Dear eQuest users,
I am having some difficulty with a project I am working on, and I was
hoping I could receive some help or any information on a building a with
packaged single zone units with DX cooling and hot water coils for
heating, with a VFD to vary the supply flow (there are no VAV boxes in
the system, and the supply reacts to overall building pressure rather
than specific zonal needs determined by pressure sensors in the
ductwork). There is no reheat in the system either.
It is a LEED for schools project where the simulation of the proposed
model shows very high unmet loads for heating and the total energy
consumption for space heating is 199 MBTU which is unrealistically low
as compared to the baseline model space heating which is 6369 MBTU. We
do not see many unmet hours in the control zone, but since the unit
supplies many zones, we are getting very high unmet heating hours in
non-control zone spaces. We are seeing on the order of 4000-5000 hours
unmet in many zones.
I have modeled the heating for all zones by specifying the
Heat Source as hot water loop,
The Zone Heat Source is not installed, as the design has no reheat.
Zone entering maximum supply temperature is at 110 degrees.
Hot deck max leaving temp is 80 degrees
Reheat delta T: n/a
Many thanks in advance,
Adam A. Gonthier
Adam:
How do you vary flow for a VFD is you aren't varying flow?
And how do you get the air to 110?F if you are leaving the "hot deck" at 80?F?
What did you change in the proposed model over the base model?
An .INP and .PD2 file of each would be helpful.
John R. Aulbach, PE, CEM
Adam,
Potential issues with how you model the variable flow...
For sure, there is a problem with supply air temperature for heating. The following is from the Help File:
"Use of HEAT-SET-T for systems SZRH and PSZ is discouraged because it simply overrides the maximum leaving air temperature specified by MAX-SUPPLY-T".
You've set your MAX-SUPPLY-T at 110 F and your HEAT-SET-T at 80 F. You are heating with only 80 F.
_____________
Demba NDIAYE
Dear John,
Have posted the proposed and the baseline case model on behalf of Adam, also
responding to your query about VFD, about varying the flows, do we need to
model a vav system?.. or I am not very familiar with this part as the specs
don?t show VAV terminal units, they have provided the VFD schedules. It
would really help if you could let us know how to to rectify the error. I
also went ahead and changed the hot deck temperature to default as suggested
by you and Demba. But it doesn?t seem to help much
Thanks for responding,
Kind Regards,
Vidya
Have posted the proposed and the baseline case model on behalf of Adam, also
responding to your query about VFD, about varying the flows, do we need to
model a vav system?.. or I am not very familiar with this part as the specs
don?t show VAV terminal units, they have provided the VFD schedules. It
would really help if you could let us know how to to rectify the error. I
also went ahead and changed the hot deck temperature to default as suggested
by you and Demba. But it doesn?t seem to help much
Thanks for responding,
Kind Regards,
Vidya
The system as Adam described it appears to be (mostly) constant-volume, variable-temperature. If there are no VAV boxes and the supply fan VFD is controlled by building pressure, then it sounds like it is supposed to be a safety back-up for the relief system. If the relief system is properly designed and balanced, I don?t see the supply volume fluctuating much. I also don?t see it fluctuating in the model if the Min Flow Ratio is left at 1.0.
The unmet hours are out of control because of the factors listed below.
? The issue John and Demba raised of an 80 F heating supply temperature.
? On top of a low heating supply temperature and no reheat, there are no baseboards or other forms of zone-level heat for RTU-4, 3, and 2. However, RTU-1 has them??
? It is extremely difficult for a single-zone system to satisfy 19 separate zones (RTU-4). It is impossible for a single-zone system to do so if there is no zone-level heating.
? Using an interior control zone that appears to be a 175 sqft storage room (RTU-1). This control zone is adjacent to ?walking cooler?, which has a connected internal load of 57 kW (34.2 kW peak demand), 1.0 sensible heat ratio, and is 170 sqft. A walk-in cooler rejects heat outside the building, so the sensible heat ratio should be zero. The control zone is also adjacent to the kitchen with 21.13 kW of connected electric power and 862 kBTH of connected gas load, both of which have sensible ratios of 1.0. This heat is having a huge influence on the control zone, which is why it is constantly in cooling mode while the rest of the zones are in heating mode.
? Not having a morning warm-up schedule
? Allowing eQUEST to allocate supply air to the zones ? entering design flow rates (supply, exhaust, and OA) at the zone level ensures each zone is getting the airflow it was designed for.
I changed the system type for the four RTUs to be PVVT with the heating supply capped at 80 F, and I was at least able to get the control zones satisfied. I would recommend discussing the design with the mechanical engineer to ensure it is properly represented in the model. The diversity of space needs and building orientation does not appear to be well-suited for single-zone control.
Thanks,
DAKOTA KELLEY
This is just basic fundamentals and has nothing to do with eQuest.
If you have 50 rooms, and want perfect control in each one you need 50
t-stats and 50 PSZ units. Once you move away from this you need a way
to independently manipulate the temperature in each room up and down.
VFD will vary volume and pressure, but will not independently vary the
volume to an individual room as you are using them. There are many ways
to do this, VAV, terminal units, multi-zones. Either your concept of
the design or the design itself is flawed. Either way there is no point
doing any more work with eQuest until you resolve the design problem.
Equest is working fine and your results are totally expected. I rarely
open files, I do not have time, but more specifically there is no point
in your case. You have junk going in, therefore you have junk coming out.
Dakota has done you a big favour, much more eloquent than I and
given some excellent pointers. You need to pick up the phone and nail
down the basics and don't forget to ask about the outside air. VAV and
OA don't always play well together. You will also have rooms with 35
kids and a room with one principal.
Sorry about the touch of flame but much of the debate on this site
lately has regressed to teaching of basic concepts or to users who seem
to have only spent a week or 2 in the trenches. It seems some of the
old hands are getting cranky because the privilege of asking questions
on this site is getting abused.
Bruce Easterbrook P.Eng.
Adam:
How do you vary flow for a VFD is you aren't varying flow?
And how do you get the air to 110?F if you are leaving the "hot deck" at 80?F?
What did you change in the proposed model over the base model?
An .INP and .PD2 file of each would be helpful.
John R. Aulbach, PE, CEM
Adam,
Potential issues with how you model the variable flow...
For sure, there is a problem with supply air temperature for heating. The following is from the Help File:
"Use of HEAT-SET-T for systems SZRH and PSZ is discouraged because it simply overrides the maximum leaving air temperature specified by MAX-SUPPLY-T".
You've set your MAX-SUPPLY-T at 110 F and your HEAT-SET-T at 80 F. You are heating with only 80 F.
_____________
Demba NDIAYE
Dear John,
Have posted the proposed and the baseline case model on behalf of Adam, also
responding to your query about VFD, about varying the flows, do we need to
model a vav system?.. or I am not very familiar with this part as the specs
don?t show VAV terminal units, they have provided the VFD schedules. It
would really help if you could let us know how to to rectify the error. I
also went ahead and changed the hot deck temperature to default as suggested
by you and Demba. But it doesn?t seem to help much
Thanks for responding,
Kind Regards,
Vidya
Have posted the proposed and the baseline case model on behalf of Adam, also
responding to your query about VFD, about varying the flows, do we need to
model a vav system?.. or I am not very familiar with this part as the specs
don?t show VAV terminal units, they have provided the VFD schedules. It
would really help if you could let us know how to to rectify the error. I
also went ahead and changed the hot deck temperature to default as suggested
by you and Demba. But it doesn?t seem to help much
Thanks for responding,
Kind Regards,
Vidya