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I'm modeling the change from a constant speed pump to a variable speed pump for a hot water loop with a standard natural gas boiler in OpenStudio. The measure is applied as expected. However, the results are not as expected. The electrical savings are as expected at ~3000 kWh, but I did not expect an energy equivalent increase in natural gas usage of 10 Million BTU (the kWh plus the efficiency of the boiler). Looking deeper, I was able to see that the output power is directly applied as an increase in heat to the water.

Looking deeper into the Energy Plus code, it appears this functionality is on purpose. This page has the source code and has a comment on line 1726 that says "We assume that all of the heat ends up in the fluid eventually since this is a closed loop". I understand there will be an increase in heating energy, but my expectation is that the heat gain is a fraction of the pumping. I assumed more heat would is lost to the surroundings in the case of a constant volume pump.

So, is EnergyPlus's assumption correct that most or all of the pumping energy ends up in the fluid eventually?

I'm modeling the change from a constant speed pump to a variable speed pump for a hot water loop with a standard natural gas boiler in OpenStudio. The measure is applied as expected. However, the results are not as expected. The electrical savings are as expected at ~3000 kWh, but I did not expect an energy equivalent increase in natural gas usage of 10 Million BTU (the kWh plus the efficiency of the boiler). Looking deeper, I was able to see that the output power is directly applied as an increase in heat to the water.

Looking deeper into the Energy Plus code, it appears this functionality is on purpose. This page has the source code and has a comment on line 1726 that says "We assume that all of the heat ends up in the fluid eventually since this is a closed loop". I understand there will be an increase in heating energy, but my expectation is that the heat gain is a fraction of the pumping. I assumed more heat would is lost to the surroundings in the case of a constant volume pump.

So, is EnergyPlus's assumption correct that most or all of the pumping energy ends up in the fluid eventually?

NOTE: I've saved files here

I'm modeling the change from a constant speed pump to a variable speed pump for a hot water loop with a standard natural gas boiler in OpenStudio. The measure is applied as expected. However, the results are not as expected. The electrical savings are as expected at ~3000 kWh, but I did not expect an energy equivalent increase in natural gas usage of 10 Million BTU (the kWh plus the efficiency of the boiler). Looking deeper, I was able to see that the output power is directly applied as an increase in heat to the water.

Looking deeper into the Energy Plus code, it appears this functionality is on purpose. This page has the source code and has a comment on line 1726 that says "We assume that all of the heat ends up in the fluid eventually since this is a closed loop". I understand there will be an increase in heating energy, but my expectation is that the heat gain is a fraction of the pumping. I assumed more heat would is lost to the surroundings in the case of a constant volume pump.

So, is EnergyPlus's assumption correct that most or all of the pumping energy ends up in the fluid eventually?

To say this another way, if you have an electric boiler, would a VFD on a hot water pump result in an increase in energy usage and not save a building owner anything?

NOTE: I've saved files here

I'm modeling the change from a constant speed pump to a variable speed pump for a hot water loop with a standard natural gas boiler in OpenStudio. The measure is applied as expected. However, the results are not as expected. The electrical savings are as expected at ~3000 kWh, but I did not expect an energy equivalent increase in natural gas usage of 10 Million BTU (the kWh plus the efficiency of the boiler). Looking deeper, I was able to see that the output power is directly applied as an increase in heat to the water.

Looking deeper into the Energy Plus code, it appears this functionality is on purpose. This page has the source code and has a comment on line 1726 that says "We assume that all of the heat ends up in the fluid eventually since this is a closed loop". I understand there will be an increase in heating energy, but my expectation is that the heat gain is a fraction of the pumping. I assumed more heat would is lost to the surroundings in the case of a constant volume pump.

So, is EnergyPlus's assumption correct that most or all of the pumping energy ends up in the fluid eventually?

To say this another way, if you have an electric boiler, would a VFD on a hot water pump result in an increase in energy usage and not save a building owner anything?