[Equest-users] Dual-Duct System: Thermostat Input

[Aaron Powers]

Hey Nick,

I've recently come across the same sort of thing with a multizone system,
so maybe some of my experiences can shed some light.  The REVERSE-ACTION
t-stat in DOE2 allows the heating action of your system to be variable
while the PROPORTIONAL stat sets the heating flow to a constant.  So any
time the slope of the heating side of your control drawings are greater
than 0, it is a REVERSE-ACTION, and if the heating side is flat it is
PROPORTIONAL.  As for your specific Titus examples, this is how I
understand it:

1. I believe you are correct.  This can be modeled with a reverse acting
stat, and the setpoints are equal in that particular diagram.  However, the
Titus blurb says that a deadband is also allowed, so the setpoints could be
different.  A key part of this strategy is that there is no enforcement of
a minimum flow, so to get this exact strategy, you also need to set the
MIN-FLOW-RATIO to zero.

2. This is the prototypical PROPORTIONAL DOE2 stat.  The cooling action is
throttled proportionally, and a constant rate is used for heating.

3. This is REVERSE-ACTION with MIN-FLOW-RATIO set to equal the low valley
part of the diagram.

4. For a constant volume MZ box, there is only one control action, so the
stat type is a moot point.  You can test this by inserting constant flow
boxes by setting MIN-FLOW-RATIO to 1.0 and running with both types.  The
results should be identical.  It's strange that the documentation says the
default is proportional, because it doesn't really matter for a CV box.
Also, remember that for CV MZ boxes, DOE2 sets the hourly setpoint to the
average of the cooling and heating setpoints and the throttle range spans
the setpoints.

As with most DOE2 stuff, I may have got one or two details wrong, but this
is how I understand it.  Hope this helps.

Aaron


---------- Forwarded message ----------
From: Nick Caton <nick at 360-analytics.com>
To: "'Equest-users'" <equest-users at lists.onebuilding.org>
Cc:
Date: Tue, 30 Sep 2014 16:50:57 -0700
Subject: [Equest-users] Dual-Duct System: Thermostat Input

Hi everyone,



I’m trying to fully understand the implications of zonal thermostat
selection with respect to dual-duct system mixing boxes (System type DDS).
The DOE2 help entry for thermostats speaks specifically to DDS, but I’m
left a bit unsure by the wording…



Here is the DOE2 help entry for reference:

With that primer, here is an excerpt from my Titus catalog illustrating
mixing box control schema for dual-duct mixing boxes.



Have I got the right idea for how to achieve each of the pictured mixing
box control strategies in distinction?



~Nick







*[image: 360 Logo cropped]*

*NICK CATON, P.E.*
*Senior Engineer*



*360 Analytics*
9750 3rd Ave NE, Suite 405

Seattle, WA 98115
office:  206.557.4732 ext. 205
www.360-Analytics.com <http://www.360-analytics.com/>
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