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Water heating above the setpoint temp and results do not show energy changes with changes in water consumption

asked 2018-06-29 16:38:39 -0500

Kay gravatar image

updated 2018-07-02 10:13:55 -0500

I am trying to create a week's energy use data for water heating in Openstudio. I have included a mixed water heater in the simulation. I have set the setpoint temperature at 130F. My water use equipment is a shower, whose temperature of use is set at 110F. I also entered a water schedule for each day of the week. I am facing two problems:

  1. The results show that the water in the water heater has a temperature between 139.4F and 130.4F. I intend to keep the water temperature constant at 130F. Or if I cant keep it constant, then let it fluctuate between 130F and 131F. I do not understand why the water temperature goes up to 139, if my setpoint is at 130F.
  2. The results also show that the water heater only uses energy when the temperature of the water falls below 130.4, and it remains on till the temperature reaches 139.4. Then it turns off, until the heat loss pushes the temp back below 130.4. But the water heater does not turn on when there is hot water use as indicated in my schedule or indicate a faster rate of heat loss in the tank due to the introduction of cold water in the tank. This seems to be perplexing. How do I make sure that the water heater energy use reflects the changes in system when hot water is consumed?

Any help would be greatly appreciated.

Thank you.


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What software are you using?

__AmirRoth__ gravatar image __AmirRoth__  ( 2018-06-30 10:29:34 -0500 )edit

Sorry for the late reply. I am using openstudio.

Kay gravatar image Kay  ( 2018-07-02 09:01:04 -0500 )edit

In the future, add the name of the software as a tag (e.g., 'openstudio'). That way, email gets sent to people who are subscribed to this tag.

__AmirRoth__ gravatar image __AmirRoth__  ( 2018-07-02 09:03:22 -0500 )edit

Thank you for the note. I have edited the question to include openstudio in it.

Kay gravatar image Kay  ( 2018-07-02 10:14:38 -0500 )edit

1 Answer

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answered 2018-07-02 09:20:51 -0500

The mixed water heater should turn on at Setpoint - Deadband and turn off at Setpoint.

  Mixed Tank,          !- Name
  0.151,                   !- Tank Volume {m3}
  Hot Water Setpoint Temp Schedule,  !- Setpoint Temperature Schedule Name
  ,                        !- Deadband Temperature Difference {deltaC}

If the deadband temperature difference is blank, the IDD shows the default as 0.0 C, however, the code uses 0.5 C to avoid problems with the algorithm.


N2 , \field Deadband Temperature Difference
   \type real
   \units deltaC
   \minimum 0.0
   \default 0.0

if (rNumericArgs(2) > 0.0001) {
    WaterThermalTank(WaterThermalTankNum).DeadBandDeltaTemp = rNumericArgs(2);
} else {
    // Default to very small number (however it can't be TINY or it will break the algorithm)
    WaterThermalTank(WaterThermalTankNum).DeadBandDeltaTemp = 0.5;

So your simulation should not be heating the water temperature above 130 C. If you cannot find the problem then the input file will be needed to further diagnose the problem.

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Thank you so much rraustad. This solved my first question.

I was curious if you might have some insight on the second question too. I have attached a shower as a water use equipment in my model and added a week's water use schedule. But when the water is used, it does not reflect a change in the energy use of the water heater or a change in the rate of heat loss of the water in the tank. The idea is that if water is removed from the tank, it would be replaced with cold water and I would the temperature in the water tank to fall faster. But I cannot see that in the results. Thats my problem.

Kay gravatar image Kay  ( 2018-07-02 10:19:57 -0500 )edit

Can you post your WaterHeater:Mixed object in your question so we can see the inputs? Indent 4 spaces on each line and it will show up in the grey box as in my answer.

rraustad gravatar image rraustad  ( 2018-07-02 10:26:29 -0500 )edit

How do I exactly copy the WaterHeater:Mixed object from the model? I cannot copy it.

Kay gravatar image Kay  ( 2018-07-02 10:42:04 -0500 )edit

If you export your model as idf, then edit with a text editor and search for WaterHeater:Mixed, You can copy-paste.

rraustad gravatar image rraustad  ( 2018-07-02 11:01:10 -0500 )edit

WaterHeater:Mixed, Water Heater, !- Name 0.151, !- Tank Volume {m3} Water Heater Setpoint Temperature, !- Setpoint Temperature Schedule Name 0, !- Deadband Temperature Difference {deltaC} 65.55, !- Maximum Temperature Limit {C} Cycle, !- Heater Control Type 845000, !- Heater Maximum Capacity {W}

Kay gravatar image Kay  ( 2018-07-02 11:10:49 -0500 )edit

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Asked: 2018-06-29 16:38:39 -0500

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Last updated: Jul 02 '18