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Why is hot water temperature in tank and the water use equipment different in openstudio?

asked 2018-07-16 16:36:59 -0500

Kay gravatar image

updated 2020-01-20 15:10:13 -0500

Hi, I was interested in the electric water heater energy consumption for a pre-defined water schedule in Openstudio. In the results section, the temperature output confused me. I got the following results for temperature: 1. The red line indicates the equipment use mixed water temperature, which is 110F, as that was my input value. 2. The blue line indicates the water temperature in the water heater tank. 3, The yellow line indicates the equipment use hot water temperature.

I am surprised that the hot water temperature in the tank and the equipment use is not the same. Even more surprising is the fact that in some situations, the hot water in the tank is lower than the hot water for the equipment use. Why could this be happening?

Thank you so much in advanvce!

Note: The equipment use cold water temperature is 50F.

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answered 2018-07-16 17:19:01 -0500

Hot water use equipment has a target faucet temperature which is lower than the hot water supply temperature from the water heater. Rarely does one have the hot water valve on full with the cold water valve shut when showering or washing hands at a faucet. Target faucet temperatures are 95°F-115°F depending on the use, while the hot water heater setpoint temperature is usually around 120°F-140°F to avoid growth of legionella bacteria. The simulation is accounting for this mixing, and water use values in your equipment specify mixed water use rate (cold + hot), not hot water use rate.

Hot water heaters are also connected to a loop, and if you are seeing a difference in tank outlet temperature and loop supply outlet temperature, my guess would be that you have a pipe bypass for your demand equipment and a constant volume pump running water through the loop continuously. This would mix and lower the hot water temperature from the tank outlet.

The dip you are seeing in hot water tank temperature is possibly an artifact of the specific output variable you are using. Are you using the WaterHeater:Mixed or WaterHeater:Stratified object? And which of these output variables are you using to signify hot water heater temperature?:

  • HVAC,Average,Water Heater Final Tank Temperature [C]
  • HVAC,Average,Water Heater Use Side Inlet Temperature [C]
  • HVAC,Average,Water Heater Use Side Outlet Temperature [C]
  • HVAC,Average,Water Heater Source Side Inlet Temperature [C]
  • HVAC,Average,Water Heater Source Side Outlet Temperature [C]
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And what's your output frequency? Use detailed. Also check out the Plant and water heater Unmet loads.

Julien Marrec gravatar image Julien Marrec  ( 2018-07-17 02:28:33 -0500 )edit

Hi Matthew,

Thank you for the answer.

I understand that the temperature in the tank and the temperature in the faucet is different; the one in the faucet is hot water + cold water. But the graph that I have included shows hot water use, not the mixed use.
My output variables are: water use equipment hot water temperature: yellow water use equipment mixed water temperature: red water heater tank temperature: blue cold water temperature is 50F

The tank outlet temperature and loop supply outlet temperature are the same. There is no difference there.

Kay gravatar image Kay  ( 2018-07-17 10:38:59 -0500 )edit

My question still remains: If yellow line (hot water) + 50F (cold water) = red line (mixed temperature at the faucet), and yellow line gets its water from the tank (blue line), why is there a difference in the yellow and blue line? How is the yellow line above the blue line even though yellow line gets its water from the blue line.

Kay gravatar image Kay  ( 2018-07-17 10:42:24 -0500 )edit

Bonjour Julien, Thank you for the answer. My output frequency is detailed yes. If the output faucet temperature is equal to the demanded temperature of 110F, what is the unmet load then? Thanks again.

Kay gravatar image Kay  ( 2018-07-17 10:44:00 -0500 )edit

Kay, can you update your question with a screenshot of your hot water loop? As mentioned in my answer, it may be related to the way your loop is set up. And please show both the "Water Heater Final Tank Temperature" and the "Water Heater Use Side Outlet Temperature".

mdahlhausen gravatar image mdahlhausen  ( 2018-07-17 11:09:35 -0500 )edit

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Asked: 2018-07-16 16:36:59 -0500

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Last updated: Jul 17 '18