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Modeling Hot Water Standby Losses Openstudio

asked 2015-12-15 15:36:28 -0500

dpud12's avatar

updated 2016-04-23 17:57:39 -0500

I would like to model a SHW system for a multifamily apartment building under two different setups:

  1. A central system with a gas boiler and central storage tank, and,
  2. In unit gas tank water heaters for each apartment.

As part of this I would like to model the losses from the piping and tanks for each system.

Is there any easier way to do this than by using the WaterHeater:Mixed object and calculating the On/Off Cycle Loss coefficient to Ambient Temperature externally in an excel sheet with some assumptions about piping/tank length, diameter, etc.

Is the WaterHeater:Mixed object currently the only option for ambient temperature losses?

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answered 2015-12-16 02:01:58 -0500

updated 2015-12-24 04:30:00 -0500

I agree with your statement about the tank losses, whether it'd be either an indirect storage tank or a tank-style water heater: you'd usually enter them in the WaterHeater:Mixed using the On or Off Cycle Loss Coefficient to Ambient Temperature. As stated in the Input/Output reference guide here:

Field: On-Cycle Loss Coefficient to Ambient Temperature

The loss coefficient [W/K] to the ambient air temperature. Often this coefficient is identical to the “UA” for skin losses. If the loss effects of the flue are being modeled in the Off-Cycle Loss Coefficient, than this field would have a different value accounting only for the skin losses.

Field: Off-Cycle Loss Coefficient to Ambient Temperature

The loss coefficient [W/K] to the ambient air temperature. Often this coefficient is identical to the “UA” for skin losses. However, it can also be used to model the loss effects of the flue in a combustion water heater, in addition to the skin losses.

For skin losses, you'll likely be already familiar with the UA coefficient if you've used other softwares such as eQuest before.

You can also implement some ambient temperature losses in Pipe:Indoor, instead of using a Pipe:Adiabatic. You'll have to set the Environment Type to Zone, specify which zone the pipe is in, create a pipe construction object, specify the inside diameter and the length of the pipe. Point is, it's surely not easier than the above. Make sure you save yourself some trouble and only model uninsulated piping.

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While not ideal; to mimic pipe losses I have sometimes slightly increased the target temperature at the fixture to mimic the temperature drop that occurs between the water heater and the fixture. Then I increased the volume of water (through fractional schedule) to mimic extra water use due to 'waiting for hot water'.

David Goldwasser's avatar David Goldwasser  ( 2015-12-24 09:55:48 -0500 )edit

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Asked: 2015-12-15 15:36:28 -0500

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Last updated: Dec 24 '15