-A system CFM and the volume of the zone
and it does seem high (2-5 ACH), but I let equest calculate the system CFM
for me.
I have one system per zone so I specified OA in the zone tab as a overall
CFM rate based on the 62.1 calculations I did.
Under the HVAC systems tab, go to the Air Flow screen and look for the field
labeled cfm/sf. It usually defaults to 0.5 cfm/sf, something I've been going
on about a bit lately on this listserv. A better number is 1 or so, usually
but it depends on building type, exposure, etc. For you, make sure it is not
too large. I suspect, however, that this is not where your problem is. I
suspect you have accidentally entered too much OA. I suggest you go back and
simply put in the Standard 62 cfm/person rate and call it good. I have never
specified it as a cfm rate but that sounds like not such a good idea for
energy modeling. See if that helps.
Do you mean under the Systems->Fans->Flow Parameters Min Flow cfm/ft2? I
left this blank thinking that equest would then size the flow rate based on
my design parameters for supply air temp and zone temp as Appendix G
requires, is that incorrect?
Yes, that is correct and obviously there isn't a large number in there. The
default is 0.5 cfm/sf. I really think it's your outdoor air. Try changing it
to cfm/person.
Michael:
Just out of curiosity, what did you calculate for the kW/CFM?
I have modeled buildings where the ventilation fans use much more energy than
the heating and cooling energy combined.
Paul Diglio
I?ll try OA as a per person number instead of a per space number and see what happens. kW/CFM varied by space between ~ .0007 and .001, and when I look at the SV-A reports the Fan Demand (kW) matches what I calculated using 90.1.
I found G3.1.2.9 could possibly be interpreted a couple different ways (for example the denominator include X BHP instead of the whole fraction), but the only one which produced a reasonable bhp for to me was . I guess this could have been where I messed up but all the other possibilities generated kW numbers which didn?t make sense (quite large or negative).
Michael:
The kW/CFM you used seems reasonable.
I don't know if you are using a CV or VAV system, but lets assume VAV.
90.1-2007 G3.1.2.9 shows the formula CFMs *.0013+A for baseline BHP. A is
calculated per Section 6.5.3.1.1. Using Table 6.5.3.1.1A VAV, we have the
formula BHP<= CFMs*0013+A. The footnote defines A=sum of (PD*CFMd/4131). So
assuming you have a fully ducted return, the PD is .5 which should be multiplied
by (CFMd/4131). If there are various pressure drops throughout the system we
need to estimate the CFM flow at each pressure drop.
As an example, let's take a 20,000 CFM VAV system with 18,000 CFM of return air
through a ducted return.
A = (.5*(18000/4131) = 2.178
BHP =20000*.0013+2.178 = 28.17
pFan= 28.17*.746/ Fan Motor Efficiency. Note that since we need kW not watts
per CFM the 746 becomes .746. That is probably the reason you generated a large
kW number. Per Table 10.8 a 30 HP baseline motor needs to be 91% efficient.
The formula now becomes pFan =28.17*.746/.91 = 23.09 kW. Our kW per CFM is
.0011.
If I am modeling a proposed project with a defined supply and return CFM, I
would expect that the baseline would be approximately the same. The CFM/Ft2
required varies greatly between building and systems. In the real world I see
1.2 or so CFM/Ft2 much, much more than I see .5 CFM/Ft2. Check your CFM/Ton to
see if you are in the ballpark.
I don't know why someone would think that outside air would cause a problem.
Air is air. Too much outside air would raise energy costs though.
Paul Diglio
The reason someone "thinks OA might cause a problem" is because of the way
that someone thinks it was entered. It sounds like it was entered using
outside-air-flow, and I may be wrong because Michael hasn't really said.
Using Outside-Air-Flow overrides any values eQUEST might otherwise
calculate, and I would suggest that might make a difference. If OA Flow-Per
was used instead, that's the one I use, it gives eQUEST the flexibility to
calculate the OA cfm itself. To my mind, that a good use of an energy model
that is not only interactive but uses 8760 hours of weather data.
Any other thoughts on this?
I thought I specified that I entered it using outside-air-flow, but I
probably was not specific enough. I think clearly the manner in which OA is
entered into the model has a huge affect on energy consumption and accuracy
of the model, however, to me it would seem that OA would not have a large
affect on the difference between cooling/heating consumption and ventilation
consumption. If you have too much OA you have to heat and cool more and
therefore the fans will run more, if you have less OA then you need less
cooling and heating, but also less ventilation. At least that makes sense
to me, but models often surprise me.
Michael
I am trying to model a CAV 90.1 baseline system 4, but it won't let me input kW/cfm, just static and efficiency. The default static value is 1.25, but I'm not sure if that's correct because I obviously don't know the external static of the duct system. I would rather input the kW/cfm. Any advice?
Brian Goldsmith
If you close equest and open the .inp file you can find the system you wish
to enter kW/cfm and one of the lines should have the static followed by two
more lines each with one of the related efficiencies. If you delete these
from the inp and open the project in equest again it should have greyed out
the static and have green values for kW/cfm and dT.
Hope that's what you're looking for.
Brian:
In detailed mode, right click the static and then choose "restore default".
Then you can enter the kW/CFM.
Paul Diglio
Yep, that worked. So simple. Thanks for the help.
Brian Goldsmith
Restore your default values. Equest will use either, static pressure
with the airflow quantity and efficiencies to calc fan power, or KW/CFM
and the airflow quantity.
Right click on the static in WG box and click restore default.
There it is, sorry, my outlook is a little slow.
I have personally never seen fan power equal the total of cooling and
heating power, except in certain climate zones where an economizer can
run often or when the exhaust/ventilation fan power, not associated with
cooling/heating, is high.
Not sure if you are stilling looking for help here.
What does your fan schedule look like? Is your fan set to run 24 hrs?
What climate zone are you in? What are your space cooling and heating
temperature set-points?
The building is a residential high rise for seniors in climate zone 3B using
PTHP for heating and cooling. For all of the residential spaces the fan
runs 24/7 as ASHRAE 90.1-2004 requires that the system run continuously
during occupied hours. Cooling set point of 78, heating setpoint of 72.
Also the apartments are continuously ventilated through exhaust risers for
the bathrooms and kitchens of each area, so I suppose it should not be
surprising to have a high vent fan consumption, however, what surprised me
was how much it increased going from the eQUEST default values to my
calculated 90.1 values.
Hi All,
If an HVAC system comprises both a supply and return fan, and the formula
for G3.1.2.9 is used, does the result, Pfan, represent ALL of the fan power
for that system including supply and return? If so, should the baseline
model only depict the supply fan values (CFM, kW/CFM, etc.) and ignore the
return?
Many thanks,
Jeffrey G Ross-Bain, PE, LEED AP, BEMP
The formula for G3.1.2.9 inlcudes the fan power for both the supply and return fans.
The baseline model should usually be modeled as per the design i.e. if there are return fans then model them. However if for some reason you would like to calculate the supply and return power together and enter it as a supply fan you can do so also. You shouldn?t ignore the return.
Christie Rimes, LEED BD+C
Hi Jeff,
For most of the baseline models I build, I put the supply and return fan efficiencies in the supply fan only kW/cfm. Since it is a return fan and not a relief fan, any time the supply fan is on, the return fan will be on, so this should work. I could see this approach not being completely accurate in designs with supply and relief fans, where the supply fan could be on, recirculating air with the relief fan off. However, this all exists in the theoretical world of "what could have been" so, I wouldn't stress too much in bring the baseline design from 95 to 100% accurate.
Neil Bulger