I'm modeling a 6-story hotel. Baseline is PTAC. I set up the baseline to
have outside air at each PTAC with the fans running continuously. The
Proposed Case is watersource heat pumps. Outside air is through dedicated
units at the corridors. I've set (at least I am trying to set) the heat
pump fans to cycle on the thermostat. I've set the heat pumps to 0 CFM
outside air. When I run the two models, the ventilation fan energy is
nearly identical. I expected to see some fan energy savings with the heat
pump fans cycling. When I look at the detailed output report, the SS-C for
the heat pumps seems like it is showing fans running continuously. I say
this because the "hours floating" are less than the "hours fans on", and the
"hours fans cycle on" is zero. This implies to me that the fans are running
continuously. However, when I look at the reports that show fan energy
consumption, the months with floating hours show significantly reduced fan
energy, which to me implies that they are cycling. The total fan kW in the
proposed model is about twice that of the baseline, so from that standpoint
maybe the kWh being about the same is correct. I set up the PTAC units at
.3 watts/CFM per ASHRAE. I set up the heat pumps fans with a static
pressure input from the design drawings rather than input the fan motor
sizes (used .2" w.g. Static).
Also, my space cooling energy is higher in the proposed case than the
baseline, even though the envelope is better, the watersource heat pumps
have a lower EIR and the design lighting power density is 30% lower. I
haven't dug into that yet because I'm still trying to verify my fan cycling
is working.
My questions are, can anyone explain how to interpret the SS-C reports in
terms of fan cycling, or point me to which report I should look to really
verify the fans are cycling? And anyone modeled a similar comparison, and
if so, what type of results did you get with respect to cooling energy and
fan power consumption? Seems like I should be seeing savings.
Hi Michael,
I would just run an hourly report for one of your heat pump systems and
pick up hourly flow rate and fan energy. This way you can see how often
your fans are running and whether they are cycling or running continuously.
With heat pump systems, there are key words under the Fans tab (for a given
system) that will indicate the control on the fans. One is night cycle
control and the other is indoor fan mode (within the Flow Parameters tab).
If your fans are not cycling, it is likely due to those two key words (or
the fan schedule you have specified).
Chris
Michael,
On a recent project I modeled this same situation. The way I was able to
force the system to only operate during a call for cooling or heating was
by defining a fan schedule with all hours set to 0. Per the DOE2.2 help
documentation on fan schedules:
- If the value is 0, the fans are off but may be turned on by
NIGHT-CYCLE-CTRL if zone temperatures warrant it.
Make sure to allow night cycle control (cycle on any). This basically
defines all hours of the day to be "night" and the fan will cycle on to
meet the load in the space.
Robby Oylear, LEED AP
Do you know why you had to do that? The DOE2 help manual states that if you
set the outside air to zero, the fan will cycle.
I tried this but it had no effect. My annual kWh equals my total fan power
times 8760, so I?m certain that the fans are on continuously. There must be
something that is keeping them on, but I cannot find it.
Michael-
For zonal systems, I'd suggest reviewing the ZONE heating and cooling
SS-P reports as another way to see what is going on. I just reviewed
these reports for a current project, and found that I get nearly the
same results for both DOE-2 system types HP and PTAC using these two fan
control options:
1. No outside air specified at SYSTEM or ZONE level, FAN-SCHEDULE set
to all 1's, and FAN-CONTROL = CYCLING
2. No outside air specified at SYSTEM or ZONE level, and as Robby
suggested, FAN-SCHEDULE set to all 0's and NIGHT-CYCLE-CONTROL =
CYCLE-ON-ANY
Attached is an xlsx summary for one zone using one of these scenarios.
As you can see, the system fans are not running 8760. The report
indicates 6010 run hrs. However, as indicated part-load hour summary,
the there are many values lower in the range, indicating the system is
only cycling on for a fraction of the hour. The total annual effective
full load hours is only 1166.
A couple things to try:
* What DOE-2 system type are you using? A zonal system, such as HP or
PTAC? If you are using a single-zone system, such as PSZ or PVVT,
then FAN-CONTROL = CYCLING has no impact. If you only want the
system to cycle on to meet loads, you have to set INDOOR-FAN-MODE =
INTERMITTENT (CONSTANT is the DOE-2 default).
* Do you specify outdoor air at the SYSTEM or ZONE level? If you
truly want no OA supplied by the system, I would default all OA
related keywords to "no_default"; a zero value may still be
recognized by the program as a value and hence, cause the system to
run 24/7.
Also, related to the night-cycle control option, remember to define a
MIN-AIR-SCH with 0's during unoccupied hours. Otherwise, when the
system cycles on at night to meet the setback temp, the OA dampers will
remain open. This is a bit of a tradeoff, since it also disables the
economizer. But for heating dominated climates, where cycling is most
often to meet heating load, this is typically the best way to go.
Otherwise, if you are comparing two designs (i.e. proposed and LEED
basline), the design with better envelope performance is more likely to
not have as many night cycle hours, and hence, use less energy heating
(or possibly cooling) ventilation air. This oversight can have a huge
impact on savings comparisons...
Hope this helps,
- David
Michael,
You should also take a look at your zone exhaust fan inputs. Since OSA will be forced on to balance any exhaust flow out, this could be related to your system operation.
David, with regard to the your below advice on MIN-AIR-SCH, I typically use the value of 0.001 as suggested in the eQuest Help file for nighttime hours - this will enable the economizer, but set the %OS requirement very low. In this case the system is allowed to open the OA damper up as needed to maintain the desired discharge temp, but is only required to use a very low percent of OSA. Do you have a reason for using "0" rather than "0.001"?
The zone exhaust fans still run during any night cycle hours - which I think forces the OSA up to match that.
Kind Regards,
Molly Curtz