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How to model Indirect Water Heaters in OpenStudio?

asked 2014-10-15 10:33:26 -0500

updated 2015-11-08 11:07:56 -0500

Hello everyone,

I'm trying to model an indirect water heater using OpenStudio 1.5.0 and Energyplus 8.1. I've got the water heater (WaterHeaterMixed object) attached to the demand side of my heating water loop, and I've input the peak water use load under the Peak Use Flow Rate entry for the water heater, as well as specifying a Use Flow Rate Fraction schedule. I've also specified a tank size and a capacity for the heater.

When I examine the output, however, I'm getting ridiculously high natural gas used for water which is constant for every month (varying slightly based on the length of the month). It's saying 100+ MMBtu per month for a 1100 m2 building.

My question is two parts:

1) If the water heater is on the demand side of a boiler loop, does specifying a capacity and a Natural Gas fuel type double count the natural gas used to heat water?

2) What inputs should I look at to control the indirect water heater properly?

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@Benjamin. Please tag this question with 'openstudio' and either 'water-heater' or 'indirect-water-heater' to allow future users to locate it easily. Thanks.

__AmirRoth__'s avatar __AmirRoth__  ( 2014-10-15 11:29:55 -0500 )edit

Sure, Amir. Is my first posting here :)

Benjamin's avatar Benjamin  ( 2014-10-15 13:39:22 -0500 )edit

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answered 2014-10-15 11:20:48 -0500

It sounds like you're trying to model a boiler that provides hot water to a storage tank, is this correct? If the water heater is providing the energy to heat the water, not a boiler, it should be on the supply side of the plant loop, not the demand side.

If you're trying to model a passive storage tank, there is a way to do this but it's not fully supported by OpenStudio 1.5.0. A work around would be to set the capacity of the water heater to zero to ensure all of the energy comes from the boiler or heat exchanger.

The two inputs I use most often to control DHW energy usage are the "Use Flow Rate Fraction schedule" and " Peak Use Flow Rate".

This video is a good resource on creating DHW systems in openstudio.

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Thanks for your answer. I suspected that it wasn't fully implemented yet, based on what I've read of the energyplus documentation.

The water isn't directly coming from the boiler and being put into the tank, but rather a hot water coil is used to heat the water in the tank, and the coil is part of the hot water loop. I've tried using a heat exchanger inserted on the demand side, just before the tank, but OS crashed repeatedly trying this. I believe there are problems with the fluid to fluid heat exchanger in how it is implemented in OS.

Benjamin's avatar Benjamin  ( 2014-10-15 13:37:33 -0500 )edit

@Benjamin The crash may be due to a bug in the fluid to fluid heat exchanger when 1.4.0 first came out. Was your crash on export or run? If so it sounds like this. It has since been fixed but if heat exchangers were added in 1.4 and then then file upgraded the issue may still be there. The cause was a missing availability schedule in our library component. You could open it in a text editor to confirm and fix, or delete and re-add the object if you are using a new version of OpenStudio.

OpenStudio Issue 1152 - Closed

David Goldwasser's avatar David Goldwasser  ( 2014-10-15 13:50:21 -0500 )edit

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Asked: 2014-10-15 10:33:26 -0500

Seen: 814 times

Last updated: Oct 15 '14