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Solving Plant temperatures are getting far too hot

asked 2021-03-26 12:48:14 -0600

Ahmad gravatar image

updated 2021-04-02 12:15:53 -0600

Hello,

Good day to all.

I am modeling an HVAC system for an offices building with 8 floors which has several AHUs and FCUs installed. The simulation stops with the following error (Copied as is):

   ** Severe  ** Plant temperatures are getting far too hot, check controls and relative loads and capacities
   **   ~~~   ** PlantLoop Name (DemandSide)= HOT WATER LOOP
   **   ~~~   ** PlantLoop Setpoint Temperature=67.0 {C}
   **   ~~~   ** PlantLoop Inlet Node (SupplySide) does not have a Setpoint.
   **   ~~~   ** PlantLoop Inlet Node (DemandSide) does not have a Setpoint.
   **   ~~~   ** PlantLoop Outlet Node (SupplySide) has a Setpoint.
   **   ~~~   ** PlantLoop Outlet Node (DemandSide) does not have a Setpoint.
   **   ~~~   ** PlantLoop Outlet Node (DemandSide) "NODE 134" has temperature=300.1 {C}
   **   ~~~   ** PlantLoop Inlet Node (DemandSide) "NODE 133" has temperature=300.2 {C}
   **   ~~~   ** PlantLoop Minimum Temperature=0.0 {C}
   **   ~~~   ** PlantLoop Maximum Temperature=100.0 {C}
   **   ~~~   ** PlantLoop Flow Request (SupplySide)=6.9 {kg/s}
   **   ~~~   ** PlantLoop Flow Request (DemandSide)=0.6 {kg/s}
   **   ~~~   ** PlantLoop Node (DemandSide) "NODE 134" has mass flow rate =0.6 {kg/s}
   **   ~~~   ** PlantLoop PumpHeat (SupplySide)=1982.2 {W}
   **   ~~~   ** PlantLoop PumpHeat (DemandSide)=185.5 {W}
   **   ~~~   ** PlantLoop Cooling Demand=6855817.6 {W}
   **   ~~~   ** PlantLoop Heating Demand=0.0 {W}
   **   ~~~   ** PlantLoop Demand not Dispatched=0.0 {W}
   **   ~~~   ** PlantLoop Unmet Demand=-6855817.6 {W}
   **   ~~~   ** PlantLoop Capacity=411062.5 {W}
   **   ~~~   ** PlantLoop Capacity (SupplySide)=411062.5 {W}
   **   ~~~   ** PlantLoop Capacity (DemandSide)=0.0 {W}
   **   ~~~   ** PlantLoop Operation Scheme=HOT WATER LOOP OPERATION SCHEMES
   **   ~~~   ** PlantLoop Operation Dispatched Load = 0.0 {W}
   **   ~~~   ** PlantLoop Operation Dispatched Load (SupplySide)= 0.0 {W}
   **   ~~~   ** PlantLoop Operation Dispatched Load (DemandSide)= 0.0 {W}

I read the other posts asking questions related to what I am asking too but I wasn't able to locate the problem.

The following are photos for the supply/demand of the loop (There are a lot of heating coils so I took two screenshots showing the essentials):

image description image description

The outlet node supply side set point is set to 80 C. The pumps are intermittent. Both AHUs and FCUs coils inlet water temperatures are set to 80 C and outlet water temperatures to 69 C. The AHUs coils capacities range between 5 kw to 10 kw (I have about 11 coils for the AHUs). The boiler design flow rate is auto-sized. The plant design loop exit temperature is 80 C and loop design temperature difference is 11 C.

I can't seem to solve my problem. Can you assist ?

Many thanks in advance.

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Comments

Why do you have a pump on the demand inlet branch?

Julien Marrec gravatar image Julien Marrec  ( 2021-03-29 03:09:08 -0600 )edit

There are two hot water pumps in the building, one primary and the second secondary. I added a pump in the demand side and used the common pipe option for the loop

Ahmad gravatar image Ahmad  ( 2021-03-29 05:40:42 -0600 )edit

I just removed it and left only the supply branch one and I still have the same problem

Ahmad gravatar image Ahmad  ( 2021-03-29 06:05:31 -0600 )edit

I would recommend to stay away from Common Pipe option, and instead use two PlantLoops connected by a HeatExchanger:FluidToFluid. You can find more info In the example 3 of Plant Application Guide: https://bigladdersoftware.com/epx/doc...

Julien Marrec gravatar image Julien Marrec  ( 2021-03-29 07:29:56 -0600 )edit

Your eplusout.err message seems to indicate there is a need for a (lot of) cooling on your demand side, not sure what's causing this for the limited amount of info I have.

Julien Marrec gravatar image Julien Marrec  ( 2021-03-29 07:31:26 -0600 )edit

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answered 2021-03-31 07:40:40 -0600

updated 2021-03-31 07:42:17 -0600

Note: I got the user's model privately.

So your problem is that all of your heating coils that are on the demand side of the Hot Water PlantLoop have an availability schedule that turns it off from April 1st to September 30. Your plant loop runaway temperature happens when the availability schedule goes to zero (with a bit of a time lag so the plant heat builds up).

You should turn off your plant loop according to the same schedule by adding an AvailabilityManager:ScheduledOff with this same schedule assigned. Then your file runs to completion.

image description

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Thanks Julien, it worked as you stated !

Ahmad gravatar image Ahmad  ( 2021-04-01 02:45:48 -0600 )edit

Please mark the answer as accepted so the thread is resolved. It's the checkmark below the Upvote / Downvote buttons.

Julien Marrec gravatar image Julien Marrec  ( 2021-04-01 02:47:02 -0600 )edit

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Asked: 2021-03-26 12:48:14 -0600

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Last updated: Apr 01