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Why is my water heater always on-cycle?

asked 2022-08-19 08:21:21 -0600

afleming's avatar

updated 2022-08-22 14:32:08 -0600

I'm trying to model a DHW system in Openstudio, and the simulation report is showing an energy expenditure that only makes sense if the water heater is on-cycle 24 hours a day. I've double checked that hot water temp is below the heater maximum temp, and I don't know what else could be happening. Is there a way to manually define cycle fraction for a hot water heater, or is it all handled autonomously?

The only clue I have to this problem is this message in the eplusout file:

* Warning * CalcEquipmentFlowRates: "COMMUNAL WATER EQUIPMENT" - Target water temperature is greater than the hot water temperature (28.55 C)

* ~~~ * Environment=MIRAMICHI ANN CLG .4% CONDNS DB=>MWB, at Simulation time=07/21 00:00 - 00:10

* ~~~ * ...target water temperature = 43.33 C

* ~~~ * ...hot water temperature = 14.78 C

* ~~~ * ...Note: target water temperature should be less than or equal to the hot water temperature

* ~~~ * ...Target water temperature should be less than or equal to the hot water temperature. Verify temperature setpoints and schedules.

EDIT: I may have solved the problem, I had assumed temperature setpoints used metric or imperial depending on the program settings (I have it set to imperial currently), but apparently they are always in Celsius? Using a Celsius value produces much more sane results.

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Can you show what inputs you put for the WaterHeater:Mixed? Are you sure it's correctly sized with respect to the WaterUse:Equipment you have? Seems like you have trouble heating up that water... I would double check the Output:Variable for the WaterUseEquipments too, make sure you somehow aren't requesting 10 times the amount you think you are.

Julien Marrec's avatar Julien Marrec  ( 2022-08-22 16:11:57 -0600 )edit

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answered 2022-08-23 06:12:56 -0600

afleming's avatar

I think I've solved the problem, I had assumed temperature setpoints used metric or imperial depending on the program settings (I have it set to imperial currently), but apparently they are always in Celsius? Using a Celsius value produces much more sane results.

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Asked: 2022-08-19 08:21:21 -0600

Seen: 7,978 times

Last updated: Aug 23 '22