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Re: [EnergyPlus_Support] set point problem
On 2 May 2007 at 15:58, Sebastian Dietz wrote:
> Hello all,
>
> I'm simulating a one zone model. The model includes a plant loop
> consisting of boiler:simple, baseboard:convective, heating coil and a
> air loop consisting of outside air mixer with heatrecovery, fan, heating
> coil and single duct:vav no reheat. I have two questions about the output.
>
> 1. I'm wondering that the output "heat exchanger total heating energy"
> has only positive values. Even in summer, when the heat exchanger has a
> cooling function for supply air. ( It should be negative because of the
> equation for air to air heatexchanger in the engineering reference
> Q_total = m * ( h_sup_in - h_sup_out))
There are separate report variables for heating and cooling, "Heat Exchanger
Total Heating Energy" and "Heat Exchanger Total Cooling Energy". You must
report both of these to see the full picture. If the heat exchanger is heating
when you do not want it to, then the controls are not set properly.
> 2. The zone mean air temperature fluctuates about 0.8 degree centigrade
> around the setpoint of 22 degree centigrade (Baseboard priority 1,
> single duct priority 2). When I flip the priorities this effect
> disappears. Does anybody know where this effect comes from?
>
> for more details I uploaded my testobj1.idf.
>
I see the problem with the controls. The air system uses SINGLE
DUCT:VAVHEATANDCOOL:NOREHEAT, with a min air fraction of 1.0, so it is a
constant volume system. The supply air temperature is controlled by a SET
POINT MANAGER:SINGLE ZONE HEATING object. Set point managers are called at the
beginning of each air loop simulation and do not update as the zone equipment
is simulated. So, when the baseboard is priority 1, the air system does not
know about the heat added by the baseboard and tries to meet the full heating
load. If you report the damper position and the supply air temperature, you
will see that the air system is at full flow at 30C (which is the maximum
supply air temperature specified in the set point manager).
So, the only option that will work is to put the baseboard as priority 2.
Because the VAV system is operating as a constant volume system, I do not think
it will matter if the air system coil carries more of the load than intended.
I think the overall energy use result will be the same as the baseboard with
priority 1 if the controls were working properly for that case.
Mike
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
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