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Re: [EnergyPlus_Support] Warmest Function Increases Boiler Natural Gas Usage?
The fan energy increase makes sense if you increase the supply air
temperature (you cannot avoid this). And the chiller savings also make
sense if you increase the chilled water temperature. A reduction in
cooling tower energy would only make sense if the load on the tower was
reduced. Since the chiller energy (and not the load in the space) is
reduced, the load on the tower is reduced.. For the natural gas
consumption, check the minimum air flow fraction on the VAV reheat
boxes. The natural gas consumption will only increase if the sensible
cooling delivered to the space increases when the VAV boxes are at the
minimum air flow rate. Check the zone sensible cooling rate for the
zones that have increased VAV box reheat energy. Find a time when the
zone VAV box is at the minimum air flow rate (you can also look at the
VAV box damper position for this check) and compare this zone sensible
cooling rate to what you get when you change the set points. You could
also reduce the minimum air flow fraction on the VAV boxes and the
natural gas consumption should decrease. This can also be accomplished
by decreasing the maximum air flow rate on the VAV boxes that are using
more reheat. You want this type of reheat to be at a minimum if at all
possible.
noelcrrl wrote:
>
> All-
>
> I am modeling the warmest function in a Hospital building. The model
> is equipped with a VAV system (pre-heat coil, cooling coil, a central
> heating coil, & reheat coils). I have the warmest setpoint node set
> at the supply fan outlet. The warmest function is set up to deliver
> supply air between 55F and 60F. Also, the chilled water is delivered
> to the cooling coils between 44F and 52F, depending on outside air
> temperature.
>
> When I increase the upper limit of the warmest function from 60F to
> 65F and the chilled water upper limit from 52F to 54F, the natural
> gas consumption of the boiler increases by 7%. In addition, the
> supply fan energy usage increases by 6%. The energy saving benefit
> of the chiller and the cooling tower, 2% and 3%, respectively, is not
> enough to overcome the boiler and fan energy usage increases. The
> overall energy usage (total facility) increases by 2%.
>
> I am confused to why this is happening? How can increasing the reset
> temperatures increase overall energy consumption instead of
> decreasing it? Why is the heating energy usage increasing in this
> manner?
>
> Thanks in advance for your help.
>
> Noel
>
>
--
Richard A. Raustad
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