Question-and-Answer Resource for the Building Energy Modeling Community
Get started with the Help page

# How does "autosized" work in EnergyPlus OpenStudio with External Inputs?

I am using a PlantComponent:TemperatureSource connected to a WaterHeater:Mixed which is then connected to a ZoneHVAC:Baseboard:Convective:Water. I had a few of these components set to autosize but realize they might not be receiving enough information during the design days. I am running the energy model in a co-simulation with another program that, during runtime, feeds in the temperature of the PlantComponent:TemperatureSource and the Mass Flow rate of a pump. Unfortunately, none of this information is passed during the design days.

My question is, how does EnergyPlus autosize equipment? Does it only look at the design days to find the most extreme conditions?

Additionally, because I'm not feeding in the temperature and flowrate values prior to the run period, is there anyway I can still have parts of the connected plant loops autosize?

edit retag close merge delete

Sort by » oldest newest most voted

### Foreword

This a rather large question, and it's going to be hard to really explain in detail how the different sizing routines work and play together. That's why the Engineering Reference has such a huge section treating about sizing.

I'll do my best to get to the point and you'll have to forgive any gross simplification.

The first of such simplifications, is that I'll take out the WaterHeater:Mixed from the problem, I'm assuming it doesn't have a built in heater and is just served by the PlantComponent:TemperatureSource[1]

### Conclusion

I'm writing the conclusion here because the section where I explain the sizing is lengthy

What you need to really remember: to size your hot water coil, the program actually gets user inputs defined in several objects and uses several sizing routines. It will use the design temperatures (and air flow rates in some cases) defined in the Sizing:Zone object and the resulting calculated laods, as well as the plant delta T you specify in Sizing:Plant.

It's up to you to determine whether you do need to feed information for the sizing routines to work properly.

My two cents is that if in your application the zone design temps aren't changing, and if you intend to keep the same constant delta T in the plant loop, you shouldn't need to feed new info for co-simulation.

If you do need to change it for the sizing routines, I'm sure there's a way (post a new question though).

### Sizing Manager

For the sizing, it's done on the design days indeed.

It's going to size stuff in this order (that's what I understand at least): Zone, System (airloop), Component, and Plant. Important: please read more in the Sizing Manager section of the engineering reference.

(FYI, it's also going to start by looking at the global Sizing:Parameters)

### Zone Sizing

First, it's going to size your zones, based on the Sizing:Zone (I/O) object, in which you enter the supply air temperature, humidity ratio, and potentially air flow rate (or tell it to calculate the air flow rate based on design days). That'll give you the heating and cooling loads.

You have no airloop here, so System is skipped.

### Component sizing

Second, it's going to look at your components, in your case the ZoneHVAC:Baseboard:Convective:Water, and try to size the heating coil. The relevant section of the engineering reference is Coil:Heating:Water Sizing > Zone Coils

Get input from Zone Sizing

First, it gets a few input stored in the Zone Sizing routine in Zone Design Data Array (read for variable definition)

It gets the zone heating coil design air inlet temperature [C]:

$$T_{in,air} = DesHeatCoilInTemp_{zone}$$

This is calculated by checking the zone temp at sizing (from the Tstat), and the potentially Outside Air Fraction and temperature (at the design day).

And the zone design heating ...

more

1

Seems the MathJax is broken once you expand the post clicking on "More". But click on "Add comment" (and no need to write anything) and it displays correctly. @Neal Kruis you may want to take a look.

( 2017-07-31 04:33:19 -0500 )edit

This is a very good write-up, thank you for taking the time to type everything out so clearly. Understanding that autosizing starts from the zone and progresses towards the plant loops helps considerably. My current co-simulation has minimal impact on the zone but a much larger alteration to the plant loop. I think autosizing the zone and baseboard would be acceptable but I'll define the design flowrates for the Temperature Source and Water Heater (no energy, storage). Thank you for all the references. I will be digging deeper into this answer in the coming months.

( 2017-07-31 08:13:15 -0500 )edit