[Equest-users] Heat Pump Heating Undersized

[joe@thespinnakergroupinc.com]

"...as your time averaged outside air requirements also come into play and may not allow the VAV system to put enough flow into a zone due to OA requirements of other zones."

Very good point.


Sent from myTouch 4G

----- Reply message -----
From: "Bruce Easterbrook" <bruce5 at bellnet.ca>
To: "Carol Gardner" <cmg750 at gmail.com>
Cc: "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>
Subject: [Equest-users] Heat Pump Heating Undersized
Date: Sat, Apr 9, 2011 10:46 am
I agree with Carol and Joe.  In almost all cases you
shouldn't need to have the cfm/SF 400% higher than the eQuest set
point.  If you do, it is likely going to be a process load that
causes this.  I have found the 0.5 cfm/SF eQuest default too low
in most cases but leave it for my first run because what you
really want are the heat/cool loads of the zones.  Joe's tip, CFM*(Tsupply-Troom)*(1.085) =
BTU/h provides your tool.  Your equipment provides
the delta T of your air heating and cooling flows, the only
variable is CFM.  Use your loads required by the building to
determine your CFM.  The old rule of thumb was around 1 CFM/SF but
in most cases this is too generous.  I think the reason eQuest
uses a low default is to help you keep the fan energy to a minimum
and maximize the efficiency of your system.  You want just enough
CFM to do the job, more is a waste, too little gives lots of unmet
load hours.  This is where I start when tweaking a system.  Your
cooling flow normally sets the flow as it has the smallest delta
T.  Heat pumps have a lower heating delta T than a furnace.  VAV
further complicates as your time averaged outside air requirements
also come into play and may not allow the VAV system to put enough
flow into a zone due to OA requirements of other zones.  OA is
also quite dominate when it comes to energy consumption.  One
other area to consider it where your controlling zone is for your
temperature control.  Selecting the proper zone can have a large
effect on unmet load hours.

eQuest is a tool and will never give you the perfect system with
the first press of the sim button.  If you are just starting with
eQuest consider yourself lucky if it even runs through.  Your
detailed reports give you the clues you need to refine your
model.  Start with the coarse variables, CFM, delta T's, note I
didn't mention throttling range.  Allowing your system to run
loose too early will hide problems in your design.  A 2 degree
throttling range is tight in my opinion but again eQuest is trying
to help with getting an efficient system.  Look for errors in
input, incorrect assumptions and numbers which don't seem right. 
Slow and plodding, one change at a time, check the effect and
ALWAYS have a backup file.

Bruce Easterbrook P.Eng.

Abode Engineering



On 08/04/2011 01:32 PM, Carol Gardner wrote:
Brian,



Hard to say without knowing more e.g. building type, internal
loads, etc., but in general, although Paul may disagree, that's
pretty high. I do not disagree with using that methodology, as
long as the cfm/sf stays within reason. 



If I were you I would start with looking at the internal loads,
like w/sf for equipment and process, etc. If they can be raised or
lowered reasonable, do so. The rule is, in general, that they be
the same in both cases, so make sure you pick the changes up in
your other model. Also look at your temperatures entered in the
zones/systems and associated schedules.



Otherwise, you can always send the .pd2 and .inp files to us.



Carol



On Fri, Apr 8, 2011 at 10:00 AM, Brian
Goldsmith <Brian.Goldsmith at atce.com>
wrote:




So if I force the
cfm/sqft to 2, it brings
the unmet load hours down to about 80, which is
good. But of course the fan
energy spikes. Each zone has its own unit. Is it
good practice to force the
cfm/sqft? Especially considering what this does to
the fan energy?

 

Brian
Goldsmith
Brian.Goldsmith at atce.com
direct  408.487.1217

 





From: Joe Fleming
[mailto:joe at thespinnakergroupinc.com]


Sent:
Thursday, April 07, 2011
1:55 PM



To:
Brian Goldsmith;
equest-users at lists.onebuilding.org

Subject:
RE: [Equest-users] Heat
Pump Heating Undersized




 
Check
the minimum
flow ratio and the minimum design flow cfm/sqft
for the trouble zones. 
 
In
the VAV system
equest distributes a percentage of the supply
airflow to each zone.  But
cfm/sqft is a priority in the hierarchy and if
you bump it up for the trouble zones
you will get higher cooling and heating airflows
at your given SAT.  The
energy you provide to the room is equal to
CFM*(Tsupply-Troom)*(1.085 or another
conversion factor depending on units
and air density).  So
if your schedules and equipment are set up
correctly you should override the
equest defaults for airflows or SAT.
 

 
Joe Fleming,
E.I., LEED AP BD+C, BEMP
Commissioning
Agent
561-602-3132
 
The Spinnaker
Group
www.thespinnakergroupinc.com

 


From: Brian Goldsmith
[mailto:Brian.Goldsmith at atce.com]


Sent:
Thursday, April 07, 2011
4:36 PM

To:
Joe Fleming;
equest-users at lists.onebuilding.org

Subject:
RE: [Equest-users] Heat
Pump Heating Undersized


 
Ya I looked at
all that, seemed to be
fine. It seems like whenever I change my system
from single zone package to
variable air volume this problem comes up. I
just changed the system type and
left everything else the same.
 

Brian
Goldsmith
Brian.Goldsmith at atce.com
direct  408.487.1217

 




From: Joe Fleming
[mailto:joe at thespinnakergroupinc.com]


Sent:
Thursday, April 07, 2011
12:14 PM

To:
Brian Goldsmith;
equest-users at lists.onebuilding.org

Subject:
RE: [Equest-users] Heat
Pump Heating Undersized

 
Usually
scheduling
(fan, available heating and t-stat don’t
coincide every hour of every day),
airflow (too low), SAT (too low), or multiple
zones on one unit.
Usually…
 


From:
equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org]
On
Behalf Of Brian Goldsmith

Sent:
Thursday, April 07, 2011
2:24 PM

To:
equest-users at lists.onebuilding.org

Subject:
[Equest-users] Heat Pump
Heating Undersized


 
I’m
having trouble with the heating in my heat pump.
I
modeled a Packaged VAV (PVAVS) with HEAT-SOURCE=
Heat Pump and ZONE-HEAT-SOURCE=
Not Installed. Set HEAT-SET-T to 120F,
DESIGN-HEAT-T to 70F, and did not
specify auxiliary heat. I get the message that
my zone has insufficient heating
capability. Looking at SS-A, is says the maximum
heating load is only -10.1
KBTU/HR. Looking at SS-P, the report says my
heating capacity is -47.8 KBTU/HR.
So why do I have 563 underheated hours? Does it
have something to do with the
system type? I tried ZONE-HEAT-SOURCE= Heat pump
and tried HEAT-SIZING-RATIO=
5, just to see if either one would do anything,
although I didn’t really expect
them to. Also tried electric supplemental heat,
which comes out to -31.715
KBTU/HR. My design supply flow is 1913 CFM, and
the outside air flow is 252
CFM. The design supply flow is the same for both
the heating and cooling
performance summaries. Should this be the case?
Thanks for any help.
 
Brian
Goldsmith, LEED® AP
Design Engineer

97 E. Brokaw Road, No 300
San Jose, CA 95112
main   
408.487.1200
direct  408.487.1217
www.atce.com

 






_______________________________________________

Equest-users mailing list

http://lists.onebuilding.org/listinfo.cgi/equest-users-onebuilding.org

To unsubscribe from this mailing list send  a blank message to
EQUEST-USERS-UNSUBSCRIBE at ONEBUILDING.ORG










-- 

Carol Gardner PE




_______________________________________________
Equest-users mailing list
http://lists.onebuilding.org/listinfo.cgi/equest-users-onebuilding.org
To unsubscribe from this mailing list send  a blank message to EQUEST-USERS-UNSUBSCRIBE at ONEBUILDING.ORG
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/attachments/20110409/bd0f61b5/attachment-0002.htm>