3.6.5.2 - Parallel fan powered box operation

[GoogleCodeExporter]

What 90.1 PRM Section(s) are relevant to this issue?
3.6.5.2 Parallel Fan box Thermostat Setpoint

Explanation of issue:
E+ does not support the control scheme specified in the PRM, which indicates 
that the zone fan is enabled whenever the the zone temperature is less than or 
equal to 2F above the zone heat Tstat setpoint (this is the DOE-2 control 
method).  E+ PIU terminals are controlled by specifying the minimum primary air 
flow fraction at which the PIU fan turns on.

My approach to date has been to default the PIU flow fraction to be 1.1x the 
terminal minimum flow fraction. For example, if the box minimum air flow 
fraction is 30% (as prescribed, assuming the minimum fraction for ventilation 
is less), then the terminal air flow fraction at which the PIU fan would run 
would be 33% or less. With this in mind, please let me know what fraction 
should be used for the 90.1 ruleset. 

Proposed resolution:
Define control flow fraction for parallel fan boxes, and ideally, provide an 
example E+ model that illustrates the E+ inputs needed to achieve the control 
scheme outlined in the PRM.

Original issue reported on code.google.com by da...@360-analytics.com on 9 Oct 2014 at 3:54

[GoogleCodeExporter]

David,
We'll have to use EMS for the requirement for zone fan control at 2F over 
heating setpoint. As far as i know OS doesn't support EMS at present, we should 
move the implementation of this task to Phase II and request the NREL team to 
add this feature. 
Using EMS, each fan would need to have a unique availability schedule, which is 
controlled by EMS program to be =1 when zone setpoint is 2F above heating 
setpoint, else is =0.

Regarding your second question, we should stick to the default flow fraction of 
0.3 for the piu min flow fraction and we'll add the EMS implementation in the 
future.

Original comment by supriya....@pnnl.gov on 11 Oct 2014 at 12:06

[GoogleCodeExporter]

Yes, you are correct, EMS is not supported in OS yet; I've cc'd Kyle to the 
issue for future reference to an EMS application that is desired.

In the near term, I will set the PIU flow fraction to be equal to the min 
terminal stop.

Original comment by da...@360-analytics.com on 11 Oct 2014 at 11:07

• Changed state: Fixed

[GoogleCodeExporter]

David, we did some tests modeling the suggested approach in E+. in E+, the zone 
fan comes on at all timesteps, based on the min flow fraction. This results in 
overestimation of fan energy- as the fan is running during deadband mode as 
well. The recommended workaround for this would be to set fan power =0. This 
could be done by setting fan pressure drop to be negligible as well as 100% fan 
and motor efficiency.
Since we have electric resistance reheat for all baseline systems with PIU 
terminals, the assumption is that the additional reheat provided represent the 
fan energy use. The end use breakdown is incorrect but overall energy use would 
be accurate enough. 
We've requested the E+ team to add a feature which controls the zone fan based 
on zone temperature. Until then, we can use this workaround. 

Let me know if you have any questions

Original comment by supriya....@pnnl.gov on 30 Nov 2014 at 9:41

[GoogleCodeExporter]

This is an OK workaround for the baseline electric PFBs, but if the proposed 
design happens to use hot water coils, the cost savings will be skewed.  Do you 
want these rules only applied to the baseline design?  In either case, the 
approach you describe can be implemented in rules; please just let me know if 
the rule applies to both the proposed and baseline.

For further consideration, is it perhaps appropriate to have the fan operating 
whenever the system is near or at min flow? The Nailor catalog describes a 
sequence on page D-7 that suggests it is:
http://www.nailor.com/onlineCatalog09/CAT-06/CATDFP.pdf

Thinking about this more, it makes sense that the PFB fan control sequence is 
highly dependent on the min primary flow ratio. 30% of primary air, as 
described in 90.1 PRM, may be what was deemed sufficient for maintaining 
adequate ventilation air distribution effectiveness during deadband, and 
therefore not require the zone fan to run.

IECC indicates the min. primary flow for PFBs is equal to the zone ventilation 
air flow rate, in which case having the parallel fan operate based on min flow 
fraction (during both deadband and heating) seems appropriate. Ultimately, I 
think these questions come down to real-world air velocities and design of the 
diffuser, both of which are not well described by the simulation program.

Original comment by da...@360-analytics.com on 1 Dec 2014 at 6:36

[GoogleCodeExporter]

[deleted comment]

[GoogleCodeExporter]

I don't think we can set the fan power to zero for hydronic systems in a 
proposed building. That would underestimate the energy use too severely. 
Regarding the terminal fans use to provide increased velocity for better 
mixing, my understanding is that if that is an issue (which it often is) a 
series terminal unit is used as that will run  at all times including during 
minimum flow. There is really no good solution to this problem until E+ gets 
fixed. I would be OK running the terminal unit fan in the proposed during 
deadband, but running it at night or at all times the terminal is off is really 
too great a penalty and would never be done in reality. There are lots of ways 
to control the PFPTU fan, but for the baseline it would make most sense to 
control it thermostatically at some setpoint just above heating setpoint. Here 
are a couple of other sequence descriptions.

******************
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&
ved=0CCoQFjACOAo&url=http%3A%2F%2Fwww.peci.org%2Fftguide%2Fftg%2FTest_guidance%2
FTests%2FTG15-Terminal_Units.doc&ei=7E5uVKXbIo2dygSIqoE4&usg=AFQjCNHEf3jSea-Yg6m
a2_vrw1M2Bo4LKA&bvm=bv.80185997,d.cGU

Parallel fan-powered terminal units:  The sequences of operations are similar 
to those outlined above for a cooling load.  When the zone temperature 
approaches the heating setpoint, the primary air damper modulates to the 
minimum cooling flow rate and the fan turns on to temper the supply air with 
air from the return plenum.  If the zone temperature continues to drops below 
heating temperature setpoint, then the primary air damper remains at the 
minimum cooling flow rate, the fan stays enabled, and the reheat valve 
modulates open as necessary to maintain zone temperature setpoint.
*************************

http://www.pcs-engineering.com/files/Sequences_from_Practical_Controls.pdf

Parallel Fan Powered Boxes with Heat
Occupied Mode
An individual zone sensor (one per zone/box) transmits zone temperature and 
zone setpoint information to the fan powered box controller. The controller in 
turn modulates the primary air damper in order to deliver the appropriate 
amount of cool air from the medium pressure duct into the zone. The air 
delivered into the zone is of variable volume and constant temperature. As the 
zone temperature falls toward setpoint, the damper is modulated closed toward 
its minimum position. When the zone temperature reaches setpoint, the damper
reaches its minimum position. If the temperature in the zone falls below 
setpoint, the fan energizes to circulate warm air from the plenum. The plenum 
air is blended with the minimal cool air from the primary air damper, and 
delivered to the zone. If the temperature in the zone continues to fall, then 
terminal unit [electric / hot water] heat is engaged. The further the 
temperature falls from zone setpoint, the more heat is engaged.
 ***************************************

Your discussion of proposed building issues makes me concerned about how series 
FPTUs are controlled in E+. I don't think we have looked at this. Do we know if 
the TU fan is also running when the main system is off?

Original comment by bikerose...@gmail.com on 1 Dec 2014 at 5:51

[GoogleCodeExporter]

In your testing of the FPTUs, did you assign an Availability Schedule that 
matches the system availability to the terminal? If not, then yes, the zone 
fans will run all the time. 

CBECC currently sets the terminal availability schedule (TrmlUnit:AvailSchRef) 
to match the system schedule. Doing so, I see that the parallel box fan only 
runs when the zone is at minimum primary air flow fraction, and that the series 
box fan only runs when the system is scheduled on.

I am currently testing whether the terminal availability is overridden by 
AvailabilityManager:NightCycle.  I believe it should be, but in reviewing 
results for other AirTerminals, I am seeing that it is not (see issue 862). I 
will have to review to see if this is also an issue for FPTUs.

Original comment by da...@360-analytics.com on 1 Dec 2014 at 7:22

• Changed state: Discussion