[Equest-users] T-Stat is acting Screwy

[Eric O'Neill]

Very interesting. It hadn't occurred to me that you might bump into the
dehumidification set-point during night setback (then again, a lot of
things haven't occurred to me). But it seems to need to be under a set
of just right conditions. When could the space cool down fast enough at
night, while still having a high enough humidity level to trigger
dehumidification? 

I don't think this would occur during the winter. Let's assume this
building is up north where shell losses can add up and cool a space down
relatively fast during winter. Even if you're humidifying to 40% at 70F
during the day (which he's not) and dropping to 60F at night, you're
only hitting 57% RH, assuming constant absolute humidity. For a lab, the
make up air (which has a low absolute humidity because of the low
ambient temperatures) would probably dry out the space before the indoor
temp hits a point low enough to set of dehumidification. 

So the other possibility I see (which is what Fred described, I believe)
is that during some nights the make up air is bringing in moist, cool
air, and the system isn't tempering it because the zone is satisfied. So
this extra air cools down the space and drags up the RH, triggering a
dehumidification mode that would be unnecessary at 70F indoor set-point.
But if this was the case, wouldn't your heating energy go down more than
slightly due to the reduced ventilating loads?

What am I missing here? How large are those internal latent loads (seems
to be a wild card to me)? I'm curious as to whether you've solved this
one Rob. 

Cheers,

Eric

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Fred
Porter
Sent: Wednesday, May 19, 2010 7:07 PM
To: Rob Hudson
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] T-Stat is acting Screwy

 

Rob, 

I'm a little unclear about statements like "I looked at my SAT and it is
controlled by a reset schedule." Does this mean that the reset schedule
a) came from "The Wizard," b) actual AHU sequences, c) a departed
modeler (you know who you are!) or some other source. This is important
and should be checked. 

 

What's happening to increase the cooling coil load is that at certain
hours the baseline reset SAT (e.g. 60F high RH) is not providing 53% RH
indoor conditions at reduced 65F zone temp even though it did at 70F. So
DOE-2.2 resets down to the MIN-SUPPLY-T (probably 55F) in the
"parametric" with the setback zone Ts. Thus, coil load shoots up at
those hours, and there are no hours where the cooling load is decreased
by changing, even increasing, the zone Ts.

 

The lab users need to decide whether the appropriate upper RH limit is
really 53%. It if can be increased, then you can save money by
increasing it, and instituting setbacks. And perhaps a setpoint decrease
to only 67-68F might give better results in reality and model land. But
it sounds like you are unsure of some of the model and the lab
requirements. If this is the case I would be very careful about making
recommendations to lab owners and users along the lines of "....reduce
zone heating setpoint to 60F," because if they could just do that, they
might not spend millions of dollars on Phoenix valves to control VAV.
You need to model the correct SATs, and only reasonable zone conditions.


 

Fred Porter

 


>>> On 5/19/2010 at 1:21 PM, in message
<AANLkTinV1_p5XWNJrIiRwocYPovZQVPlvHc1Uzu0DixR at mail.gmail.com>, Rob
Hudson <rdh4176 at gmail.com> wrote:

I'm trying to wrap my head around this the best i can. 

I looked at my SAT and it is controlled by a reset schedule. the
temperature of the space never reaches anything above about 72 deg,
which makes me think that what you are saying is very true and the
heating t-stat schedule is in control all the time. 

So can you tell me what it is that i can do to fix this? 

On Wed, May 19, 2010 at 2:52 PM, Fred Porter <FPorter at archenergy.com>
wrote:

Rob,

OK, I can't resist a bit of lunchtime "Stump the Chump." First I'd say
verify the setpoint change in the runs is actually working by examining
SS-O for the baseline and parametric run. But I'll assume it is, and
offer my explanation.

One piece that is left out in all this is the SAT control.... very
important for a CV-reheat application. And LOCATION! 

My bet is that your SAT is resetting down at night based on hitting the
upper RH limits during the setback scenario. This is why the CHW load
increases. This is also why HW savings might not be as great as
expected. 

Holding all else the same, the hourly temperature in a lab zone will
with minimal internal gains as described will follow the heating
setpoint down; the cooling setpoint will not be in control. Generally
this will lead to high internal relative humidity (which is why this
setback is only done carefully and to a limited extent in labs) from the
OA for many hours, even if internal latent gains are nil. If the
"baseline" model has any SAT reset, the MAX-HUMIDITY resets the hourly
SAT back down to the MIN-SUPPLY-T when the model zone RH exceeds the RH
setpoint, in this case a relatively low 53%. This increases in frequency
as the zone temperature decreases.

Fred Porter


>>> On 5/19/2010 at 9:24 AM, in message
<AANLkTinsmv-lXFuk3A65lvU2asMT_VshFXVOYiSk6xwB at mail.gmail.com>, Rob
Hudson <rdh4176 at gmail.com> wrote:

Thanks for your thoughts and here is your answers:

1) yes, but there is no night set back time.
2) everything is 100% oa and constant volume 24/7/365
3)no exhaust heat recovery, and steam preheat. the thermostat is set for
70 - 73 heating and cooling and the set back is to 60 and 80 at night
for my parametric run.
4) the cooling comes from a central chilled water plant for the entire
campus, and i just created a chilled water loop, added a meter and a
pump and it seems to be running happily.
5) the fans run all the time, day and night
6) I do have internal loads with latent heat, and it cycles down at
night to almost nothing. These loads are currently the same for the
model and the parametric run.
7) i have set humidity levels to 20% and 53%, which also do not change
with my parametric run. 

hope this give you enough information.

thanks again

On Wed, May 19, 2010 at 11:13 AM, David Bastow
<dbastow at mcclure-engineering.com> wrote:

Rob,

I have a number of questions:

1.	Do you have many lab hoods? And are exhaust and make-up air
revised for the lab hoods at the night set back time?
2.	What is the percent of outside air to the lab? Does this
percentage change during the night set back time?
3.	Do you have a pre-heat or exhaust heat recovery and pre-cooling
on your make-up air? What is the temperature setting of this pre-heat or
heat recovery and pre-cooling? Does the temperature settings on these
change at the night set back time?
4.	Do you utilize an air or water side economizer for 1st stage
cooling? Does this use change any at night set back time?
5.	Does the fan run continuously day and night? Have you ran models
with the fan running continuously, and with fan cycling based on demand
at night and off completely at night, to see how the results compare?
6.	Do you have a internal load watts per square foot and latent
load on the space from interior lighting and equipment? Do these loads
change at the night set back time?
7.	Are you adding humidification or dehumidifying the space based
on some humidity settings? Does the humidity settings change during the
night set back time?

One of these things is probably causing the increased chilled water
cooling load. Often if you have high internal loads, even at night, then
changes to the fan cycling and the amount of outside air brought in at
night, will increase the chilled water cooling load. I would review all
of these areas and run various test models to see how they each affect
your energy usage when modified.

Our firm just completed modeling some very large lab facilities with
more than 55 exhaust hoods in the building, with high internal loads
24/7 and 100% outside air. As long as the models are set up correctly
they are normally right. It takes some real design and thermal dynamic
thought and often may models to really get your mind right with what is
truly going on with the facility. Its important to keep an open mind to
what is going on. Having been doing computer hourly modeling for over 17
years, it is often easy to think you have a handle on what is going on
with the building, but it is important to keep an open mind and
investigate all the different avenues that you can think of until figure
out what is going on. I have 99% of the time that I have blamed the
screwy program having problems that I have found that I just didn't look
at all the different angles enough.

David A. Bastow 

McClure Engineering, Inc. 

 

________________________________

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Rob
Hudson
Sent: Wednesday, May 19, 2010 7:46 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] T-Stat is acting Screwy

I have a lab space that is kept between 73 and 70 degrees all year long.
One of my parametric runs has the cooling and heating T-stat schedules
changing to have night time setbacks to 80 and 60, respectively. When i
use these, i get more energy spent overall. specifically, i have a
chilled water meter, steam meter, electric meter and hot water meter to
monitor everything. The chilled water increases while the others
slightly decrease when i use the set back schedules. Any ideas?

-- 
Rob Hudson




-- 
Rob Hudson




-- 
Rob Hudson

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/attachments/20100521/95936f74/attachment-0001.htm>