I am looking for a little help in trouble shooting a couple of setpoints on the attached model.
The first concern I have is my throttling setpoint. I have it 8 degrees right now. The reason for that was to eliminate unmet cooling hours. Is this a feasible approach? Is there something else that I should be looking at?
Second - Fan design KW/CFM - after reading through the archives I have figured out that I just need to restore the defaults in the static in WG box to allow the KW/CFM box to become active. Is the next step just to calculate the bhp for each unit per table G3.1.2.9, then calculate fan power from section G3.1.2.9?
I did this for my first unit and got a value of .000768318 KW/CFM. This was for system S1 with a baseline "design" (from the air side summary report) CFM of 1,986 CFM. Does that seem like a reasonable #? I have attached my calc spreadsheet for reference, it is a little crude but I think it is correct.
Thanks again in advance.
Travis
Travis,
Bumping your throttling range up will definitely reduce your unmet cooling/heating hours, but it's not usually the correct approach to do so. You're basically saying that the system is trying to maintain 75?F in cooling (hypothetically), but the cooling demand is met as long as it's less than 75 + 8?F = 83?F. Is 83?F a comfortable temperature for your occupants? Probably not.
I'd check to make sure you have adequate zoning by looking at the SS-R report and seeing if the unmet hours are similar between the spaces in your zone. What can happen is your system will maintain temperature in the control zone (where the thermostat is) and the other spaces can suffer. If all of the spaces have unmet hours, your system capacity might off. There are lots of things that could be wrong, but the reports are your best friend while troubleshooting..
Regarding your kw/cfm calc - I didn't look at it, but the number looks reasonable if you're modeling a 90.1 baseline system. If you're modeling an actual design, you want to use the kW (from bhp) your fan is operating at during the design condition.
Greg Collins
Greg, it looks like it was all in the zoning. Designating the trouble areas with their own system allowed me to reduce the throttling range to a more reasonable number (I left it at 2?). Thanks for your help.
I have a couple of other issues that I can't figure out. Looking at the ES-D report, it looks like the virtual rate between the proposed and baseline case are different. Why would that be? Everything is set to the local energy company and nothing has been changed in the utility and economics tab. Not exactly sure what a virtual rate is to be honest.
The other question I have is with regards to some energy usage showing up on the baseline model under pumps and aux. The baseline model does not have any pumps associated with it. Any thoughts on what exactly the auxiliary load is accounting for?
Thanks again for your help.
Travis
The virtual rate is a straight $/unit rate for each utility, so $/kWh and $/therm for electricity and natural gas. The reason they are different in this case is due to a monthly charge that is applied to each utility no matter the amount of energy used. You can see this in the utility rate properties. So your charge to units is not exactly proportional because of that monthly charge. Having a demand charge for kW would also have an effect on your virtual rate.
Regarding the Pumps and Aux usage, this is coming from your Crankcase Heat that is associated with your DX cooling for each system. This input is under Cooling, Unitary Power, Crankcase Power. There are a number of these kind of parameters that end up in the Aux part of Pumps and Aux usage.
Thanks,
Matthew Larson, LEED AP BD+C
Dear all,
I'd like to ask for a clarification of the throttling range in eQuest. I had been working under the assumption that your zone setpoint is at the middle of your throttling range, meaning the conditioned space temp drifts by +/- 0.5*TR (modifying the example from Greg's email...with a 75F set point and a throttling range of 8F, the system will begin cooling when the space hits 79 and turn off when it hits 71). This seems to be somewhat supported by the DOE-2 description which reads:
"The zone temperature heating or cooling set point is assumed to be at the midpoint of the throttling range."
In this case, you get an unmet hour when the space gets above 79F.
However, that same DOE-2 description continues with this statement:
"If the zone temperature is more than one THROTTLING-RANGE above the cooling set point, the zone for that hour is considered to be undercooled."
That would suggest that Greg's example was correct - for a 75F setpoint, you get an unmet hour when the space hits 75+8=83F. I did a google and archive search and found statements supporting both interpretations. Can anyone clarify this with any sort of authority?
Jason Kirchhoff
Jason,
I searched the archives to find this explanation of throttling range written
by Steve Gates. He explains how it works and how eQUEST counts hours outside
of the throttling range. This should help.
Carol
Throttling range is the temperature band between no output and full output,
with the setpoint in the middle of the range. If the VAV cooling setpoint
is 76F with a throttling range of 2F, then the VAV box is at minimum position
at75F, and at maximum at 77F.
The BEPS and BEPU reports count hours as outside the throttling range if the
zone temperature is more than 1F outside the temperature band. In the above
example, a zone greater than 78F or less than 74F would be considered out of
control.