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2019-02-14 08:39:13 -0500	received badge	● Popular Question (source)
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2016-01-08 14:36:25 -0500	commented answer	Why does a constant speed pump to variable speed pump result in electrical energy savings and an equivalent heating energy increase? Yes, VFDs are commonly installed on heating hot water pumps and modulated to meet a differential pressure setpoint. Here is an article on that. From the article you posted, there are reheat savings associated with a VFD due to lower flow through the pipe. The model I had would not have accounted for it, as there is only adiabatic pipe. Adding pipe between the pump and the boiler doesn't seem to result in Heating Energy Savings, either.
2016-01-08 13:09:15 -0500	received badge	● Editor (source)
2016-01-08 12:50:08 -0500	commented answer	Why does a constant speed pump to variable speed pump result in electrical energy savings and an equivalent heating energy increase? Yes, those are the coefficients used. I'll post the measure and OpenStudio Model to the original question shortly.
2016-01-08 11:21:14 -0500	commented answer	Why does a constant speed pump to variable speed pump result in electrical energy savings and an equivalent heating energy increase? Thanks for responding and expanding on what I wrote! My model has a FracMotorLossToFluid of 0, so the shaft power heats the fluid. When changing from a constant speed pump to a variable speed pump, and not changing the motor efficiency, all the energy savings from a VFD result in a heating penalty. My question is, should all electrical savings from a VFD should result in a heating penalty? I've never seen the heating penalty mentioned or discussed before.
2016-01-08 10:35:18 -0500	received badge	● Student (source)
2016-01-08 10:34:49 -0500	asked a question	Why does a constant speed pump to variable speed pump result in electrical energy savings and an equivalent heating energy increase? 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?