Question-and-Answer Resource for the Building Energy Modeling Community
Get s tarted with the Help page
Ask Your Question

only 1 chiller is used if autosizing chiller capacity

asked 2015-08-25 09:37:56 -0500

updated 2020-03-10 09:50:43 -0500

The cooling load of my project is 500 ton, I would like to use 2 chillers for my baseline case in OS1.7 so that each chiller is less than 300 ton. However, if I autosize the chiller, the part load ratio show that only 1 chiller will be used whose size is 500 ton. How should I change the control mode so that the load could distributed evenly to each chiller?

edit retag flag offensive close merge delete

1 Answer

Sort by ยป oldest newest most voted

answered 2015-08-25 11:23:22 -0500

updated 2015-08-27 09:38:35 -0500

In this case, only one chiller is being used because both chillers are probably sized to meet the full load of 500 tons.

Every chiller, boiler, etc. object has a field for "Sizing Factor" which is used to allocate the design load across multiple pieces of equipment. This defaults to 1.0, and the autosizing does not have a way to check if there are multiple chillers or boilers present. The detailed inputs for the chillers in OpenStudio should give you access to set the Sizing Factor for each one. For example, if you want 2 chillers of equal size, then set the sizing factor of both chillers to 0.5. If the total design chiller load is 500 tons, then this would result in 2 chillers of 250 tons.

edit flag offensive delete link more


@MJWitte, sorry my description may be confusing. I mean autosizing the capacity, not the sizing factor.

Yan gravatar image Yan  ( 2015-08-25 11:52:53 -0500 )edit

If you set the chiller's sizing factor to 0.6, then a 500 ton size will come out as 300 tons.

Archmage gravatar image Archmage  ( 2015-08-25 12:29:16 -0500 )edit

@MJWitte, I am not sure if my understanding is correct. Do you mean that in this baseline case, I should first check the cooling load, then divide 300 ton by the load and get a factor, finally input the factor into sizing factor?

Yan gravatar image Yan  ( 2015-08-25 13:12:19 -0500 )edit

@MJWitte, or do you mean dividing 1 by number of chiller and use this result as sizing factor?

Yan gravatar image Yan  ( 2015-08-25 13:21:51 -0500 )edit

@Yan, as @Archmage said the sizing factor is multiplied by the total design load. You can allocate the chillers any way that you want. 0.5 and 0.5, or 0.4 and 0.6, or 0.6 and 0.6 for some extra capacity. You don't need to do a preliminary run unless you have a target size you want to reach.

MJWitte gravatar image MJWitte  ( 2015-08-27 09:29:06 -0500 )edit

Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer


Question Tools

1 follower


Asked: 2015-08-25 09:37:56 -0500

Seen: 211 times

Last updated: Aug 27 '15