Confusing indirect water heater outputs

[shorowit]

A user reported that the hot water gallons outputs for an indirect water heater (base-dhw-indirect.xml) are much lower than that of a conventional water heater (base.xml). We've seen this before for HPWHs (see here), but in that case it is much more complicated due to the use of a stratified tank. In this case, we have a mixed tank, so it should be more straightforward.

@yzhou601 and I briefly started to investigate and noticed that the Water Heater Use Side Outlet Temperature for the indirect water heater is 1 deg-C higher than the standard water heater, so that's part of it. That appears to be caused by the fact that we are using the same (upper) tank setpoint in both cases, but the indirect water heater setpoint does not actually use a deadband? We quickly tried to address that, but the hot water gallons outputs were still far too low (though they did increase a bit).

So the question is... are we modeling the wrong hot water use? Or is it just a reporting issue? The hot water delivered energy is pretty similar between the two cases, so that may suggest that it's just a reporting issue.

[yzhou601]

A few findings after investigating more into the results:

1. We can actually simplify our logic in the workflow by using IndirectHeatPrimarySetpoint source side flow control mode, so that we don't need to create two setpoint schedules for the tank, and the deadband can be implemented correctly, similar to the fix in prior discussion but more clean.
2. The hot water gallons still looked off after fixing the ddb, this seems to be caused by the wateruse:equipment object. Looking into the timeseries results, the mixing of hot water and cold water looks weird, it's not meeting the target temperature, see below an example:

So it's mixing too much cold water and there's "unmet load", so the hot water gallons are low. However, looking at the tank temperature, it has the ability to meet those loads. Other than the "dhw fixtures", equipment like "dishwasher" also has some cold water mixing happening, those kinds of equipment are supposed to draw 100% hot water at usage:

So this definitely looks like an E+ issue to me.

[yzhou601]

#2190