Explicit Tankless Water Heater Model #11399
Description
General Summary
EnergyPlus does not have a tankless water heater model. This is a pretty common upgrade for residential water heaters (or basically a requirement in certain places like CA for new construction). Workarounds generally model a tank with negligible volume and some derate factor baked in, but more accurate models exist and would need to be implemented in EnergyPlus.
Detailed Description
Tankless water heaters are a relatively common upgrade for homes looking to reduce fuel use for water heating. They have much larger burners and little to no storage volume to meet hot water loads almost instantaneously. While the burner efficiency is well characterized, there is some losses due to energy that goes into heating up the large internal heat exchanger or is lost as that heat exchanger cools down after hot water is drawn. ANSI-RESNET 301 specifies a derate factor that differs depending on whether EF or UEF is used for rating the product to account for how much losses there are with different draw profiles. That is, the different numbers used there indicate a constant derate doesn't apply for any hot water usage profile.
A relatively simple first principles model for a tankless water heater has been published since 2008, but has never been implemented in EnergyPlus. I know this feature was also requested last year and never got properly implemented. Adding this issue to track the implementation and support another request to add this functionality into EnergyPlus. This would avoid the need for workarounds that generally assume a constant derate factor is applied to the rated efficiency when modeling a tank with ~0 volume and no tank losses.
Possible Implementation
No response