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Boiler inlet temperature doesn't drop as expected

asked 2023-03-03 11:27:45 -0500

GSirotti's avatar

updated 2023-03-04 06:17:40 -0500

Hi, I am a beginner with e+ and OS. I would like to ask you for some advice on a project I am completing with OS. I am trying to simulate an HVAC system served only by two pipe fan coils. I modelled the system as suggested in this discussion (Here) and it seems to work during the summer, but I have some problems during the heating period.

First of all, I attach the layout of the system and would like to ask if it is correct not to define an air loop. I have only assigned FourPipeFanCoil units for each thermal zone and then connected them to the chilled and hot water loop, so without defining a dedicated air loop for each unit (fan coils do not have a dedicated air loop).

Chilled water loop here

Hot water loop here

Then I would like to ask why, if I graph the inlet/outlet temperature of the boiler, I have these results.

Boiler outlet temperature

Boiler inlet temperature

It seems to me that the temperature does not drop as I would expect, and I do not know why in summer it does not drop accordingly with the shutdown of the system (I forced the plant off with availability scheduled off manager). In general, the model simulates less gas consumption than the real one and I think that this could be the problem.


I m going to attach a more readable graph of temperatures and water flow rate through the boiler

Standard Winter Week Behaviour

During Summer Behaviour

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@GSirotti can you create a plot that has three lines:

  • boiler outlet temp
  • boiler inlet temp
  • boiler water flow rate (inlet node or outlet node would work)

Perhaps the boiler is shut off in the summer, so flow rate is 0 but temperatures are still constant from the last timestep where the boiler operated (spring).

Aaron Boranian's avatar Aaron Boranian  ( 2023-03-03 15:52:11 -0500 )edit

Hi Aaron, thanks for your reply. I've edited the question with two new graphs where you can see the mass flow rate through the boiler. As you can see the temperature is constant at 75°C during summer and whenever the plant is shut down, as I have scheduled the setpoint of outlet hot water at 75°C. Then during standard working days, it is strange that the temperature drop is really small (about 74°C at the inlet and 75°C at the outlet), I have several fan coils and I expect a bigger drop in temperature.

GSirotti's avatar GSirotti  ( 2023-03-04 06:20:18 -0500 )edit

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answered 2023-03-04 14:34:01 -0500

When there is NO flow through a boiler (like your second image for summer), the inlet and outlet temperatures will remain at the same value from the previous timestep where there WAS flow. The boiler does not store a volume of water, so there are no losses to surroundings like for a water heater. You can read more details about the hot water boiler model within EnergyPlus in the Engineering Reference.

When there IS flow through a boiler (like your first image for winter), you should have a temperature difference between inlet and outlet. If you are concerned that the temperature difference between inlet and outlet is smaller than you would expect, you have a few things to investigate:

  1. Are there cooling loads from the fan coils during the winter week? If there is enough solar exposure and internal gains, this could be the case. This would cause a lower overall heating load for the hot water system.
  2. Is the boiler flow rate (~35 kg/s or ~16 gpm) what you would expect? If not, then you should review the design water flow rate of the boiler.
  3. Are the sizing inputs for the hot water loop defined correctly? You can access these by clicking on the horizontal dashed line in the OpenStudio Application diagram for the hot water system found in the HVAC Systems tab. You can read more about these inputs in another Unmet Hours post.

It's hard to tell without reviewing your model. In the future, you can also add a link to your post where others can access the input file from Google Drive, Dropbox, or a similar file sharing service.

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Thanks for your support Aaron. I've changed the circuit's mass flow rate, which was really too high, and now the temperatures are more realistic.

GSirotti's avatar GSirotti  ( 2023-03-07 02:22:59 -0500 )edit

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Asked: 2023-03-03 11:27:45 -0500

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Last updated: Mar 07 '23