[Equest-users] T-Stat is acting Screwy

[Fred Porter]

Rob,
OK, I can't resist a bit of lunchtime "Stump the Chump." First I'd say verify the setpoint change is actually working by examining zone SS-Os 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 supply air temperature (SAT) control, which is very important for a CV-reheat application. And of course location. 
 
My bet is that eQuest is resetting your SAT down at night based on hitting the upper RH limits during the zone setback scenario. This is why the CHW load increases. This is also why HW savings might not be as great as expected. 
 
Why? 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) due to the moisture in the OA, even if internal latent gains are nil. If the 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 mode occurs more often  in the "parametric" case as the zone temperature decreases.
 
Fred Porter
 
P.S. The only way to truly follow the SAT is to set up some hourly reports.  I recommend the use of variable 225 for your AHU. 
 

>>> 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:
Do you have many lab hoods? And are exhaust and make-up air revised for the lab hoods at the night set back time?
What is the percent of outside air to the lab? Does this percentage change during the night set back time?
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?
Do you utilize an air or water side economizer for 1st stage cooling? Does this use change any at night set back time?
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?
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?
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. 
 ( http://www.mcclure-engineering.com/ )

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
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