space heating between natural gas VAV RTU and electric resistance terminal units

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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.

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.

Ramana Koti's picture
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Joined: 2011-09-30
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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

David Reddy3's picture
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Joined: 2011-09-30
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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.

>> Christopher Jones, P.Eng.

Chris Jones's picture
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