OpenStudio Extreme Peaking Loads in Hot Water System

asked 2021-10-06 04:28:05 -0600

Alejandro Lopez's avatar

updated 2022-02-08 09:08:11 -0600

After including an electric hot water system, my model began having excessive electricity peaking loads, as seen in the graph below. NOTE: The peaks in this graph occur throughout the entire year, I zoomed in so that the peaks could be appreciated.

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System Description: I have designed a very simplified hot water loop for a 15-story residential building, containing apartments and offices. It contains (in clockwise order) an electric water heater, a setpoint manager, two water use connections, and a constant-speed pump. The water demand for all apartments (112 with 3.25 people per apartment) and all offices (8 with 10 people per office) is represented by each water use connection. Using the ASHRAE Modified Hunter Curve - Flow Chart and Fixture Units, I have selected a peak flow rate of 0.004227 m3/s (for all 112 apartments) and 0.000631 m3/s (for all 8 offices). The schedules I have used for hot water are trimodal (morning, afternoon, and evening peaks) with no mode exceeding 0.8 of the peak loads stated above.

It is clear that the hot water system is causing this, as when it is deleted, the electricity consumption becomes more regular. The graph below shows the building's electricity consumption over the same period:

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This post ( suggests that inappropriate heater sizing can lead to extreme peak loads. I tried doing the same thing they tried at the start, i.e. decrease peak flow rate and schedules, but still no major change. I also tried implementing the solution to the post (i.e. over-size the heater), but it actually caused peaking electricity demand for hot water to skyrocket (from 300kW to 1000kW). Why is this happening? And, how can it be solved?

To address Julien Marrec's comment (13/10/2021):

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So you have 112 in-unit water heaters in your model, all with the same hot water draw schedule, and many of the water heaters will fire at the same time resulting in large electricity spikes. I don't understand what the problem is?

shorowit's avatar shorowit  ( 2021-10-07 13:03:28 -0600 )edit

There are only 2 hot water connections each with one water facility (i.e. faucet) representing the two different demands, the demand from the apartments and the demand from the schedules. Hence, it is a simplified model, as the water demand for all 112 apartments is virtually coming out of one faucet. The hot water demands are being met by a single 30 m3 boiler. However, I think the problem lies in the sizing of the system.

Alejandro Lopez's avatar Alejandro Lopez  ( 2021-10-07 13:50:34 -0600 )edit

30m3 boiler? That looks very excessive. A rule of thumb would be 200L of DHW for a family of 3.25 per day, so about 20m3 of dhw per day. So your boiler is bigger than the daily load?

Can you post a picture of your DHW load schedule? (The one assigned to your wateruseequipment). Also, post a picture or osm snippet of your waterheater characteristics so we see how it's sized. Or better yet, share the entire model...

Julien Marrec's avatar Julien Marrec  ( 2021-10-13 03:23:18 -0600 )edit

Thank you for reaching back @julien-marrec ! I have uploaded the screenshots requested, I am currently uploading the model files to Google Drive, and will share the link once it's sorted. Are there any quicker ways of doing this? (uploading the model files directly onto this post did not work)

Alejandro Lopez's avatar Alejandro Lopez  ( 2021-10-13 04:58:37 -0600 )edit