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Design size nominal capacity have very large differences

asked 2018-03-14 05:00:55 -0500

Mans_sopers gravatar image

updated 2018-03-14 09:00:27 -0500

When running the simulation of a building with 8 conditioned spaces, I wanted to see what the capacity of the heaters should be to fulfill the temperature setpoints I set. The only comparible data I could find in the results are the design size nominal capacity compared to the user-specified nominal capacity. The results I got are in the table below. The minimum temperatures for the PTAC and Coil 6 are set for 15 degrees C for the entire year. The other setpoints are either at 5 degrees for the entire year, or 20 degrees once a week during office hours.

I expected some differences in the results, but not this large. Also, for the bathroom (coil 5) apparently I need more than 3 times the capacity than for the pantry (coil 7), while they have the same setpoints and the pantry has a larger space area. This too seems odd. Could someone please explain why these numbers are off (by this much) and if the design size nominal capacity is the correct way to look at what capacity I would need to fulfill the setpoints?


Design Size Nominal Capacity [W]    User-Specified Nominal Capacity [W]
COIL HEATING ELECTRIC 1 5614.64 1000.00
COIL HEATING ELECTRIC 2 5699.91 1000.00
COIL HEATING ELECTRIC 3 2291.29 2000.00
COIL HEATING ELECTRIC 4 2893.05 2000.00
COIL HEATING ELECTRIC 5 9292.89 1000.00
COIL HEATING ELECTRIC 6 17418.09    1000.00
COIL HEATING ELECTRIC 7 3020.91 2000.00
PTAC ELEC HTG COIL  25180.63    2000.00

I added my model in case this is not clear:

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answered 2018-03-14 12:28:10 -0500

updated 2018-03-14 12:30:53 -0500

It looks like you specified 1,000W or 2,000W for heating capacity in OpenStudio. Where did these values come from? Is this an existing building?

Are Coils 1 - 7 part of air terminals connected to central systems, or zone-level equipment like the PTAC?

The "Design Size Nominal Capacity" outputs are the result of auto-sizing, where EnergyPlus uses parameters specific for the design day separately from the annual simulation. These parameters are defined across multiple objects, including:

  • Sizing:Zone - supply air temperature, fresh air requirements, etc. for each conditioned zone to maintain over the design day. In OpenStudio, these are defined in the Thermal Zones model data tab on the left. image description
  • SizingPeriod:DesignDay - outdoor air conditions during design day. In OpenStudio, these are defined in the Site model data tab on the left. image description

I would start by checking the inputs of these objects. If those look alright, then check your heating thermostat schedule of temperature values. There are separate schedule profiles to define for the summer design day, winter design day, and annual simulation (run period). In OpenStudio, these are defined in the Schedules model data tab on the left. image description

If the thermostat schedules look OK, then examine the summer and winter design day profiles for your other schedules (equipment, occupants, lights, etc.).

Finally, consider the differences in the placement (core vs perimeter, north vs south, etc.), internal gains (equipment, occupants, lights), and floor areas between your conditioned spaces to explain these differences in auto-sized heating coil capacity.

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I downloaded the design days from the website displayed at the site tab. They're from Amsterdam since the building is supposed to be placed in the Netherlands. The building is not yet built and the 1000 and 2000 W coil heaters are just to see what happens. I know the size is a bit low. As to the coils, they are supposed to be zone level. I don't know if I did that wrong and if that could cause the problem. I will check the sizing:zone, but if this is from the same DDY file (Amsterdam) they should be alright, right? I manually created the thermostat schedules, so those temperatures should be ok

Mans_sopers gravatar image Mans_sopers  ( 2018-03-14 12:57:09 -0500 )edit

I havent checked the winter and summer schedules for the other equipment, since these are constant throughout the year (in the scenario I wrote). The mayor difference between the very high sized capacity spaces and the rest is the thermostat I think. Could you maybe take a look because I've been over these tabs already a few times. The link to the file is in my original question. Thanks for the clear answer though, it is still really helpful in understanding OpenStudio better.

Mans_sopers gravatar image Mans_sopers  ( 2018-03-14 13:00:19 -0500 )edit

I tried adjusting the Sizing:Zone parameters a little, but this does not change the fact that some spaces need very large capacities while others don't. For the ACDC and secondary room this might be explained by the fact that it runs 24/7, but I have absolutely no idea how it is possible that the bathroom would need such high capacity. 9000W for a bathroom seems rediculous. It's just a plain bathroom.

Mans_sopers gravatar image Mans_sopers  ( 2018-03-15 05:16:55 -0500 )edit

To continue on my last comment. I checked all schedules, setpoints etc. For the bathroom, the lighting schedule is different than the heating schedule (lighting 6 times at a workday spread over the day, heating the entire workday (8 am to 6 pm). There is no other internal equipment other than lighting. The bathroom is the smallest room in the model and is placed entire inside the building (only outside boundary is with the roof). I also tried changing the wall conductivities but it seems unaffected. Really running out of options now so I appreciate the help.

Mans_sopers gravatar image Mans_sopers  ( 2018-03-15 08:59:43 -0500 )edit

Final comment, sorry for the spam but I'm trying to figure this out: I assigned the heater to the thermal zone "bathroom", which consist of three spaces (toilet 1, 2 and shower). Is it possible it assigns a heater to each space, therefore acting like I need three times the heating capacity? And if so, could this be solved by setting the multiplier at 0.33?

Mans_sopers gravatar image Mans_sopers  ( 2018-03-15 09:16:46 -0500 )edit

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Asked: 2018-03-14 05:00:55 -0500

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Last updated: Mar 14 '18