Very low heating consumption with heat exchanger - Openstudio

asked 2016-10-07 09:36:29 -0500

dpud12 gravatar image

updated 2017-05-03 20:36:42 -0500

I am working on modeling a multistory apartment building with a Water Source Heat Pump system. I am modeling the system with a heat pump loop connected to separate heating and cooling loops with heat exchangers as shown in the picture below. The heating loop has a natural gas boiler and the cooling loop has a cooling tower.

When I run the model in this configuration, I get a very low gas heating consumption. In contrast, when I run the model with the Boiler and Cooling tower directly in the heat pump loop, I get a much higher natural gas consumption. It appears to me that Openstudio is not counting the gas consumption of the boiler when it is in the heating loop. I ran a test by removing the heat exchanger between the heat pump loop and the heating loop, and compared the system node temps of the heat pump loop with and without the heat exchanger for the heating loop connected. As you can see from the graphs below, the boiler does appear to be maintaining the loop temps at the setpoint when connected through the heat exchanger, however, there is no difference in the natural gas consumption between these two scenarios.

Thanks in advance for any insight you may have into what is occurring!

Primary Heat Pump Loop

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With Boiler connected through heat exchanger, heat pump loop system node temp

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Without Boiler connected through heat exchanger, heat pump loop system node temp

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Is there anything on the heating loop when you take the boiler off (heating loop in graph #2?). It seems that the heat pump is doing the majority of heating - how much Electricity do you have going towards heating for both scenarios?

If you are modeling a water-source heat pump, what is the ultimate source of the heat? ground? A body of water? A waste heat loop somewhere?

To model this, I wouldn't use a boiler on the heating loop. I'd use a district heating object, with s setpoint manager to follow outdoor air temperature or a schedule temp to match your heat source - whatever it is.

mdahlhausen gravatar image mdahlhausen  ( 2016-10-07 14:45:09 -0500 )edit

One of the things that I am trying to understand is where the heat is ultimately coming from, as currently there are no heating components tied to the heat pump loop, and so no clear explanation for why loop temperature does not continue to drop lower than -30 C. Perhaps it is from the pump, however, the pump consumption does not change significantly between the two scenarios (229 MWh vs 195 MWh). The electricity for heating is 33 MWh for the scenario where I have the boiler in heating loop, versus 154 MWh when I don't have the heating loop connected to the heat pump loop.

dpud12 gravatar image dpud12  ( 2016-10-11 10:38:15 -0500 )edit