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Extremely high HVAC nominal capacity than hourly loads

asked 2022-02-21 09:04:42 -0500

Debayan93's avatar

updated 2022-03-15 08:55:37 -0500

Hi all,

I am trying to understand why my Autosized HVAC system's nominal capacities are so higher than the hourly load values of the heating/cooling system in DesignBuilder v7. I am using the following template (Fan Coil Unit with Electric Boiler & Air Cooled Chiller):

HVAC System

In the Output summary tab, I have nominal capacities in this range:

Nominal Capacity

However, my hourly load profiles are significantly smaller than nominal capacities from E+ reports in Results Viewer.

Boiler Chiller

My HVAC system is autosized with Global sizing factors for heating and cooling 1.25 and 1.15, respectively.

I am trying to understand the reason behind the significant difference between them. Am I looking at the wrong outputs to understand the sizing capacity and system load profiles? In that case, can anyone suggest what terms I should be looking for? Or else, the autosizing of nominal capacity is being done very differently in the simulation, which I expect to have a closer value to the peak of the individual hourly load.

Any insights would be helpful.



Edit: The EnergyPlus file is attached here:

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answered 2022-02-21 09:21:32 -0500

updated 2022-02-21 09:25:55 -0500

You should look at these plots of data during the design days. Set Run Simulation for Sizing Periods to Yes if not already set.

  Yes,                     !- Do Zone Sizing Calculation
  Yes,                     !- Do System Sizing Calculation
  Yes,                     !- Do Plant Sizing Calculation
  No,                      !- Run Simulation for Sizing Periods
  Yes,                     !- Run Simulation for Weather File Run Periods

Check the averaging window used for sizing. If you leave this field blank the averaging window will equal 15 minutes. Try using 4 time steps in the averaging window to dampen peak loads.

Timestep, 4; (15 minutes)

  1.25,                    !- Heating Sizing Factor
  1.15,                    !- Cooling Sizing Factor
  ;                        !- Timesteps in Averaging Window Field: Timesteps in Averaging Window The number of load timesteps in the zone design flow sequence averaging window. The default is 1, in which case the calculated zone design flow rates are averaged over the load timestep. The zone design air flow rate calculation is performed assuming a potentially infinite supply of heating or cooling air at a fixed temperature. Thus the calculated design air flow rate will always be able to meet any load or change in load no matter how large or abrupt. In reality air flow rates are limited by duct sizes and fan capacities. The idealized zone design flow calculation may result in unrealistically large flow rates, especially if the user is performing the sizing calculations using thermostat schedules with night setup or setback. The calculated zone design flow rates are always averaged over the load timestep. The user may want to perform a broader average to mitigate the effect of thermostat setup and setback and prevent the warm up or cool down flow rates from dominating the design flow rate calculation. Specifying the width of the averaging window allows the user to do this. For example, if the load calculation timestep is 15 minutes and the user specifies the Timesteps in Averaging Window to be 4, the zone design air flows will be averaged over a time period of 1 hour. Specifying 8 would result in averaging over a 2 hour period.

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Hi @rraustad,

I have the timesteps as 6 in my simulation. However, even after seeing the peak load on the design day, nominal capacity is still 8-9 times of peak load (case of Boiler). I am unable to understand why it is such significantly higher, not being a closer number to the peak load on design day multiplied by the sizing factor. I am trying to do the undersizing of my HVAC later based on autosized values of nominal capacity.

I have attached the .idf file in the original post. Maybe you can have a look and let me know.

Best, Debayan

Debayan93's avatar Debayan93  ( 2022-02-21 09:58:10 -0500 )edit

The eio shows that the heating coil is the same size as the boiler.

Component Sizing Information, Coil:Heating:Water, 10 FAN COIL UNIT HEATING COIL, Design Size Rated Capacity [W], 29343.71797
Component Sizing Information, Coil:Heating:Water, 10 FAN COIL UNIT HEATING COIL, Design Size Maximum Water Flow Rate [m3/s], 7.13145E-004

Component Sizing Information, Boiler:HotWater, BOILER, Design Size Nominal Capacity [W], 29343.71797
Component Sizing Information, Boiler:HotWater, BOILER, Design Size Design Water Flow Rate [m3/s], 7.13145E-004
rraustad's avatar rraustad  ( 2022-02-21 10:31:17 -0500 )edit


Should I also hardsize then the Fan Coil Unit Heating Coil to undersize the entire heating system?

Best, Debayan

Debayan93's avatar Debayan93  ( 2022-02-21 10:39:15 -0500 )edit

The HW coil is setting the size of the boiler. Now need to figure out why the heating coil is sizing that large. If you change zone heating SAT from 50 to 30 you get 13.5 kW heating coil/boiler size. I'd review the sizing inputs.

    30.00,                   !- Zone Heating Design Supply Air Temperature {C}
rraustad's avatar rraustad  ( 2022-02-21 10:48:43 -0500 )edit


Thanks, reducing it to further lower values like 20 gives a fairly lowered size. However, I was wondering is there any simpler way to auto-calculate these inputs so the nominal capacity matches near peak design day load. I am trying to minimize the need for user-provided inputs to retrieve the nominal capacity based on which I can undersize the components then.

Best, Debayan

Debayan93's avatar Debayan93  ( 2022-02-21 11:18:36 -0500 )edit

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Asked: 2022-02-21 09:04:42 -0500

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Last updated: Feb 21