[Equest-users] space heating between natural gas VAV RTU and electric resistance terminal units

[Chris Jones]

The VAVS and PVAVS systems do not correctly 
implement the central heating coil as documented.
Ideally one would like the VAV system to control 
the supply air temperature to meet the cooling 
load of the "warmest" zone with its box 
open.  The supply air reset temperature is controlled by
COOL-MAX-RESET-T.

One would expect the heating coil to kick on 
during heating season so the supply air 
temperature is reset up to somewhere between 
MIN-SUPPLY-T and COOL-MAX-RESET-T.  From the documentation under HEAT-CONTROL:

"In SYSTEM TYPEs VAVS, RHFS, PVAVS, and PIU the 
HEAT-CONTROL keyword is activated when COOL-CONTROL
is set equal to WARMEST and a central heating 
coil has been specified by giving HEAT-SET-T a
value greater than MIN-SUPPLY-T; in this case the 
only non-ignored value for HEAT-CONTROL is COLDEST.
When coldest/warmest control is specified the 
controller first looks to see if any ZONE has a cooling demand; if
there is a ZONE cooling demand the AHU discharge 
temperature is reset to attempt to just satisfy the ZONE with
the largest cooling demand. If no ZONE has a 
cooling demand the controller looks to satisfy the ZONE with the
greatest heating demand but without forcing any 
ZONE into creating a cooling demand. If neither a heating or
cooling demand is found for any ZONE mixed air is passed."

That does not happen.  If you do the hourly 
report runs and check the supply air temperature, 
the OA T, the mixed air temperature, and some of 
the zone loads, you will find that the supply air 
temperature doesn't get reset as expected.

Note that you do need to set a wide throttling range - 4° - 6°F.

You can also try changing the RESET-PRIORITY keyword to see if that helps.


I have found the only way to avoid over use of 
the electric baseboards is to add a pre-heat coil 
and set the temperature to the MIN-SUPPLY-T less 
the fan delta-t.  At least you will a supply air 
temperature that is the minimum.  You will still 
get some over use of the electric baseboards.

You can try a COOL-CONTROL = RESET and play with 
the temperature schedules and BASEBOARD-CTRL = 
RESET, and adjust the schedules.


0At 06:56 PM 22/06/2012, David Reddy wrote:
>Ramana-
>
>It appears you have not defined at Hot Deck Max 
>Leaving temp (HEAT-SET-T), which needs to be 
>specified for the central Furnace coil to be 
>energized in the simulation.  Also, for PIU (and 
>I believe all the other multizone systems), the 
>central coil LATs will track the Cool Control 
>properties, which in your case, is an OA Reset 
>Schedule.  I suggest setting up an hourly report 
>for the air handler coil leaving air 
>temperatures, and observing what is happening 
>are to understand how the control set points are being implemented.
>
>Hope this helps,
>David
>
>On 6/22/2012 1:25 PM, Ramana Koti wrote:
>>Dear all,
>>
>>My question relates to distributing space 
>>heating between the central VAV RTU (natural 
>>gas furnace) and PIU terminal units (electric 
>>resistanct heating). My elecriticy consumption 
>>for space heating end use seems very high from 
>>November to April which makes me suspect that 
>>the system is not set up accurately - majority 
>>of space heating is wrongly being picked up by 
>>electric resistance in terminal units as 
>>opposed to being picked up by the natural gas furnace in the RTU.
>>
>> From the BEPS report, of the total annual 
>> energy consumption of 372.7 MBTU, 148.7 MBTU 
>> is electricity space heating and 7.6 MBTU is natural gas space heating.
>>
>>I have attached eQUEST screenshots of my 
>>heating and cooling cap/control settings for 
>>this system. Any input on how this situation 
>>can be rectified will be highly appreciated.
>>
>>Thank you,
>>Ramana Koti.
>>
>>
>>
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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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