How to combine conventional water-heater with heat-pump water-heater?

image description I would like to simulate a configuration where a conventional water heater running on specified fuel type and a heat pump water heater are combined in such a way that both share the same water tank. This water tank then supplies heating and hot water. (The water tank is modeled also by a weater heater with zero Watts heating capacity).

Although in OpenStudio 1.6.0 there is no heat pump water heater in the menues, it is possible to create a preliminary scheme in OpenStudio with two conventional water heaters and then to adapt the loops in IDF-editor replacing one water heater with a heat pump water heater.

The supply side loop looks like this:

  o---[Water Heater]------------o
  |                             |
  o---[Heat Pump Water Heater]--o---[Water Tank]--o
  |                                               |
                                                  |
                                               [Setpoint MGR]
                                                  |

However, this scheme did not work satisfactorily, since the setpoint manager apparently has not been able to keep temperature in the Water Tank within desired bandwidth.

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OS-user-AT

asked 2015-05-12 05:48:57 -0500, updated 2015-05-14 05:36:31 -0500

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By default (if you have a single setpoint manager on the supply outlet node) OpenStudio will create a load based operation scheme. I think you might want to use a component setpoint operation scheme instead. To do this in OpenStudio simply drop setpoint managers on the immediate outlet nodes of the water heater components. OpenStudio will interpret this as you wanting to control those components to setpoint and will therefore use the component setpoint operation scheme when creating the idf file.

update

I have found a bug when you attempt component setpoint operation on a component directly upstream of the supply outlet node. The workaround is to insert a pipe between the setpoint component and the supply outlet node.

image description

Kyle Benne

answered 2015-05-13 09:20:21 -0500, updated 2015-05-15 08:53:42 -0500

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Thanks, that's instructive. However, after inserting setpoint managers at the outlet nodes of water heaters, I got the following error message:

ERROR:Invalid: Load range based components are separated by other control type components. Load Range Based should be grouped together on each flow path.
   ...Summary of Errors that led to program termination:...

I have uploaded a screenshot of plant loop taken after I tried your advice.

OS-user-AT (2015-05-14 05:34:30 -0500) edit

maybe you need not consider the error messages as a bug, because when testing your workaround, my simulation results show that the supply loop setpoint manager has practically no effect in this scheme, apparently because it cannot overrule the component-based setpoint managers. The idea that we could use two water heaters feeding a common buffer water tank (with no heating capacity, but desired temperature bandwidth) is probably not feasible in such a scheme.

OS-user-AT (2015-05-18 05:32:40 -0500) edit

If I understand correctly you are correct. In the layout I show in the picture above, the setpoint manager on the supply outlet node (the node directly after the pipe) has no impact at all on the simulation. The hot water heaters are entirely controlled by their respective component setpoints. The setpoint manager on the supply outlet is merely there to satisfy EnergyPlus requirements.