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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-CONTROL, HEAT-SET-T, COOL-MAX-RESET-T and
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.<br><br>
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.<br><br>
sa <br>
At 04:04 PM 15/02/2012, Rob Hudson wrote:<br>
<blockquote type=cite class=cite cite="">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.<br><br>
Hope this helps!<br><br>
Rob H.<br><br>
<br><br>
<br><br>
<br>
On Wed, Feb 15, 2012 at 2:31 PM, R B
<<a href="mailto:slv3sat@gmail.com">slv3sat@gmail.com</a>>
wrote:<br>
<dl>
<dd>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.<br>
<dd> <br>
<dd>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 <br>
<dd> <br>
<dd>If I use single zone reheat, cannot get the fan to be variable speed.
<br>
<dd> <br>
<dd>PVVVT might work (never tried it) but it does not use chilled
water.<br>
<dd> <br>
<dd>I could use a dual duct single fan and the min flows be zero fo both
ducts?<br>
<dd> <br>
<dd> <br>
<dd>Thanks<br>
<dd>-Rohini<br><br>
<dd>_______________________________________________<br>
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</dl><br><br>
<br>
-- <br>
Rob Hudson<br>
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<x-sigsep><p></x-sigsep>
>><br>
Christopher Jones, P.Eng.<br>
Suite 1801, 1 Yonge Street<br>
Toronto, ON M5E1W7<br>
Tel. 416-203-7465<br>
Fax. 416-946-1005<br>
email cj@enersave.ca<br>
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