HeatExchanger:FluidtoFluid not correctly delivering hot water load to boiler, causing energy not balanced

[yzhou601]

Issue overview

System description:

I am modeling a combi water heating system which is connected to a boiler serving hot water for both space heating and water heating. I did it in this way:

I modeled a hot water tank (no water heater) in a plant loop demand side + a heat exchanger in supply side, and then I put this heat exchanger to boiler plant loop demand side. The tank source side supplies 52.6667C hot water and boiler plant loop supplies 82+C hot water. Whenever there's hot water draw, the tank + heat exchanger plant loop turned on to abstract heat from boiler. The tank source side flow rate needed is large to guarantee it's meeting hot water use demand. So the Indirect Water Heating Recovery Time is set to small.

An hourly time step simulation is run.

And there're some problems running this model:

1. The first problem is that DHW SOURCE HX:Fluid Heat Exchanger Heat Transfer Energy and WATER HEATER:Water Heater Source Side Heat Transfer Energy [J] never gives the same annual number, which indicates energy not balanced.

2. Might relate to the first problem, when I change tank source side mass flow rate (that is, reduce indirect water heating recovery time), but keep the hot water use profile the same, something weird happened under this condition. Model 1: Combi-system 50 gallon tank: Model2: Combi-tankless-system with 1gallon tank:

Model1 tank Energy unbalanced: Water Heater Source Side Heat Transfer Energy - Fluid Heat Exchanger Heat Transfer Energy:

Model2 Energy unbalanced: Water Heater Source Side Heat Transfer Energy - Fluid Heat Exchanger Heat Transfer Energy:

These graphs showed problem 1, energy is not balanced looking at tank source side heat transfer energy and heat exchanger heat transfer energy. It looks like heat exchanger is not delivering the correct hot water load to boiler, both in storage tank and tankless case. In any of test runs above, the hot water demand is the same, the large discrepancy in heat exchanger and boiler side looks very confusing. And the inconsistency is way larger than negligible scale. The only difference between model1 and model2 is that the tanks are of different sizes. But the discrepancy is not following a certain pattern, so no clue what happened behind, which is suspicious.

The gas energy consumed by boiler shows a large discrepancy (up to 10+GJ) between different recovery time settings(size of source side) though any of them looks to be well maintaining hot water use if looking at tank side. I have no idea how this could happen.

Attached IDF&EPW file Click here

Details

Some additional details for this issue (if relevant):

• Platform (Windows, 10)
• Version of EnergyPlus (E+ 9.2)
• Unmethours link or helpdesk ticket number

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• [x] Defect file added (list location of defect file here)
• [ ] Ticket added to Pivotal for defect (development team task)
• [ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[yzhou601]

@mjwitte @rraustad @Myoldmopar Do you mind taking a look at this issue? HeatExchanger:FluidtoFluid looks to be the correct one to use for this system (that requires hot water plant loop to be connected with boiler plant loop). But the large energy discrepancy between Fluid Heat Exchanger Heat Transfer Energy and Water Heater Source Side Heat Transfer Energy did look confusing.

[yzhou601]

The energy issue can be mitigated by increasing min plant iterations or system time step. It looks like the issue is related to temperature convergence tolerance, because for each hour, the temperatures are only slightly different looking at HX nodes and tank source nodes. The large mass flow rate magnified the energy issue. I've found another more dynamic solution to this system. I am closing this issue.