[Equest-users] Which system is best to use?

Chris Jones cj at enersave.ca
Thu Mar 22 08:21:46 PDT 2012

It's a frustrating issue.  In a typcal "real" 
building, the VAV central coil resets based on 
the zone with the highest cooling demand.  On 
cold mornings, the reset can be as high as 70° 
F.  According to the documentation in 
DOE22Vol2-Dictionary, the keywords COOL-CONTROL, 
HEAT-MIN-RESET-T and some others should allow you 
to simulate a VAV system that has a heating coil 
that can reset the supply air temperature.  I 
have never been able to find that magic 
combination that results in the hourly reports 
the heating coil providing heating when I would expect it to.

The issue with the preheat coil is that you have 
to set the exiting temperature a few degrees 
lower than the cooling MIN-SUPPLY-T to account 
for fan heat so you don't increase heating 
significantly.  This still results in too much reheat required at the zones.

At 04:04 PM 15/02/2012, Rob Hudson wrote:
>When using the VAV or PVAV system types the 
>heating coil can be sized by using the leaving 
>air temperature conditions, but never turns 
>on.  The true way to use a heating coil for a 
>VAV is to call it a preheat coil, and specify a 
>leaving air temperature close to what you have 
>in reality.  Occasionally, the reality is higher 
>than what eQuest allows (something like 70 
>degree F) and then you need to "fake" it by 
>making up the rest of the heating load in the reheat coils.
>Hope this helps!
>Rob H.
>On Wed, Feb 15, 2012 at 2:31 PM, R B 
><<mailto:slv3sat at gmail.com>slv3sat at gmail.com> wrote:
>I have a system with VFD on the fan, central 
>heating and cooling coils (no reheat), and 
>chilled water for cooling. Curious to see how 
>others would model this, or if I am missing something very obvious.
>If I use VAV, even though there is central coil, 
>it does not come on. The heating seems to be 
>only through the reheat coil. I could do the 
>warmest zone control, but I think this uses only 
>return air, not heating coil for the
>If I use single zone reheat, cannot get the fan to be variable speed.
>PVVVT might work (never tried it) but it does not use chilled water.
>I could use a dual duct single fan and the min flows be zero fo both ducts?
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>Rob Hudson
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Christopher Jones, P.Eng.
Suite 1801, 1 Yonge Street
Toronto, ON M5E1W7
Tel. 416-203-7465
Fax. 416-946-1005
email cj at enersave.ca
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