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Heat Exchanger to Preheat Service/Domestic Hot Water

asked 2017-12-05 17:11:53 -0500

updated 2017-12-06 13:22:23 -0500

I would like to preheat water into a WaterHeater:Mixed with the use of a fluid-to-fluid heat exchanger. The WaterHeater is used for service hot water. Is there a way I can setup the supply side to preheat the domestic supplied water?

Right now I have this setup but no preheating is occurring before the WaterHeater:Mixed. Hot Water Plant Loop

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answered 2017-12-06 03:30:22 -0500

updated 2017-12-06 03:31:57 -0500

What is your HeatExchanger:FluidToFluid Control Type? Make sure you use something like HeatingSetpointModulated (any of the heating ones).

Check the resulting IDF too, the HX currently ends up by default on a PlantEquimentOperation:Uncontrolled, while ideally it would end up first in line on the PlantEquipmentOperation:HeatingLoad (FYI, this is going to be the case now that my PR #2851 got merged in), but this isn't necessarily mandatory.

Start with he control type, if it still doesn't work, I'd try to change the PlantEquipmentOperation manually, using the ruby bindings: create a PlantEquimentOperation:HeatingLoad, put the HX then the water heater, and set that to the plant Loop. Set the PlantLoop's Load Distribution scheme to SequentialLoad (make sure you do end up with a PlantEquipmentList that has the HX first, then the WaterHeater:Mixed)

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Hi Julien, thank you for your response. I had the Control Type set to HeatingSetpointOnOff, I will try HeatingSetpoint:Modulated.

I was not aware of the PlantEquipmentOperation logic not prioritizing a HX for the HeatingLoad. I am not familiar with how to change the PlantEquipmentOperation using a ruby binding. Do you have any recommendations or tutorials?

Though I think I am familiar with making the edits directly to an exported IDF file. I will give it a try.

JustinShultz gravatar imageJustinShultz ( 2017-12-06 14:33:16 -0500 )edit

I setup a PlantEquipmentList with the HX before the boiler but found that the HX still has no effect on reducing the natural gas usage of the boiler. Following the setup in the image and the HX in the HeatingLoad list, will the HX preheat the boiler's cold water in or is the cold water coming from some other source? The diagram looks as if there is a return water stream but there isn't because the hot water from the boiler is going to service hot water. I would like to preheat the cold water that is going into the WaterHeater:Mixed to replenish the volume.

JustinShultz gravatar imageJustinShultz ( 2017-12-06 19:19:19 -0500 )edit

I'd have to look at the source code again to know at which node exactly is the cold water introduced, but I'm pretty sure at least on the supply inlet node it should be there. Every liter of hot water used by SHW is replaced by cold water (which temperature is set by the Site:WaterMainsTemperature or defaulted to 10°C if you didn't supply it)

Julien Marrec gravatar imageJulien Marrec ( 2017-12-10 14:34:21 -0500 )edit

Without seeing your full model (including but not limited to the source plant loop serving the HX), I can't tell you what's wrong (and this is going further than what I'm comfortable doing for free on this site)

Julien Marrec gravatar imageJulien Marrec ( 2017-12-10 14:35:25 -0500 )edit

The cold water is passed on the water use equipment outlet node, which amounts to updated temperature at the demand side outlet node. See here

Julien Marrec gravatar imageJulien Marrec ( 2018-02-14 03:14:15 -0500 )edit

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Asked: 2017-12-05 17:11:53 -0500

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Last updated: Dec 06 '17